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

3. 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

4. 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.

5. 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.

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

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

8. Reaction: road pushes on tires Action: tires push on road

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

10. 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.

11. Chapter 5 Review Questions

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

13. 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

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

15. Quantities that require a magnitude and direction for specification are called vectors.
Those quantities that have no direction are called scalars.

16. 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

17. 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.

18. 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.

19. 5 miles
3 miles
36.90
4 miles

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

21. We say that the 5 miles at 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.

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

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

25. Components of Vectors
Consider the vector y x

26. Velocity Components in Projectile Motion