# Homework Help: Physicsal of bullet

1. Mar 12, 2005

### reggie

In police practice, bullets od identical mass are fired towards a concrete qall at different speed. It is found that a bullet travelling horizontally at 200m per second can penetrate the wall by a distance of 01.m.

1.In penetrating the wall, the kinetic energy of the bullet decreases. Account for this
'energy loss'.

2.What distance can another bullet of the same mass moving at 240m per second penetrate
in the wall?

3.Mass of a bullet is found to be 0.01 kg. What is the magnitude of retarding force applied
to the bullet by the wall to stop it?

2. Mar 13, 2005

### Davorak

Asking for the answer will not get you any where with most people on this board. Read the sticky at the top of this form if you have already. If we do not see what work you have already done then we can not tell where you are misunderstanding things. Most every one here is busy and donâ€™t have time to other peoples homework.

Write down the equation of motion(i.e. F = ma = ???) the wall. Since the problem did not mention gravity I would ignore it. This is a second order differential equation. Since this is a drag problem the bullets position will asymptotically approach a position in the wall.

Solve the above equation will give you the general solution for drag problems. Use the fact that "200m per second can penetrate the wall by a distance of 01.m." to fix the unknown variables in the problem.

3. Mar 13, 2005

### Andrew Mason

In what form can the kinetic energy be dissipated? Where can it go?

We aren't given enough information. We could assume that the stopping force is constant, but we don't really know that. If one assumes that it is constant, what is the work done by the bullet to the wall? Where does that ability to do work come from?

What is the relationship between kinetic energy and force and distance? Work out force from that.

AM