Chapter 5Chapter 5

Newton's Third Law

of Motion

1. FORCES AND INTERACTIONS1. FORCES AND INTERACTIONS

Force is not a thing but is an interaction between one thing and another.

Examples: 2 people pushing each other on skate boards, driving a nail, punching a bag

No single isolated force Demo - Bathroom ScalesDemo - Bathroom Scales

Law 3 - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Action-reaction pairs never act on same object.

2. NEWTON'S THIRD LAW OF MOTION

2. NEWTON'S THIRD LAW OF MOTION

Defining Your SystemDefining Your System

Two objects define a system for a Newton’s third law interaction.

We are not considering (necessarily) the net force acting on an object.

An object cannot exert a force on itself to cause an acceleration.

Action: earth pulls on you

Reaction: you pull on earth

Action and Reaction on Different Masses

Consider you and the earth

Recoil

Video – Scooter PropulsionVideo – Scooter Propulsion

Cannot touch without being touched

F m =

F m=a a

Action: tires push on roadReaction: road pushes on tires

Action: rocket pushes on gases

Reaction: gases push on rocket

3. SUMMARY OF NEWTON'S THREE LAWS

3. SUMMARY OF NEWTON'S THREE LAWS

Law 1 (Law of Inertia) Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.

amF

Law 2 Law 3 - Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Chapter 5 Review Questions

A bug and a car collide. Which experiences the greater force?

(a) bug(b) car(c) neither, they both experience the

same magnitude of force

Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction force.

(a) the force applied to the bat by the hands(b) the force applied to the bat by the ball(c) the force the ball carries with it in flight(d) the centrifugal force in the swing

N

4. VECTORS4. VECTORS Imagine that you have a map that

leads you to a buried treasure. This map has instructions such as

15 paces northwestof the skull.

The 15 paces is a distance and northwest is a direction.

Quantities that require a magnitude and direction for specification are called vectors.

Those quantities that have no direction are called scalars.

Examples of scalars in physics aremass timedistance densitywork energy

Examples of vectors in physics aredisplacement velocityacceleration forcemomentum angular momentum

The math associated with scalars is familiar to everyone.

The math associated with vectors is more involved.

Let’s explore the graphical addition of vectors.

Let’s use a treasure map again as an example of the addition of vectors.

Let’s imagine the instructions tell you to go 4 miles east then 3 miles north.

4 miles

3 miles

5 miles

36.90

In this case you could have gone 3 miles north first and then 4 miles east next and still end up at the same location.

Your final position is 5 miles at 36.90 north of east.

It would have saved time if that had been the one distance and one direction traveled in the first place.

We say that the 5 miles at 36.90 north of east is the vector sum of the 4 miles east vector and the 3 miles north vector.

The order of the addition does not matter.

Examples of addition of vectors follows. The method used will be the head-to-tail.

4 Newtons east

3 Newtons north 5 Newtons @

370 north of east

Force Vectors

What is the resultant force?

530 north of east

Velocity Vectors

80 km/h northwrt the wind

Wind at 60 km/h east

100 km/h @

What is the actual velocity?

y

x

Components of Vectors

R

xR

yR

Consider the vector

R

Velocity Components in Projectile Motion