Chapter 18 Section 3

1. Chapter 18 Section 3 Collisions

2. Mass • Mass is the amount of matter in an object • The mass of an object affects how easy it is to changes its motion.

3. Inertia • Inertia is the tendency of an object to resist a change in its motion. • The amount of resistance to a change in its motion increases as an object’s mass increases.

4. Momentum • The momentum of an object is a measure of how hard it is to stop the object. • Increasing the speed or velocity of an object makes it harder to stop. • Momentum has direction that is the same as the direction of the velocity.

5. Momentum Equation • Momentum( kg x m/s) = mass (kg) x velocity (m/s) p=momentum m=mass v=velocity P M V

6. Solve the Equation 1 • Calculate the momentum of a 14kg bicycle traveling north at 2 m/s.

7. Answer 1 • (14kg) (2 m/s north) = 28 kg·m/s north

8. Solve the Equation 2 • A 10,000 kg train is traveling east at 15 m/s. Calculate the momentum of the train.

9. Answer 2 • 10,000 kg x 15 m/s east = 150,000 kg·m/s east

10. Solve the Equation 3 • What is the momentum of a car with a mass of 900 kg traveling north at 27 m/s?

11. Answer 3 • 900 kg x 27 m/s north = 24,300 kg · m/s north

12. The Law of Conservation of Momentum • In any collision, momentum is transferred from one object to another. • The momentum lost by one object is equal to the momentum gained by the other object. • The law states that the total momentum of a group of objects remains constant unless outside forces act on the group.

13. Example of the Law • In a game of billiards (pool), when the cue ball hits the other billiard balls, it slows down because it transfers some of its momentum to the other billiard balls. • What would happen to the speed of the cue ball if all its momentum were transferred to the other billiard balls?

14. An Example of an Outside Force • An outside force, like friction, can change the total momentum of the group of objects.

15. Types of Collisions • Bounce off of each other, like bowling ball and pins (elastic) • Collide and stick to each other, like one football player tackles another. (inelastic)

16. Example of Momentum Conservation • Suppose two balls approach each other at 1 m/s from opposite directions. Their total momentum is zero. After collision, they both zoom off at 1 m/s in opposite directions. What is their momentum? What do you know about the mass of the two balls?

17. Answer • If the total momentum is zero and the two balls have the same speed in the opposite direction, the balls must have the same mass.

18. Predictions • Then law of momentum conservation can be used to predict the velocity of objects after they collide.

19. Suppose a 2 kg backpack initially has a velocity 5 m/s east. Your mass is 48 kg, and initially you’re at rest then the total initial momentum is- • 2 kg x 5 m/s + 48 kg x 0 m/s = 10 kg · m/s east

20. P= mv • You can now use the equation for momentum to find the final velocity. 10 kg · m/s/east = 2 kg + 48 kg x velocity 10 kg · m/s/east = 50 kg x velocity 0.2 m/s east = velocity The ending velocity is smaller (0.2 m/s east) is smaller than the initial velocity (5 m/s east)

21. Visualize what will happen. • If small car is stopped at a red light is hit from behind by a truck, what will happen?

22. Answer • Both vehicles will move forward with the car moving faster than the truck.

23. Visualize what will happen. • What will happen if two marbles of the same mass, traveling towards each other, collide?

24. Answer • The marbles will collide and reverse direction, moving away from each other at the same speed as before the collision.

25. Conclusion • Momentum equals the mass of an object times its velocity • Momentum is transferred from one object to another in a collision. • The total amount of momentum of a group does not change unless acted upon by an outside force/s.