why does a ball bounce?

[SimonInSimon]

especially those sports ball. please help me with this. I'm not familiar with the terms they mention in the internet. thanks!

[swansont]

We're probably going to use the same terminology, and at what scale do you want the explanation?

 

If a ball bounces it's because the collision is (mostly) elastic. The material has a large coefficient of restitution, so when it deforms, most of the energy stored is released again when the ball retakes its original shape. This is called elastic deformation. The molecules (usually longer chains) don't have a rigid structure; you don't break bonds when you apply the stress.

[the tree]

It's the returning to it's original shape which pushes the ball away from the ground again.

[pioneer]

If you look at the molecular level, rubber, for example, is sort of like a bunch of long molecular chains. When we bounce the ball, deformation at the bottom of the ball compresses these chains and places them into higher energy packing states. They quickly try to release this stored energy and return back to the lower energy state. The result is an expansion to release the stress that propels the ball back upwards.

 

Equal and opposite force applies so you can only get out what you put in. The ball also propagates some of its stress upward into the ball, with the result the ball will rarely bounce to the same height that it fell, unless you add some extra force to compensate for the impulse loss into the ball.

 

An interesting material is silly putty made from a silicone type rubber. This material will deform like putty in your hands, but if you throw it on the ground it will bounce like a solid ball. This has to do with the material only being able to deform with force impulses that are stretched out over time. If the impulse is too fast, it acts more like a solid material since the molecules are not able to slide past each other, storing local force.

 

For example, if you take some silly putty and stretch it slowly, it will stretch and elongate. But if you stretch it with a quick snap, it will shear off in a very smooth plane, as though one has sheared a solid material. The bounce force impulse acts the same way, with the compression too fast to allow the molecules to slide past each, causing a localized force build up, that will cause the putty material to bounce like it is a solid ball.

 

A kung fu master uses the force/time principle for breaking objects. The idea is to propagate a force impulse so fast that the material can not spread out the force and then bounce it back. The result is a fracture along a plane of the material. If you go too slow, the material will spread out the force and then bounce back the force into your hand, causing bone to fracture. The trick they use is to visualize the stopping point of your hand beyond the object. This assures that your hand won't slow down to soon especially at the point of possible force bounce back. There are demonstrations of some masters breaking a meter of ice. They know their physics, intuitively, and simply exploit a weakness in the material. It takes a lot of faith that science will work under these hazardous conditions.

[Innit]

If you look at the molecular level, rubber, for example, is sort of like a bunch of long molecular chains. When we bounce the ball, deformation at the bottom of the ball compresses these chains and places them into higher energy packing states. They quickly try to release this stored energy and return back to the lower energy state. The result is an expansion to release the stress that propels the ball back upwards.

 

Equal and opposite force applies so you can only get out what you put in. The ball also propagates some of its stress upward into the ball, with the result the ball will rarely bounce to the same height that it fell, unless you add some extra force to compensate for the impulse loss into the ball.

 

An interesting material is silly putty made from a silicone type rubber. This material will deform like putty in your hands, but if you throw it on the ground it will bounce like a solid ball. This has to do with the material only being able to deform with force impulses that are stretched out over time. If the impulse is too fast, it acts more like a solid material since the molecules are not able to slide past each other, storing local force.

 

For example, if you take some silly putty and stretch it slowly, it will stretch and elongate. But if you stretch it with a quick snap, it will shear off in a very smooth plane, as though one has sheared a solid material. The bounce force impulse acts the same way, with the compression too fast to allow the molecules to slide past each, causing a localized force build up, that will cause the putty material to bounce like it is a solid ball.

 

A kung fu master uses the force/time principle for breaking objects. The idea is to propagate a force impulse so fast that the material can not spread out the force and then bounce it back. The result is a fracture along a plane of the material. If you go too slow, the material will spread out the force and then bounce back the force into your hand, causing bone to fracture. The trick they use is to visualize the stopping point of your hand beyond the object. This assures that your hand won't slow down to soon especially at the point of possible force bounce back. There are demonstrations of some masters breaking a meter of ice. They know their physics, intuitively, and simply exploit a weakness in the material. It takes a lot of faith that science will work under these hazardous conditions.

 

That was brilliant! Thanks for that!

[iNow]

A kung fu master uses the force/time principle for breaking objects. The idea is to propagate a force impulse so fast that the material can not spread out the force and then bounce it back. The result is a fracture along a plane of the material. If you go too slow, the material will spread out the force and then bounce back the force into your hand, causing bone to fracture. The trick they use is to visualize the stopping point of your hand beyond the object. This assures that your hand won't slow down to soon especially at the point of possible force bounce back.

Wouldn't it be simpler to say that the faster the arm is moving, the more force it delivers to the object being struck. That the more "snap" in the martial artists punch (the quicker they remove their own fist from the object struck), the less that force will be returned to the striker and more will remain in the object which was struck.

 

In fact, this applies to your silly putty example too. The faster it's pulled, the greater the force (due to velocity), which is why it snaps. And it's elastic, which is why it bounces somewhat.

 

And, your second paragraph would be more accurate if you referenced the second law of thermodynamics, instead of just the propogation of stress through the ball (think "heat" and you'll catch my point).

 

And... ellipsis.