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National 4/5 Physics

- In addition to set homework you will be expected

to - finish off class notes and regularly review work

against - the learning outcomes.
- You will be expected to take responsibility for

your own - learning and for seeking help when you need it.

At the - end of each section, you must ensure all notes

are - completed and examples attempted.

In unit 1 we will learn about the physics of

motion. We will focus on the language, principles

and laws which describe and explain the motion of

an object. Kinematics, also known as Mechanics is

the science of describing the motion of objects

using words, diagrams, numbers, graphs and

equations.

The goal is to develop mental models which

describe and explain the motion of real-world

objects.

- Key words vectors, scalars, distance,
- displacement, speed, velocity.
- By the end of this section you will be able to
- Describe what is meant by vector and scalar

quantities - State the difference between distance and
- displacement
- State the difference between speed and velocity
- State that force is a vector quantity
- Use a scale diagram to find the magnitude and

direction - of the resultant of two forces acting at right

angles to - each other.

- Key words average speed
- By the end of this section you will be able to
- Describe how to measure an average speed
- Carry out calculations involving distance, time
- and average speed.

Which of these are units of speed?

metres

gallons

- miles per hour

seconds

minutes

amperes

miles

kilometres per second

miles per minute

watts

metres per second

Newtons

Speeds in.

- In Physics we normally use units
- m/s for velocity.

Average speed (m/s)

Snail

High speed train

Sound

270 m/s

13.4 m/s

0.006 m/s

UK town

Fast jet

747 jumbo jet

29790 m/s

10.3 m/s

97 m/s

Air molecule

Earth in orbit

Falcon

60 m/s

648 m/s

7500 m/s

Olympic sprinter

Earth satellite

UK motorway

500 m/s

300000000 m/s

340 m/s

Walking speed

Light speed

Concorde

1.7 m/s

31 m/s

833 m/s

Average speed ( m/s )

Light speed 300000000 m/s

Earth in orbit 29790 m/s

Earth satellite 7500 m/s

Concorde 648 m/s

Air molecule 500 m/s

Fast jet 833 m/s

747 jumbo jet 270 m/s

Sound 340 m/s

Falcon 97 m/s

High speed train 60 m/s

UK motorway 31 m/s

UK town 13.4 m/s

Walking speed 1.7 m/s

Olympic sprinter 10.3 m/s

Snail 0.006 m/s

What is speed?

- When we talk about speed we mean
- the distance covered by an object in a
- given time.

What is speed?

- If Hamish (the dog) runs 10 metres in 2
- seconds, what is his speed?

What is speed?

- His speed is 5 metres per second.
- So speed is

What is speed?

- If you forget the formula think of cars

travelling at 30 kilometres per hour

- Key words average speed
- By the end of this section you will be able to
- Describe how to measure an average speed
- Carry out calculations involving distance, time
- and average speed.

distance

time

speed

Speed Calculations

- A cyclist travels 100 m in
- 12 s. What is her speed?

- Step 1 write down what you know.

d 100 m t 12 s speed ?

- Step 2 write down your formula. You can use

the triangle to help you but remember you get

no marks for this!

d 100 m t 12 s speed ?

d speed x t

- Step 3 substitute in your values.

d speed x t

d 100 m t 12 s speed ?

- Step 4 rearrange

d speed x t 100 speed x 12

d 100 m t 12 s speed ?

- Step 5 calculate

d speed x t 100 speed x 12 speed

8.33

d 100 m t 12 s speed ?

100

12

- Step 6 units!!!!

d speed x t 100 speed x 12 Speed

8.33 m/s

d 100 m t 12 s speed ?

100

12

- Key words average speed, instantaneous
- speed
- By the end of this section you will be able to
- Describe how to measure instantaneous speed.
- Identify situations where average speed and
- instantaneous speed are different.

Instantaneous and average speed

- Are instantaneous and average speed the same?

Instantaneous or average?

- A cars speed between North Berwick and
- Edinburgh
- Average

Instantaneous or average?

- The speed read from a cars speedometer
- Instantaneous

Instantaneous or average?

- A tennis balls speed as it crosses the net
- Instantaneous

Instantaneous or average?

- A racing cars speed over a lap of the track
- Average

Instantaneous or average?

- A parachutists speed as he/she lands
- Instantaneous

Scalars and Vectors

- Imagine a boat
- making a distress
- call to the
- coastguard.
- The boat tells the
- coastguard he is 60 km
- from Aberdeen.

Scalars and Vectors

- Is this enough
- information for the
- coastguard to find
- the boat?

Scalars and Vectors

Scalars and Vectors

- The coastguard needs both
- distance (size)
- and
- direction
- to find the boat.

Scalars and Vectors - Definition

- A scalar is a quantity which has only
- magnitude (size). It is defined by a
- number and a unit.
- A vector is a quantity which has
- magnitude (size) and direction. It is
- defined by a number, a unit and a
- direction.

Distance and Displacement

A pupil walks from her house to her school. Her

brother makes the same journey, but via a shop.

How far has the girl walked? How far has her

brother walked?

500 m

300 m

400 m

Distance and Displacement

The girl has walked 500 m. Her brother has walked

700 m.

Distance is a scalar quantity it can be defined

simply by a number and unit.

500 m

300 m

400 m

Distance and Displacement

Distance is simply a measure of how much ground

an object has covered.

500 m

300 m

400 m

Distance and Displacement

But how far out of place is the girl? And her

brother? Displacement is a vector which requires

number, unit and direction.

Distance and Displacement

The girl has a displacement of 500 m at a bearing

of 117 East of North.

500 m

300 m

400 m

Distance and Displacement

What is her brothers displacement?

500 m

300 m

400 m

Distance and Displacement

Her brother has a displacement of 500 m at a

bearing of 117 (117 East of North).

500 m

300 m

400 m

Distance and Displacement

Their displacement (how far out of place they

each are) is the same.

500 m

300 m

400 m

Speed and Velocity

Speed is a scalar quantity requiring only

magnitude (number and unit). Velocity is a

vector, requiring magnitude and direction.

Speed and Velocity

Speed tells us how fast an object is

moving. Velocity tells us the rate at which an

object changes position.

Speed and Velocity

- Imagine a person stepping one step
- forward, then one step back at a speed of
- 0.5 ms-1.
- What is the persons velocity? Remember
- velocity keeps track of direction. The
- direction of the velocity is the same as
- the direction of displacement.

Speed and Velocity

- Key words vectors, scalars, distance,
- displacement, speed, velocity.
- By the end of this section you will be able to
- Describe what is meant by vector and scalar

quantities - State the difference between distance and
- displacement
- State the difference between speed and velocity
- State that force is a vector quantity
- Use a scale diagram to find the magnitude and

direction - of the resultant of two forces acting at right

angles to - each other.

Distance and Displacement

Speed and Velocity

A physics teacher walks 4 meters East, 2 meters

South, 4 meters West, and finally 2 meters North.

The entire motion lasted for 24 seconds.

Determine the average speed and the average

velocity.

The physics teacher walked a distance of

12 meters in 24 seconds thus, her average speed

was 0.50 m/s. However, since her displacement

is 0 meters, her average velocity is 0 m/s.

Remember that the displacement refers to the

change in position and the velocity is based upon

this position change. In this case of the

teacher's motion, there is a position change of 0

meters and thus an average velocity of 0 m/s.

Scalar or Vector?

- Key words vectors, scalars, resultant, scale

diagram - By the end of this lesson you will be able to
- Describe what is meant by vector and scalar

quantities - State the difference between distance and
- displacement
- State the difference between speed and velocity
- State that force is a vector quantity
- Use a scale diagram to find the magnitude and

direction - of the resultant of two forces acting at right

angles to - each other.

Vectors

- Vectors can be represented by a line
- drawn in a particular direction.
- The length of the line represents the
- magnitude of the vector.
- The direction of the line represents the
- direction of the vector.

Addition of Vectors

- When two or more scalars are added
- together, the result is simply a numerical
- sum.
- For example a mass of 3kg and a mass of
- 5 kg, when added, make a mass of 8kg.

Addition of Vectors

- When two or more vectors are added
- together, providing they act in the same
- direction, the addition is straightforward.

5 N

3 N

8 N

Addition of Vectors

- If they are acting in opposite directions

5 N

3 N

2 N

- Key words vectors, resultant
- By the end of this section you will be able
- to
- Use Pythagoras and Trigonometry to find
- the magnitude and direction of the
- resultant of two forces acting at right
- angles to each other.

Addition of Vectors

- The resultant of two or more vectors
- which act at angle to each other can be
- found either using a scale diagram, or by
- Pythagoras and trigonometry.

To find the resultant of a set of vectors using a

scale diagram

- 1. Decide on a suitable scale and write this
- down at the start
- 2 Take the direction to the top of the page as
- North. Draw a small compass to show this.
- 3 Draw the first vector ensuring it is the
- correct length to represent the magnitude
- of the vector, and it is the correct
- direction.

To find the resultant of a set of vectors using a

scale diagram

- Draw an arrow to represent the second
- vector starting at the head of the first.
- Vectors are always added head to tail.
- 5 The resultant vector can now be determined
- by drawing it on the diagram from the tail
- of the first to the head of the last vector.
- The magnitude and direction of this vector
- is the required answer.
- 6 The final answer must have magnitude and

direction either a bearing from North or an

angle marked clearly on the diagram

Scale Diagrams

- Scale remember if the question is in ms-1 then

your scale should be a conversion from cm to

ms-1. - Direction draw compass on page
- 1st vector length and direction
- 2nd vector tail of 2nd starts at tip of first
- Resultant vector tail of 1st to tip of last
- Answer must include magnitude (including units)

and direction

Scale Diagrams

- Direction should be given as a three
- figure bearing from North
- e.g. 045 or 175 or 035
- If you give any other angle, you must
- clearly mark it on the scale diagram.

- A car travels 100 km South, then 140 km
- East. The time taken for the whole
- journey is 3 hours.
- Using a scale diagram (and the six step
- process) find
- the cars total distance travelled
- its average speed
- its overall displacement
- its average velocity

Scale Diagrams

- Scale diagrams are used to find the
- magnitude and direction of the resultant
- of a number of a set of vectors.

The tropical island of Sohcahtoa

The tropical island of Sohcahtoa

The tropical island of Sohcahtoa

The tropical island of Sohcahtoa

hyp

opp

?

adj

- The Old Arab Carried A
- Heavy Sack Of Hay
- Tan Opp/Adj Cos Adj/Hyp
- SinOpp/Hyp

hyp

opp

?

adj

N

3 km North

E

4 km East

Remember The vectors above are not tip to tail.

You must join them tip to tail

N

R ?

3 km North

?

E

4 km East

Bearing of 053.10

6N

6N North, 8N East - what is the resultant force

R ?

We ADD vectors HEAD to TAIL tip to toe

8N

R

?

- Key words acceleration, velocity
- By the end of this section you will be able to
- Explain the term acceleration
- State that acceleration is the change in
- velocity per unit time
- Carry out calculations involving the relationship
- between initial velocity, final velocity, time

and - uniform acceleration.

Measuring Acceleration Activity

What do you expect to happen to the value of

acceleration as the light gate is moved further

up the slope?

Position of light gate from bottom of slope Position of light gate from bottom of slope Position of light gate from bottom of slope Position of light gate from bottom of slope Position of light gate from bottom of slope

Acceleration (m/s2) 1st attempt 2nd attempt 3rd attempt Position 1 m Position 2 m Position 3 m Position 4 m

Acceleration (m/s2) 1st attempt 2nd attempt 3rd attempt

Average acceleration (m/s2)

What is acceleration?

- Acceleration is the change in velocity of an

object per second (in one second). - Is acceleration a vector or scalar quantity?

Acceleration

- What is the definition of acceleration?
- Is it a vector or a scalar?

Acceleration is the rate of change of velocity

per unit time OR change in velocity per unit time.

Vector since velocity is a vector.

What is acceleration?

- The rocket starts off at 0 m/s and 1
- second later is travelling at 10 m/s.
- What is its acceleration?
- 10 metres per second per second
- 10 m/s2

change in speed

in one second

Calculating acceleration

- We need to know
- the change in velocity so
- initial velocity (u)
- final velocity (v)
- and
- time (t)

(No Transcript)

change in velocity

in one second

Acceleration

- a acceleration measured in m/s2
- u initial velocity measured in m/s
- v final velocity measured in m/s
- t time measured in s

Units of acceleration

final velocity initial velocity

a

time

acceleration is measured in m/s2

If the speed is measured in kilometres per hour,

acceleration can be measured in kilometres per

hour per second.

Acceleration

- An object accelerates at a rate of 4 m/s2.
- What does this mean?
- The object goes 4 m/s faster each
- second.

Acceleration

- The object goes 4 m/s faster each
- second.
- If the object is initially at rest, what
- is its velocity after
- 1s? 4 m/s
- 2s? 8 m/s
- 3s? 12 m/s
- 4s? 16 m/s

Acceleration

- What does it mean if an object has a negative
- value of acceleration?
- It means that it is slowing down.
- For example an object which has an
- acceleration of -2 m/s2 is becoming 2 m/s
- slower each second.

Acceleration Calculations

- A car, starting from rest, reaches a
- velocity of 18 m/s in 4 seconds. Find the
- acceleration of the car.
- What do I know?
- Initial velocity u 0 m/s
- Final velocity v 18 m/s
- time t 4 s

Acceleration Calculations

- What do I know?
- Initial velocity u 0 m/s
- Final velocity v 18 m/s
- time t 4 s
- Formula?

Acceleration Calculations

- A cheetah starting from rest accelerates
- uniformly and can reach a velocity of 24
- m/s in 3 seconds. What is the
- acceleration?
- Use technique and show all working!
- Units!!

Acceleration Calculations

- A student on a scooter is travelling at
- 6 m/s. 4 seconds later, she is travelling at
- 2 m/s. Calculate her acceleration.
- Use technique and show all working!
- Units!!
- What do you notice about her change in
- velocity?

Rearranging the acceleration equation

v-u a

t

Rearranging the acceleration equation

- Key words acceleration, velocity
- By the end of this section you will be able to
- Explain the term acceleration
- State that acceleration is the change in
- velocity per unit time
- Carry out calculations involving the relationship
- between initial velocity, final velocity, time

and - uniform acceleration.
- Graph results

Acceleration using two light gates

- The length of the mask is 5 cm. Calculate
- the acceleration.
- Remember calculate u (initial velocity) and
- v (final velocity) and use

Acceleration using a double mask

- The length of each section mask is 4 cm. The gap

is also 4 cm. Calculate the acceleration. - Remember calculate u (initial velocity) and
- v (final velocity) and use

- Key words acceleration, velocity, displacement
- By the end of this seection you will be able to
- Draw velocity-time graphs of more than one
- constant motion.
- Describe the motions represented by a
- velocity-time graph.
- Calculate displacement and acceleration, from
- velocity-time graphs, for more than one constant
- acceleration.

Graphing Motion

- Information about the motion of an
- object can be obtained from velocity-time
- graphs.
- Similarly, we can graph motion based on
- descriptions of the motion of an object.

Velocity-time graph

- The motion of a moving object can be
- represented on a velocity time graph.

Vectors and Direction

- When dealing with vector quantities we
- must have both magnitude and
- direction.
- When dealing with one-dimensional
- kinematics (motion in straight lines) we
- use and to indicate travel in opposite
- directions. We use to indicate acceleration
- and to indicate deceleration.

Velocity-Time Graphs

Describe the motion of this object.

Constant velocity does not change with time

0

0

Velocity-Time Graphs

Describe the motion of this object.

Increasing with time constant acceleration

0

0

Velocity-Time Graphs

Describe the motion of this object.

Decreases with time constant deceleration

0

0

Velocity-Time Graphs

Describe the motion of this object.

0

0

Speed-Time Graphs

Calculate the distance covered by the object in

the first 10 s of its journey.

The area under the graph tells us the

distance travelled.

0

0

Speed-Time Graphs

Calculate the distance covered by the object in

the first 10 s of its journey.

The area under the graph tells us the

distance travelled.

0

0

- Key words forces, newton balance, weight, mass,

gravitational field strength. - By the end of this section you will be able to
- Describe the effects of forces in terms of their

ability to - change the shape, speed and direction of travel

of an object. - Describe the use of a newton balance to measure

force. - State that weight is a force and is the Earths

pull on an - object.
- Distinguish between mass and weight.
- State that weight per unit mass is called the

gravitational - field strength.
- Carry out calculations involving the relationship

between weight, mass and - gravitational field strength including situations

where g is not equal to 10

What effect can a force have?

- Force is simply a push or a pull.
- Some forces (e.g. magnetic repulsion, or
- attraction of electrically charged
- objects) act at a distance.

What is force?

- A force can
- change the shape of an object
- change the velocity of an object
- change the direction of travel of an object

Units of Force?

- Force (F) is
- measured in
- newtons (N).

Measuring Forces

- A Newton (or
- spring) balance can
- be used to measure
- forces.

Mass and Weight

- We often use the words mass and weight
- as though they mean the same
- but do they?

Mass and Weight

- An objects mass is
- a measure of how much stuff makes up
- that object how much matter, or how
- many particles are in it.
- Mass is measured in
- grams or kilograms.

Mass and Weight

- An objects weight is
- the force exerted by gravity on a mass.
- Since it is a force, weight must be
- measured in
- newtons.

Investigating the relationship between mass and

weight

- How can we find the relationship between
- mass and weight?
- A newton balance can be used to find the
- weight of known masses.

Results

Mass Weight in N

100g

200g

300g

400g

500g

1kg

2kg

5kg

Relationship between mass and weight

- From this we can see a relationship
- between mass and weight
- 100g 0.1 kg -gt 1 N
- 1kg -gt 10 N
- To convert kg -gt N multiply by 10
- To convert N -gt kg divide by 10

Gravitational Field Strength (g)

- Gravitational field strength on Earth is
- 10 N / kg

What is gravitational field strength?

- This is the pull of gravity on each
- kilogram of mass.
- So on Earth, the pull of gravity on a 1kg
- mass is

10 N

What is gravitational field strength?

- and the pull of gravity on a 2 kg mass is

20 N

Definition

- A planets gravitational
- field strength is the
- pull of gravity on
- a 1 kg mass.

Gravity in the universe

- Is gravitational field strength always the
- same?
- No! It varies on different planets.
- http//www.exploratorium.edu/ronh/weight/index.htm

l

Your weight on different planets

- Use the website to find your weight on
- different planets for a mass of 60 kg (a
- weight of 600 N on Earth).
- From this calculate the gravitational field
- strength for each planet.

- Mass on Earth 60 kg
- Weight on Earth 600 N
- Gravitational field strength
- Weight on Mercury 226.8 N g
- Weight on Venus 544.2 N g
- Weight on the Moon 99.6 N g
- Weight on Mars 226.2 N g
- Weight on Jupiter 1418.4 N g
- Weight on Saturn 549.6 N g

Units for g

- We found g by dividing weight in newtons
- by mass in kilograms.
- What are the units for g?

10 N / kg

- Which of the planets has the greatest
- gravitational field strength?
- Why do you think this is the case?

Weight, mass and gravity

- We have seen that there is a link between
- weight, mass and gravity.
- On Earth
- 1 kg acted on by 10 N / kg weighs 10 N

m x g W

mass

Gravitational field strength g

weight

Weight, mass and gravity

Why is weight measured in newtons?

- W mg

Gravitational field strength measured in N / kg

Mass measured in kg

Weight measured in newtons

- Key words friction, force
- By the end of this section you will be able to
- State that the force of friction can oppose
- the motion of an object.
- Describe and explain situations in which
- attempts are made to increase or decrease
- the force of friction.

Frictional Forces

- Moving vehicles such as cars can slow
- down due to forces acting on them.
- These forces can be due to
- road surface and the tyres
- the brakes
- air resistance.

Frictional Forces

- The force which tries to oppose motion is
- called the force of friction.
- A frictional force always acts to slow an
- object down.

Increasing Friction

- In some cases, we want to increase
- friction. Some examples of this are
- Car brakes we need friction between
- the brake shoes and the drum to slow
- the car down
- Bicycle tyres we need friction to give
- grip on the surface

Increasing Friction

- On the approach to traffic lights and
- roundabouts, different road surfaces are
- used to increase friction compared with
- normal roads.

Decreasing Friction

- In some cases, we want to decrease
- friction. Some examples of this are
- Ice skating
- Skiing
- Aircraft design

Reducing Friction

- Friction can be reduced by
- Lubricating the surfaces this generally
- means using oil between two metal
- surfaces. This is done in car engines to
- reduce wear on the engine metal parts
- arent in contact because of a thin layer
- of oil between them.

Reducing Friction

- Friction can be reduced by
- Separating surfaces with air (e.g. a
- hovercraft).
- Making surfaces roll (e.g. by using ball
- bearings).

Reducing Friction

- Friction can be reduced by
- Streamlining. Modern cars are designed
- to offer as little resistance (or drag) to
- the air as possible, reducing friction on
- the car.

Streamlining

- Cars, aeroplanes and rockets are streamlined

(that is, have their - drag coefficient reduced) by
- Reducing the front area
- Having a smooth body shape

- Key words force, vector, balanced
- forces
- By the end of this section you will be able
- to
- State that force is a vector quantity.
- State that forces which are equal in size but
- act in opposite directions on an object are
- called balanced forces and are equivalent to
- no force at all.
- Explain the movement of objects in terms of
- Newtons first law.

Force

- Force is a vector quantity. What do we
- mean by this?
- To describe it fully we must have size
- and direction.

Balanced Forces

F

F

- Balanced forces are EQUAL FORCES which act in

OPPOSITE DIRECTIONS. They CANCEL EACH OTHER OUT.

- If balanced forces act on a STATIONARY OBJECT,

it REMAINS STATIONARY.

If balanced forces act on a MOVING OBJECT, it

continues moving in the same direction with

CONSTANT VELOCITY.

- This is summarised by NEWTONS FIRST LAW which

states

An object remains at rest, or moves in a

straight line with constant velocity unless an

UNBALANCED FORCE acts on it.

- To understand NEWTONS FIRST LAW remember

An object tends to want to keep doing what it is

doing (so if it is sitting still it wants to stay

that way, and if it is moving with constant

velocity it wants to keep going).

- This reluctance to change motion is known as

inertia. - The greater the mass, the greater the

reluctance. - Think! Is it easier to stop a tennis ball

travelling towards you at 10 m/s or to stop a car

travelling towards you at 10 m/s?

Forces and Supported Bodies

- A stationary mass m
- hangs from a rope.
- What is the weight of
- the mass? In what
- direction does
- this act?
- W mg downwards

m

Forces and Supported Bodies

- The mass is stationary.
- Newtons law tells us
- that the forces must
- be
- balanced forces.
- The weight is
- counterbalanced by a
- force of the same size
- acting upwards due to
- the tension in the
- string.

m

Forces and Supported Bodies

- A book of mass m
- rests on a shelf.
- What is the weight of
- the book? In what
- direction does
- this act?
- W mg downwards

m

Forces and Supported Bodies

- The mass is stationary.
- Newtons law tells us
- that the forces must be
- balanced forces.
- The weight is
- counterbalanced by a
- force of the same size
- acting upwards due to
- the shelf.

m

What forces are acting on this stationary

hovering helicopter?

W mg

lift

W mg

Newtons First Law

- Newtons first law tells us that when the
- forces on an object are balanced, a
- stationary object will remain stationary.
- But it also says that if when forces are
- balanced, an object moving at constant
- velocity will continue in the same direction
- with the same velocity.

A moving car If a car moves with constant

velocity, then what forces are acting on it?

The ENGINE FORCE and the FRICTION FORCE

must be equal.

- Newtons Law Car Seat Belts

If a car stops suddenly, someone inside the car

appears to be thrown forwards. In fact, they

simply carry on moving with the cars previous

speed. A seat belt prevents this happening by

applying an unbalanced force to the person, in

the direction opposite to motion. This causes

rapid deceleration.

No seatbelt whats going to happen when the car

hits the wall? Explain this in terms of Newtons

1st law.

Whats going to happen when the motorbike hits

the wall? Explain this in terms of Newtons 1st

law.

Air bags Air bags produce a similar effect to

seatbelts. They apply a force which opposes the

motion, causing rapid deceleration. The large

surface area also spreads the force of impact,

reducing the pressure and reducing injury.

Forces in a Fluid

Terminal velocity Any free-falling object in a

fluid (liquid or gas) reaches a top speed, called

terminal velocity.

Terminal Velocity The air resistance acting

on a moving object increases as it gets

faster. Terminal velocity is reached when the

air-resistance (acting upwards) has increased to

the same size as the persons weight (acting

downwards)

time 0s, velocity 0 m/s, friction 0 N

Friction Ff(air resistance) 0 N

a -10 m/s2

W weight

Ff

a lt -10 m/s2

v

W weight

Equal opposite forces Acceleration

zero Terminal velocity

Ff

a 0 m/s2

v

W weight

Velocity Time Graph

velocity (m/s)

Terminal velocity

0

0

time (s)

air resistance

Terminal velocity is reached when the air

resistance balances the weight.

weight

Terminal Velocity

- What effect does opening a parachute
- have on the terminal velocity?
- When the parachute is opened, air resistance
- increases a lot. There is now an unbalanced force
- upwards, which causes deceleration. The velocity
- decreases, and the air resistance decreases until
- the forces are balanced again. The parachutist
- falls to the ground with a lower terminal

velocity.

- Key words Newtons second law,
- unbalanced forces, mass, force,
- acceleration
- By the end of this section you will be able to
- Describe the qualitative effects of the change of
- mass or of force on the acceleration of an object
- Define the newton
- Carry out calculations using the relationship
- between a, F and m and involving more than
- one force but in one dimension only

- The example of the parachutist accelerating

until the forces are balanced helps us to

understand NEWTONS SECOND LAW which states

When an object is acted on by a constant

UNBALANCED FORCE the body moves with constant

acceleration in the direction of the unbalanced

force.

Force, mass and acceleration

Acceleration (m/s2)

- F ma

Force (N)

mass (kg)

Force, mass and acceleration

- One newton (1N) is the force required to
- accelerate 1 kg at 1 m/s2

F ma

- Find the unbalanced force required to accelerate

a 4 kg mass at 5 m/s2 - What do I know?
- m 4kg
- a 5m/s2

F ma F 4 x 5 F 20 N

- Key words free body diagrams, resultant
- force
- By the end of this section you will be able
- to
- Use free body diagrams to analyse the forces
- on an object
- State what is meant by the resultant of a
- number of forces
- Use a scale diagram, or otherwise, to find the
- magnitude and direction of the resultant of
- two forces acting at right angles to each
- other.

Newtons First Law

- A body remains at rest, or continues at
- constant velocity, unless acted upon by an
- external unbalanced force.
- (that is objects have a tendency to keep
- doing what they are doing)

Newtons Second Law

- Newtons Second Law is about the
- behaviour of objects when forces are not
- balanced.
- The acceleration produced in a body is
- directly proportional to the unbalanced
- force applied and inversely proportional to
- the mass of the body.

Newtons Second Law

- In practice this means that
- the acceleration produced increases as
- the unbalanced force increases
- the acceleration decreases as the mass of
- the body increases

Which forces?

- An object may be acted upon by a number
- of forces but
- only an overall unbalanced force
- will lead to acceleration in the direction
- of that force.

Forces are measured in?

- Newtons Second Law can be written as
- or more commonly

Forces are measured in?

- which gives us the definition of the Newton
- 1N is the resultant (or unbalanced)
- force which causes a mass of 1kg to
- accelerate at 1m/ s2

Quick Quiz

Unbalanced force (N) Mass (kg) Acceleration (m/ s2)

10 2

20 2

20 4

2 5

10 10

5

10

5

10

1

Direction of force

- Consider the oil drop trail left by the car
- in motion.
- In which direction is the acceleration?
- In which direction is the unbalanced
- force?

To the right

To the right

Direction of force

- Consider the oil drop trail left by the car
- in motion.
- In which direction is the unbalanced
- force?

To the left the car is moving to the right and

slowing down.

Newtons First and Second Laws

- Remember
- Forces do not cause motion
- Forces cause acceleration

Free-Body Diagrams

- Free body diagrams are special
- examples of a vector diagram.
- They show the relative magnitude
- and direction of all forces acting
- on an object.
- They are used to help you identify
- the magnitude and direction of an
- unbalanced Force acting on an
- object.

Using Newtons Second Law

- In the simplest case

m

Fun

Using Newtons Second Law

Direction of acceleration? Direction of

unbalanced force? Formula for calculating

acceleration?

Solving Problems

- Always draw a diagram showing all known
- quantities (forces magnitude and
- direction, resultant acceleration and
- direction, mass of object(s) )
- Remember that Funma can be applied to
- the whole system
- When working in the vertical direction
- always include the weight

- Key words acceleration, gravitational
- field strength, projectiles
- By the end of this section you will be able
- to
- Explain the equivalence of acceleration due to
- gravity and gravitational field strength
- Explain the curved path of a projectile in
- terms of the force of gravity
- Explain how projectile motion can be treated
- as two separate motions
- Solve numerical problems using the above method
- for an object projected horizontally.

Acceleration due to Gravity

Definition A planets gravitational field

strength equals the force of gravity PER UNIT

MASS. Units? N/kg

To calculate an objects weight, use this

equation -

Acceleration due to Gravity

- Near a planets surface all objects experience

the same gravitational acceleration. - This acceleration is numerically equal to the

planets gravitational field strength.

Acceleration due to Gravity

For example, on Earth g 10

N/kg A free-falling object will experience

acceleration of a -10 m/ s2 What does the

ve sign tell you?

Gravitational field strength

- Is the gravitational field strength the same on

each - planet?
- How does distance affect gravitational field

strength? - It decreases the further away you are from the

planets - surface.
- What will happen to the weight of an object as it

gets - further from the surface? Explain your answer.
- It will decrease.

- The force of gravity near
- the Earths surface gives
- all objects the same
- acceleration.
- So why doesnt the
- feather reach the ground
- at the same time as the
- elephant?

- Why are the gaps
- between the balls
- increasing?

- An object is released from rest close to the

Earths - surface. Which formula can be used to find its

velocity - at a given time?
- v u at
- where v ? u 0 a t

- What is its velocity
- At the time of release?
- After 1 second?
- After 2 seconds?
- After 3 seconds?
- After 4 seconds?

Projectiles

Forces acting on projectiles

- What would happen to a ball kicked off a
- cliff, in the absence of gravity?

Forces acting on projectiles

- There would be no vertical motion
- therefore the ball would continue at
- constant speed in a straight
- line (remember Newtons first law)

Objects projected horizontallyThink about

- What is the initial vertical speed of a
- projectile fired horizontally?
- How will the horizontal speed vary during
- the objects flight?

0 m/s

It will remain the same as the initial horizontal

speed.

Objects projected horizontallyThink about

- Describe the vertical motion of an object
- projected horizontally

It will accelerate downwards due to gravity.

Objects projected horizontallyThink about

- What formula can be used to find the
- horizontal displacement of an object
- fired horizontally if horizontal velocity
- and time of flight are known?

time of flight (s)

horizontal displacement (m)

sh vht

horizontal velocity (m/s)

Which ball will hit the ground first?

http//www.fearofphysics.com/XYIndep/xyindep_corre

ct.html

Summary

Horizontal motion Vertical motion

Forces Are there forces present? If so, in what direction are they acting? No Yes The force of gravity acts downward

Acceleration Is there acceleration? If so, in what direction? What is the value of the acceleration? No Yes Acceleration "g" downward at 10 m/s2

Velocity Constant or changing? Constant Changing by 10 m/s each second

Solving Numerical Problems

- Always write down what you know many questions

have a lot of text surrounding the Physics so

pick out the information from the question - Write down other relevant information you have

e.g. acceleration due to gravity - Select formula this isnt a test of memory so

while you should learn your formulae, dont be

afraid to check against the data book or text

book - Substitute values and rearrange formula
- Write answer clearly remembering magnitude and

direction, and units.

Example

- A flare is fired horizontally out to sea from a
- cliff top, at a horizontal speed of 40 m/s. The
- flare takes 4 s to reach the sea.
- What is the horizontal speed of the flare after 4

s? - There are no forces acting in the horizontal. The
- horizontal speed remains the same 40 m/s.

Example

- (b) Calculate the vertical speed of the flare

after 4s - final speed v ?
- initial vertical speed u 0 m/s Initial

vertical speed is always 0 m/s! - acceleration a 10 m/s2
- time t 4 s
- v u at
- v 0 10 x 4
- v 40 m/s

Example

- (c) Draw a graph to show how vertical speed

varies with time. - Initial vertical speed 0 m/s
- Final vertical speed 40 m/s

Example

- (d) Use this graph to calculate the height of the

cliff.

Displacement area under velocity-time graph ½

bh ½ x 4 x 40 80 m Height of cliff 80 m

- Key words Newtons third law, newton
- pairs
- By the end of this section you will be able to
- State Newtons third law
- Identify Newton pairs in situations involving
- several forces
- State that momentum is the product of mass
- and velocity.
- State that momentum is a vector quantity.

Forces acting between objects

- Newton realised that

When a body is acted upon by a force there must

be another body which also has a force acting on

it. The forces are equal in size but act in

opposite directions.

Newtons Third Law

- If object A exerts a force on object B, then B

exerts an equal and opposite force on A

Forces always occur in equal and opposite pairs

For every action there is an equal and opposite

reaction

Firing a gun

- Force of GUN on BULLET

Force of BULLET on GUN

Starting a sprint

- Force of RUNNER on BLOCKS

Force of BLOCKS on RUNNER

A falling apple

- Force of EARTH on APPLE

Force of APPLE on EARTH

A Rocket

Force of GAS on ROCKET

- Force of ROCKET on GAS

- Key words work done, energy, force,
- distance, power, time
- By the end of this section you will be able
- to
- State that work done is a measure of the
- energy transferred.
- Carry out calculations involving the
- relationship between work done, force and
- distance.
- Carry out calculations involving the
- relationship between work done, power and
- time.

Work done?

- What is meant by work done in Physics?
- When a force acts upon an object to
- cause a displacement of the object, it is
- said that work was done upon the object.

Work done?

- There are three key ingredients to work
- force, displacement, and cause.
- In order for a force to qualify as having done
- work on an object, there must be a
- displacement and the force must cause the
- displacement.

Note at this level, we can use distance instead

of displacement.

Work done?

- Formula linking work done, force and

displacement? - Examples of work done?
- a horse pulling a plough through the field
- a shopper pushing a grocery cart down the aisle

of a supermarket - a pupil lifting a backpack full of books upon her

shoulder - a weightlifter lifting a barbell above his head
- an Olympian launching the shot-put, etc.
- In each case described here there is a force

exerted upon an - object to cause that object to be displaced.

Work done

- A dog pulls a 4 kg sledge for a distance on
- 15 m using a force of 30 N. How much
- work does he do?
- What do I know?
- F 30N
- d 15m

Work done

- What do I know?
- F 30N
- d 15m
- Formula?

Power

- Power is the rate of doing work i.e. if
- work is done then the work done per
- second is the power.

Energy in joules

time in seconds

Power in watts (joules per seconds)

Power

- A dog pulls a 4 kg sledge for a distance on
- 15 m using a force of 30 N in 20 s.
- Calculate the power of the dog.
- What do I know?
- F 30N
- d 15m
- t 20s

Power

- What do I know?
- F 30N
- d 15m
- t 20s
- Formula?

Power

- What do I know?
- F 30N
- d 15m
- t 20s
- Ew 450J
- Formula?

- Key words gravitational potential energy,
- mass, gravitational field strength, kinetic
- energy
- By the end of this section you will be able
- to
- Carry out calculations involving the relationship
- between change in gravitational potential energy,
- mass, gravitational field strength and change in
- height.
- Carry out calculations involving the relationship
- between kinetic energy, mass and velocity.

Gravitational Potential Energy

- is the potential energy
- gained by an object when
- we do work to lift it
- vertically in a gravitational
- field.

Gravitational Potential Energy

- The work done in lifting an
- object vertically

What force is required?

Gravitational Potential Energy

To lift the object we must overcome the weight

Wmg

Gravitational Potential Energy

Vertical distance we call this height h

Gravitational Potential Energy

Kinetic Energy

- is the energy associated with a moving object.

Kinetic Energy

- depends on
- The mass of the object

Kinetic Energy

- depends on
- The velocity of the object

Kinetic Energy

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