1. 2/24/17 Today we will identify action/reaction pairs Warm Up:
Identify the action/reaction pairs below:
1. A foul ball hits a fan in the stands.
2. Mrs. Nairn drops her computer out the window

2. Identifying Action and Reaction
Action/Reaction Pairs are not always the most obvious forces

3. Newton's Third Law of Motion: Astronauts in Outer Space

4. Falling Boulder Reaction Force: Boulder exerts a force on the Earth
Interaction: Gravitational attraction between the boulder and the Earth
Action Force: Earth exerts a force on the boulder
Reaction Force: Boulder exerts a force on the Earth

5. Earth’s mass so large that it has a very small acceleration; Rock small mass big acceleration

6. Example
Compare force, mass and acceleration of cannon and cannonball

7. What is the action? What is the reaction? Cannon pushes cannonball.
Cannonball pushes cannon.

8. How do the forces acting on each compare?
Equal but opposite

9. How do the masses compare?
Cannon has a much larger mass than cannonball

10. Do both the cannon and cannonball accelerate?
Yes! (we know cannonball does…we see it fly from the cannon…) (cannon has a recoil/kickback)

11. How do their accelerations compare?
Cannonball will accelerate more due to its small mass compared to the cannon

12. Compare the accelerations if the cannon and cannonball had the same mass.
Acceleration would be the same

13. A moving car hits a bug flying across a road.
What is the action-reaction pair?
Which feels the greater force?
Why does the bug have a bad day, but the car does not?

14. How does one team win a game of tug-of-war?
It can’t be that one team pulls harder than the other, why can’t it be that?
Forces would be equal and opposite
Example: piece of paper
The team that wins has a greater force with the ground! (more friction)
Think back to the video with the astronaut

15. A rocket does not need the air around it to move. Why?
The rocket and the fuel inside that is being released are part of the action, reaction pairing.
Example: a Balloon
Why does a balloon fly around the room when you let go of it?

16. Do action and reaction pairs cancel?
Since action and reaction pairs are equal but opposite, why don’t they cancel?
Simple….they are acting on two different objects!

17. Look back at the cannon and cannonball
Cannon exerts a force on the cannonball
Cannonball exerts a force on the cannon
In order to be considered an action-reaction pair, the forces must be on each other!

18. Back to the cannon…
So both the cannon and the cannonball have an external force on them
Remember Newton’s first law – object will accelerate when there is an external force on it

20. The Horse-Cart Problem

21. Book Work
Page 83: 1-8, 12-26,