# Homework Help: Force and Motion Problem

1. May 20, 2014

### Emperor

1. The problem statement, all variables and given/known data

A rifle bullet leaves the muzzle of a gun at 800. m/sec. The rifle barrel is 80. cm long and the mass of the bullet is 20.g. What force was needed to accelerate the bullet?

2. Relevant equations

Formulas:

F(Total)= ma

F= mg

F = micrometers (m)(g)

3. The attempt at a solution

t= 80 $\div$ 800 = .1s

a= 800 - 0 $\div$ .1 = 8000 m/s^2

The answer is provided on my sheet as 8000 N, but I don't have a clue as to how to do that. Its the closest I've gotten to the answer.

2. May 20, 2014

### Simon Bridge

Welcome to PF;
The equation you used was for time assumed a constant speed along the barrel - but if the bullet travels at a constant speed, then the force must be zero.

if you imagine the bullet has constant acceleration from rest to 800m/s along the length of the barrel, then you use a kinematic equation to find the acceleration.

note: it is best practice to complete the algebra before using the numbers.

3. May 20, 2014

### Emperor

Thanks for the help. Did you get the answer "8000 N" just like I did this time around?

4. May 20, 2014

### Simon Bridge

I do not, as a rule, actually complete the problems here.