1. Newton’s Third Law of Motion- Action and Reaction
Chapter 4
Newton’s Third Law of Motion-
Action and Reaction

2. 4.1 Force is Part of Interaction
Force is more than a push or pull
Force is mutual action, or interaction, between two things
Forces occur in pairs
Examples= car crash (2 cars), you walking, car tires, swimming (what are other examples?)
Doesn’t have to be living to exert force!
Do not possess force, exert it (do possess momentum, kinetic/potential energy)

3. 4.2 Newton’s Third Law
Newton’s Third Law- Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first
Aka = To every action there is always an opposed equal reaction (V for Vendetta, Count of Monte Cristo)
Forces are called the action force and the reaction force

4. 3rd Law continued
Doesn’t matter which is called reaction, just have to keep in mind that both are needed (You complete me) or neither would exist
But what happens with falling (floor is not equal and opposite to me)?

5. 4.3 Simple Rule to Identify A & R
To identify action and reaction:
Action- object A exerts force on object B
Reaction-object B exerts force on object A
Can switch around
Action is what is not always present
See drawings on board

6. 4.4 Action and Reaction on Different Masses
Firing cannon, cannon recoils (kicks)
Equal forces, different effects (examples)
Cannon has more mass, cannonball has more acceleration
How would this work for gas expelled from rocket?
Used to think rocket needed to push against air, so couldn’t go to moon. Rocket just needs to push against expelled gases (and minimal air drag)

7. 4.5 Action and Reaction on Different Objects
Why don’t forces cancel?
Bodies are different
Example: kicking ball with one foot, two feet (see drawing on board)
Reaction slows foot down
Only cancel if same body, but there still is action and reaction
Other examples

8. 4.6 Horse-Cart Problem
Why pull cart if would be cancelled out by equal and opposite pulls (horse on cart and vice-versa)
3 systems
Cart (farmer)
Horse
Horse-cart
How does the horse move forward? What is needed?

9. More Horse-cart As a whole, can cancel horse vs. cart forces
Think about pushing a cart against the ground
See drawing on board
P. 61
How does this apply to Newton’s other laws?

10. 4.7 Action = Reaction
Pull on rope = tension in rope, otherwise acceleration and …
Tug of war- both pulls in same magnitude
So, how does one win? Force against ground, not rope!
What if one side wears socks, the other shoes? Who would win? Why?
Helicopter blades force air down (action) and air makes blades go up (reaction), or have lift. Greater lift equals rising helicopter

11. More action, reaction
Birds fly by pushing air downward, so air pushes up
When birds create updraft (swirl of air) for other birds, get the V
Airplanes
Why can’t we fly?
Can’t fight way out of paper bag, even if champion boxer!
Problems on board with pulleys

12. 4.8 Summary
1st Law = at rest, stay at rest; in motion, stay in motion, unless change occurs
2nd law = law of acceleration, F = ma (a = g in vacuum) and have to take into consideration air drag if falling from air (terminal speed if constant)
3rd law = law of action-reaction: you push something, it pushes back equally and in opposite direction