# Force, gravity question

1. Feb 2, 2008

### Physicsissuef

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?

2. Feb 2, 2008

### John Creighto

The rock bounces at approximately 30 degrees because the force acting on the rock during the collision is perpendicular to the wall and if energy is conserved the velocity perpendicular to the wall will have the same magnitude before and after the collision.

As for why some collisions are more elastic then others that depends on the structure of the rock and wall and I'll leave that for someone else to answer.

3. Feb 2, 2008

### Physicsissuef

But if the force is perpendicular to the wall, it will bounce of for 90 degrees? Or the force is not big enough to bounce it for 90 degrees?

4. Feb 2, 2008

### LURCH

No, the force si perpendicular so the rck's motion perpendicular to the wall should remain the same. Let's say you throw the rock at 60kph at an angle of 45o; then the rock is only moving 30kph relative to the wall's vertical plane (and the other 30khp is vertical or horizontal motion, or some combination of these). If the bounce off the wall is perfectly efficient, and no energy gets absorbed into the structures or lost as heat and sound, then the rock's motion away from the wall after the impact should still be 30kph, and the total velocity of the rock should still be 60. therefore, you end up with an angle of 45o, just like before the impact.

5. Feb 2, 2008

### Physicsissuef

why some materials bounce off more than the others?

Why some materials bounce (jump) off more than the others? What happens in the microstructure?

6. Feb 2, 2008

### JayKo

interesting thought, i would like to find out as well :D

7. Feb 2, 2008

### Feldoh

It depends on something called the coefficient of restitution (or essentially a measure of elasticity between two surfaces). If the coefficient of restitution between two surfaces is one, then energy is completely conserved.

This is why if you drop a tennis ball it will bounce up but only to a portion of the height from where it was originally dropped. In real-world situations, such as throwing a rock against a wall, it is not a perfect elastic collision, because the coefficient of restitution is not 1 energy will not be completely conserved. The coefficient of restitution is different for all objects so as a result objects that hit each other will all react differently.

8. Feb 2, 2008

### Physicsissuef

But what happens with the microstructure?

9. Feb 2, 2008

### dst

That's a mathematical tool and it doesn't really explain what's going on.

Imaginably, it would be the same thing as standard elastic deformation - chains of molecules flex around to distribute the force/energy, and then flex back due to electrostatic repulsion and other intermolecular forces between chains/parts of the structure. I mean, graphite has a very rigid structure as does diamond and most sort of things.

In addition, some things (i.e. flesh) compress in other ways.

As for rocks, they have a very rigid structure and so they obviously don't flex much. It's all to do with how electrons on the surface interact; the electrons within the surface of the rock will repel the electrons within the surface of the wall due to their charges, so you will get it bouncing back. And conservation of momentum is the "reason" why it will bounce at a similar angle. There are all sorts of things to consider.

Last edited: Feb 2, 2008
10. Feb 2, 2008

### John Creighto

I don't think determining the coefficient of restitution is easy. Here is one paper for some types of materials:

http://www.sciencedirect.com/scienc...serid=10&md5=8138502b41d03bdd3a7ed8e2f91568fe

11. Feb 2, 2008

### John Creighto

This paper sounds somewhat interesting:

http://engr.smu.edu/me/syslab/papers/p5.pdf [Broken]

Last edited by a moderator: May 3, 2017
12. Feb 2, 2008

### John Creighto

Here are some of my thought on how you could lose energy. If you break a piece of it then that should be in-elastic. When you compress the air in a tennis ball it creates heat. If any of that heat escapes to the air then you lose energy as the compression is no longer adiabatic. If there is viscous flow like in the case of plastics you lose energy.

13. Feb 2, 2008

### John Creighto

Some other modes of energy transport are should waves. When the rock collides with the wall it creates compressive waves in the wall which carry energy away from the rock. When some of this energy is reflected back I presume it bounces the rock away from the wall but not all of this vibrational energy is transfered back into the rock.