Newton's Third Law of Motion Chapter 5 Newton's Third Law of Motion

1. 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 Scales

2. NEWTON'S THIRD LAW OF MOTION 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.

Defining 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 and Reaction on Different Masses Consider you and the earth Action: earth pulls on you Reaction: you pull on earth

a m F F a Recoil Video – Scooter Propulsion = = Video – Scooter Propulsion Cannot touch without being touched

Reaction: road pushes on tire Action: tire pushes on road

Reaction: gases push on rocket Action: rocket pushes on gases

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. 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.

4. VECTORS Imagine that you have a map that leads you to a buried treasure. This map has instructions such as 15 paces to the north of the skull. The 15 paces is a distance and north is a direction. N

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 are mass time distance density work energy Examples of vectors in physics are displacement velocity acceleration force momentum 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.

5 miles 3 miles 36.90 4 miles

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.

Force Vectors What is the resultant force? 3 Newtons north 5 Newtons @ 370 north of east 4 Newtons east

Velocity Vectors What is the actual velocity? 80 km/h north wrt the wind 100 km/h @ 530 north of east Wind at 60 km/h east

Components of Vectors Consider the vector y x

Velocity Components in Projectile Motion

Chapter 5 Review Questions

A bug and a car collide. Which experiences the greater force? (b) car (c) neither, they both experience the same magnitude of force (c) neither, they both experience the same magnitude of force

Consider hitting a baseball with a bat 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 (b) the force applied to the bat by the ball