1. Chapter 7

2. Suppose a large truck and a compact car have a head-on collision
Suppose a large truck and a compact car have a head-on collision. During the collision, is the force of the truck on the car large, smaller, or equal to the force of the car on the truck?

3. Imagine you are running on gravel
Imagine you are running on gravel. Which way do you kick any loose gravel and why?

4. Is the tension in rope 2 greater, less than or equal to the tension in rope 1?

5. They are the same!!

6. All 3 50kg blocks are at rest
All 3 50kg blocks are at rest. Is the tension in rope 2 greater than, less than or equal to the tension in rope 1?

7. Equal too!!

8. Notes!!!
Any time that object A pushes or pulls on object B, object B pushes or pulls back. These objects are interacting with one another with mutual influence.

9. The forces in this interaction occur simultaneously or not at all
The forces in this interaction occur simultaneously or not at all. We cannot truly say which wrestler is the action and which is the reaction.

10. Did the earth pull on the moon before or after the moon pulled on the earth?

11. We do not truly know!! But we do know they this action/reaction pair exists because without it there would be no change on tides.

12. Analyzing Interacting Objects and Their ”Environment”
Person pushing a crate.
2. The person and the crate are a “system” but they are interacting with the floor and earth as well.
3. Free body diagram for each object in the system is then created.

13. Notice when the man pushes the crate, he creates propulsion through bio-chem means to drive himself forward.

14. Other examples of “internal” propulsion are:
Animals pulling wagons
Rocket powered missiles
Cars
Trucks

15. Towing or pulling adds a third object to any system!!

16. The members of an action reaction pair have equal magnitudes, meaning FAonB=FBonA

17. Just because the forces are equal in an action reaction, acceleration and observable results do not necessarily equal one another!!!

18. An exception…
If two objects are touching or the rope between them is under tension, then objects of different mass can have the same acceleration due to the action reaction pair.

19. Boxes A and B are sliding to the right on a frictionless table
Boxes A and B are sliding to the right on a frictionless table. The hand is slowing them down. Mass A is greater than mass B. Rank in order from largest to smallest the horizontal forces on A,B and the hand.

20. H on B = B on H > B on A = A on B

21. Blocks A and B are connected by a massless string through a pulley.
What does the force diagram for each block and string look like?
What does the force diagram of the string look like?

22. Does the system move?

23. YES!!!
Downward due to gravity on block B!!

24. Homework page 204: Conceptual # 1,5,6,7,11,12,14,15