1 / 17

Chapter 4 Force and The Laws of Motion

- Physics
- Teacher Mrs. R. Williams

Background

- Sir Isaac Newton (1643-1727) an English scientist

and mathematician famous for his discovery of the

law of gravity also discovered the three laws of

motion. He published them in his book

Philosophiae Naturalis Principia Mathematica

(mathematic principles of natural philosophy) in

1687. Today these laws are known as Newtons

Laws of Motion and describe the motion of all

objects on the scale we experience in our

everyday lives.

Newtons Laws of Motion

- 1. An object in motion tends to stay in motion

and an object at rest tends to stay at rest

unless acted upon by an unbalanced force. - 2. Force equals mass times acceleration (F

ma). - 3. For every action there is an equal and

opposite reaction.

SECTION 4.1--Force

- Definition Force is the cause of an

acceleration or the change in an objects

velocity - A force can be a push, a pull, and pressure

force can act directly through physical contact

or at a distance like a magnetic field. - We measure force in NEWTONS.
- A newton is the force needed to accelerate a 1

kilogram mass by 1 meter per second squared - 1N 1kg x 1m/s2

Newtons

- Newtons ARE NOT units of weight or mass!
- Weight What you weigh is a measure of your mass

on the planet earth. Your weight can change

relative to your environment! Mr. Price weighs

200 pounds on earth, but on the moon he would

weigh only 33 pounds! - While your weight can change your mass is

constant regardless of your environment.

Although Mr. Price would weigh 33 pounds on the

moon his MASS would still be 200 pounds on the

moon, Jupiter or in a black hole! - Newtons are a measure of forcewe can use newtons

to measure how much force the earth is exerting

on your body (your weight) but newtons are units

of force! - Conversion Factors
- 1 pound 4.448 newtons
- 1 newton 0.225 pounds

Forces

- Two Types of Forces
- 1. Contact Forces The force that arises from

the physical contact of two objects. - 2. Field Forces The force that can exist

between objects even in the absence of physical

contact (like gravity and magnetism) - The 4 Fundamental Forces of the Universe
- Gravity
- Electromagnetism
- The Strong Nuclear Force
- The Weak Nuclear Force

Force Diagram

- Force Diagrams
- FORCE IS A VECTOR with both a magnitude and a

direction - Forces are represented by arrows which indicate

the direction of the force. The length of the

arrow indicates the magnitude of the force. - Diagrams are often used to analyze situations

where more than one force is acting on an object.

These are known as force diagrams - Simple force diagrams of single objects and the

forces acting on them are called free-body

diagrams.

Section 4-2Newtons First Law

- Inertia An object at rest remains at rest, and

an object in motion continues in motion unless

the object experiences a net external force. - Acceleration is determined by net external force.
- Newtons first law implies that the net external

forces on an object (with a constant velocity)

must be equal to zero. - net external forces means the sum of all of the

individual forcesadd those vectors! - The Greek letter sigma (S) is used to designate a

sum.

What does this mean?

- Basically, an object will keep doing what it was

doing unless acted on by an unbalanced force. - If the object was sitting still, it will remain

stationary. If it was moving at a constant

velocity, it will keep moving. - It takes force to change the motion of an object.

What is meant by unbalanced force?

If the forces on an object are equal and

opposite, they are said to be balanced, and the

object experiences no change in motion. If they

are not equal and opposite, then the forces are

unbalanced and the motion of the object changes.

Some Examples from Real Life

- A soccer ball is sitting at rest. It takes an

unbalanced force of a kick to change its motion.

Two teams are playing tug of war. They are both

exerting equal force on the rope in opposite

directions. This balanced force results in no

change of motion.

Newtons First Law

- Mass is a measurement of inertia
- The inertia of an object is proportional to its

massbigger objects are harder to move and harder

to stop. - Dont confuse size, mass and density
- Equilibrium
- Equilibrium exists when the net forc3es on an

object equal zero. - Equilibrium can occur when an object is at rest

or moving with a constant velocity.

Section 4-3--Newtons 2nd and 3rd Laws

- Newtons 2nd Law Force Mass x Acceleration
- SF ma
- For objects at equilibrium a0
- Newtons 3rd Law For every action there is an

equal and opposite reaction. - Newtons third law implies that forces always

exist in pairs. - We often split these pairs up into the action

force and the reaction force, which are always

have the same magnitude but opposite directions. - Action/reaction pairs can cancel each other out

which results in equilibrium. - Action/reaction pairs can also accelerate objects

(see action/reaction handout) - Field Forces also Exist in Pairs

Section 4-4Everyday Forces

- Weight
- Weight is defined in physics as the force of

gravity exerted on a mass. Weight has magnitude

but no direction making it a scalar quantity. - Remember the force of gravity is g 9.81m/s2
- This value changes inversely to the distance from

the center of the earth (you weigh less at higher

altitudes!)

THE NORMAL FORCE

- The Normal Force
- The Normal Force A force exerted by one

object on another in a direction perpendicular to

the surface of contact. (The Floor Force) - The mathematical definition of normal is

perpendicular. - The normal force is always perpendicular to the

surface of contactNOT always opposite the force

of gravity. - Fn m g cosT

FRICTION

- The Force of Friction
- Whenever two objects come into contact some

energy is lost to friction. - Friction is defined as the resistive force that

opposes the relative motion of two contacting

surfaces. - Two Types of Friction
- Static Friction is the friction experienced by

two or more objects that are in contact and at

rest. - Kinetic Friction is the friction experienced by

two or more objects that are in contact and are

moving relative to each other. - Kinetic friction is less than static friction
- Friction must always be considered in order to

exactly calculate net forces.

Force of Friction

- The force of friction is proportional to the

normal force (i.e. more massive objects

experience more friction and a larger normal

force) - Friction depends on the surfaces in contact (i.e.

smooth surfaces experience less friction than

rough surfaces.) - The Coefficient of Friction (the Greek letter mu

µ) - The coefficient of static friction (µs) is the

ratio of the maximum value of the force of static

friction to the normal force. - The coefficient of kinetic friction (µk) is the

ratio of the force of kinetic friction to the

normal - µ Friction force
- Normal force