Why some things bounce and some things doesn't?
2014-06-07 10:46 am
If i throw a ball down and another thing which has same weight as the Ball in a same speed from a same height then according to the Newton's law both of them are supposed to receive same ammounts of forces after hitting the ground. But although the Ball bounced back but the other thing that i used didn't bounce. Why?
回答 (2)
2014-06-07 5:38 pm
In a word "deformation." "When a sufficient load is applied to a metal or other structural material, it will cause the material to change shape. This change in shape is called deformation." [See source.]
There are two kinds: elastic and plastic. Elastic deformation is when the deformation is self-reversing upon removal of the stress (force) on the material. While the atomic bonds of the material are stretched, they are not broken. And the stored energy while deformed is converted into kinetic energy as the bonds return to normal. That KE results in the bounce.
Plastic deformation is permanent and not self reversing. In this case some atomic bonds of the material are actually broken. They cannot restore; so the energy that went into breaking the bonds remains as stored energy or is converted into heat or other high entropy energy. So there is no conversion into kinetic energy and no bounce.
BTW, while compression, the deformation, is a prerequisite for a bounce, the degree of compression is not the real indicator of how elastic a body might be. The real indicator is how much energy is stored in that compression and, of course, the self reversing.
My point, is that a solid cue ball in billiards and that shows little compression on impact has more elasticity than many rubber balls that have noticeable compression on impact. Again, it's not the degree of compression so much but the energy stored by the compression. In general a soft material needs more deformation than a hard material to store comparable energy.
There are two kinds: elastic and plastic. Elastic deformation is when the deformation is self-reversing upon removal of the stress (force) on the material. While the atomic bonds of the material are stretched, they are not broken. And the stored energy while deformed is converted into kinetic energy as the bonds return to normal. That KE results in the bounce.
Plastic deformation is permanent and not self reversing. In this case some atomic bonds of the material are actually broken. They cannot restore; so the energy that went into breaking the bonds remains as stored energy or is converted into heat or other high entropy energy. So there is no conversion into kinetic energy and no bounce.
BTW, while compression, the deformation, is a prerequisite for a bounce, the degree of compression is not the real indicator of how elastic a body might be. The real indicator is how much energy is stored in that compression and, of course, the self reversing.
My point, is that a solid cue ball in billiards and that shows little compression on impact has more elasticity than many rubber balls that have noticeable compression on impact. Again, it's not the degree of compression so much but the energy stored by the compression. In general a soft material needs more deformation than a hard material to store comparable energy.
2014-06-07 11:00 am
In a word, elasticity, which is the tendency of material to return to its original shape after being distorted. Elastic materials store energy; when you compress or stretch them you put energy in, and that energy can be released again.
A rubber ball is very elastic, a pebble is not. When the ball hits the ground most of the kinetic energy of the ball gets stored in compressing the rubber. That energy is released as the rubber re-expands, which is what causes the ball to bounce back. A pebble is not very compressible, so it can't store much kinetic energy. Instead the energy gets converted into heat, the pebble just gets a little bit warmer when it hits the ground.
A rubber ball is very elastic, a pebble is not. When the ball hits the ground most of the kinetic energy of the ball gets stored in compressing the rubber. That energy is released as the rubber re-expands, which is what causes the ball to bounce back. A pebble is not very compressible, so it can't store much kinetic energy. Instead the energy gets converted into heat, the pebble just gets a little bit warmer when it hits the ground.
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