# force, gravity question

• February 2nd, 2008, 03:41 AM
scientist91
force, gravity question
Hi! I want ask you something. Why when I throw (lets say rock) in some wall from angle of 30, the rock will bounce off, also for approximately 30? Why some object bounce off more than the other ones? Why the angle of bouncing is like that? What happens with the micro structure of the rock and the wall?
• February 4th, 2008, 07:46 PM
VTMechEng
Well that all depends on the elasticity of the object being thrown against the wall. Some materials are better at deforming and reforming their original shapes than others, which is what makes them "bouncy."

The further and faster an object can morph and reshape, the more it will bounce.

It has nothing to do with gravity as both objects would fall at the same rate. It's just a matter of how much horizontal force the impact makes.

As far as the angle of bouncing goes, that will get even more complicated. Objects thrown at the same angle will initially bounce off the wall at the same angle. It's just the fact that an object that doesn't bounce well will take a more arched path than say a rubber ball.
• February 5th, 2008, 11:03 AM
JimboWizbo
Quote:

Originally Posted by VTMechEng
As far as the angle of bouncing goes, that will get even more complicated. Objects thrown at the same angle will initially bounce off the wall at the same angle.

As far as I remember from studying mechanics 4 or 5 years ago the angle is not necessarily the same, if you look up the coefficient of restitution (a value that varies depending on the materials involved) you will find that it can be used to calculate the velocity vector of the object after the impact.

The topic is common in A level mechanics so there's lots of information and examples on it.

First post :-D Hi all.
• February 14th, 2008, 02:05 PM
Cynapse
A guess is that this is the restricive coefficient of the angle produced. This would depend on the geometric shape of the object (a brick would transfer much horizontal momentum), the friction it produces against the reflection.

Classically (and using ideal circumstance) a reflection will occur at exactly the angle you throw it at, with respect to the normal. However something like a rock is not ideal; when you throw it there may be anguler momentum (spinning it) which will either aid or hinder a bounce angle. Finally and most importantly, even if it is not thrown with anguler momentum, because of it's irregular shape when an edge strikes the wall, not all of the rock will be going along the same point path, so some wont have struck the wall yet: it will keep going. The edge that has struck however cant go anywhere (wall is in the way :P), therefore the rock rotates about this edge giving angular momentum, therefor either aiding or hindering the angle it rebounds.

Different shaped objects hit the wall differently so their angle is changed according to the solution of all of the above factors!