# Why do objects bounce?

1. Jul 24, 2009

### kashiark

Why do objects bounce? Let's say you throw a tennis ball at a wall. Why doesn't the ball just transfer its momentum to the wall and then fall on the ground? Does the wall receive more momentum than the ball originally had?

2. Jul 25, 2009

### Cyrus

3. Jul 25, 2009

### Staff: Mentor

Think about conservation of momentum and conservation of energy. If the ball has some energy that is absorbed by compressing itself against the wall, what must happen next...?

4. Jul 25, 2009

### Joans

oR nEWtons 3'rd law?

5. Jul 25, 2009

### YellowPeril

Think about the following real world situations:

A rubber bouncy ball bouncing on a floor follows an unpredictable path, this is because there is change in angular momentum due to friction with the floor.

A tennis ball follows a more predictable path, except in extreme situations e.g. pro tennis. How does the change in momentum with a top spin on a tennis ball for example compare to the change in momentum for a bouncy ball with top spin on a kitchen floor.

Putty thrown against a wall might not bounce.

A tennisball thrown into a net might not bounce.

Idealistically for direct impact of a sphere onto a fixed plane, v = eu where v is the final velocity, u is the initial velocity and e is the coefficient of restitution, e is 1 for perfectly elastic collisions, e is zero if the two bodies do not separate after collision.

Note: In the real world, it is impossible that the wall remains totally still, you only have to look at skyscrapers swaying in the wind to appreciate that fact.

6. Jul 25, 2009

### arildno

As Cyrus has said, think about the ball AND the wall as springs.

When there is no longer any velocity difference between the wall and the ball, no force acts to encourage further deformation, and the stored potential elastic energy reconverts (partially) into kinetic energy.

7. Jul 25, 2009

### Bob S

If a tennis ball bounces off a wall with 100% coefficient of restitution, the momentum transferred to the wall is twice what it would be if the ball just fell to the ground.

8. Jul 25, 2009

### Nirgal

It really depends on the materials that make up the ball and the wall. Depending on the materials you will get an elastic, inelastic, or somewhere-in-between collision. I don't think this answers your question though. Say the collision is completely elastic so that the ball rebounds completely. The theory behind why it rebounds has to do with electromagnetism and quantum mechanics. The atoms in the ball must repel those in the wall. The degree to which this happens depends on the materials that make up the ball and the wall as stated above. You're asking a rather fundamental and very important question. But to tell you the truth, I don't believe you can answer this question completely, without invoking concepts from quantum mechanics.

9. Jul 26, 2009

### vin300

It depends on how elastic a material is used and how much force is imparted to it. A steel ball is more elastic than he tennis, you may want to use a steel ball in a game that needs to impart as much force to distort it and it exerts an equal restoring force so you get a stunning bounce as well, but this does not mean rigid bodies do not bounce, they just cannot change their configuration