1. 4.3 Elastic and Inelastic collisions

2. Consider this… Think very carefully about a system composed of two objects that are on a collision course

3. Think about what you know In a football game, a 70 kg quarterback squares off against a 150 kg linebacker – assume both can run at the same speed What will happen if the 70 kg quarterback tries to tackle the 150 kg linebacker? What will happen if the 150 kg linebacker tackles the 70 kg quarterback?

4. Based on what we know… Can you predict what will happen? What factors will contribute to the “after” picture in this situation? Based on what we have talked about before, what laws can you apply in order to help you determine what will happen?

5. Collisions The outcome of a collision is dependent on the variables at the very start of the system These variables include:  The mass of each object  The velocity of each object These variables help to determine the inertia as well as the momentum of the starting objects involved in the collision – which will determine what happens to each object afterwards

6. Mass has impact Remember that mass helps to determine the amount of inertia that an object has An object with a small mass colliding into a large mass has to deal with more inertia Whereas an object with a large mass colliding into a small mass has to deal with less inertia

7. Small things can compensate Small objects, however, can have an effect in a collision Imagine a bullet being fired into a block of wood What will happen to the block of wood? High velocities can give objects a large momentum which will also affect their behaviour in collisions

8. Analyzing collisions When analyzing collisions, you are always looking at the “before” and “after” pictures of the colliding objects For example, imagine two carts in a system that is free from outside influences The two carts have the same mass Cart B starts at rest Cart A has some velocity What will happen?

9. A B VAVA The before picture VBVB

10. The after picture A V A ’ = 0 m/s B VB’VB’

11. What do you notice? Cart A upon hitting cart B stays still Cart B moves forward with a new velocity By comparing the values in the before and after picture, you can calculate values for velocity based on a few basic facts

12. Conservation It goes without saying that the Law of Conservation of Energy and Momentum are valid when dealing with collisions If we isolate a system of two colliding objects, then whatever energy and momentum the system started with should exist in the system afterwards

13. Is this always true? Think about a real life situation – a car collides into another car What happens to the cars in the process of the collision? Is all the energy that started in the system preserved in the velocity observed afterwards?

14. Imperfect collisions? Remember that when you analyze collisions, you are ultimately focused on two major variables in the system: the velocity and the mass Because of this, if some of the kinetic energy is lost from the system (because it was converted to other forms) it will not appear in the system after the collision

15. Kinetic Energy lost That means that if you try to apply the Law of Conservation of Energy when analyzing ONLY the kinetic energy before and after, you will find that they don’t equal each other Therefore, when analyzing collisions, they are divided into 2 categories:  Elastic  Inelastic

16. Elastic collisions In these collisions, kinetic energy remains in the system because mechanical forces that may rob the system of mechanical energy don’t exist These are obviously very rare

17. Inelastic collision More realistic – occurs when objects convert some of the kinetic energy present into the system into other forms of energy Think about car collisions – what happens to the car? Where does the energy come from to cause damage to the cars? The most inelastic collision occurs when the two objects that collide stick together after the collision Why do you think that’s the case?