1. Conservation of Momentum
Collisions and Explosions

2. Review of Newton’s 3rd Law
While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?
Trick Question! Each force is the same size. For every action, there is an equal ... (equal!). The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction. Besides, fireflies have guts and bug guts have a tendency to be splatterable. Windshields don't have guts. There you have it.

3. Review of Part 1
Ft=mΔv

7. Conservation of Momentum
When two objects collide
Force between the 2 is the same (newton’s 3rd)
The time they collide is the same
This means the impulses are equal

8. Conservation of Momentum
Thus, since each object experiences equal and opposite impulses, it follows logically that they must also experience equal and opposite momentum changes
Think of this as a system, not individual pieces

9. Check Your Understanding
When fighting fires, a firefighter must use great caution to hold a hose that emits large amounts of water at high speeds. Why would such a task be difficult?
The hose is pushing lots of water (large mass) forward at a high speed. This means the water has a large forward momentum. In turn, the hose must have an equally large backwards momentum, making it difficult for the firefighters to manage.

12. Demo
Same size carts
Small runs into large
Large runs into small

16. 200-kg cannon and 1-kg cannonball (400 m/s)
v = 0
What is the speed of the cannon?

17. 200-kg cannon and 1-kg cannonball (400 m/s)
v = 0
0 =
What is the speed of the cannon?
(200)v (1)(400)
v = -2 m/s

18. Check Your Understanding
A large truck and a Volkswagen have a head-on collision.
a. Which vehicle experiences the greatest force of impact?
b. Which vehicle experiences the greatest impulse?
c. Which vehicle experiences the greatest momentum change?
d. Which vehicle experiences the greatest acceleration?
a, b, c: the same for each.
Both the Volkswagon and the large truck encounter the same force, the same impulse, and the same momentum change (for reasons discussed in this lesson).
d: Acceleration is greatest for the Volkswagon. While the two vehicles experience the same force, the acceleration is greatest for the Volkswagon due to its smaller mass. If you find this hard to believe, then be sure to read the next question and its accompanying explanation.

19. Conservation of Momentum
Sample Problems

42. http://www. khanacademy

48. m