# Homework Help: Kinetic Energy of bullet fired from gun

1. Aug 17, 2014

### lolbob07

Hi everyone. I'm quite troubled with this physics question.

A .035 kg bullet is fired from a .23 m barrel. The bullet experiences a force of 4500 N while in the gun barrel. What is the kinetic energy of the bullet as it leaves the gun barrel?

I don't understand how to figure out the velocity to calculate the KE. Is it possible to calculate the KE from GPE or Work?

Our teacher has not taught us this and I don't understand why he is expecting us to answer this question....
(btw. I'm in year 10)

Thanks.

2. Aug 17, 2014

### Staff: Mentor

Yes. Look up the work-energy theorem. (See: Work-Energy Principle)

Calculate the work done by that force. (Assume that it's constant.)

3. Aug 17, 2014

### lolbob07

So, If I assume It is constant. The answer = 4500 N x .23 m = 1035 J?

Last edited: Aug 17, 2014
4. Aug 17, 2014

### Staff: Mentor

Yep. You got it.

5. Aug 17, 2014

### lolbob07

Thanks. I don't understand why our teacher gave us this question when we hadn't learnt the work energy theorem... Anyways thanks.

6. Aug 17, 2014

### CWatters

You could also solve it by applying Newtons laws and equations of motion.

7. Aug 17, 2014

### lolbob07

We have not learned equations of motion. I have no clue about them.

8. Aug 17, 2014

### BvU

Pity you didn't use the template. Under 2) relevant equations you would have filled in something, right? I mean by the time you are in year 10 there must have been something comining by that can be related to this exercise ?
What would you have filled in if you absolutely would have needed to fill in something ?

9. Aug 18, 2014

### dean barry

Classics :

force (N) = mass (kg) * ( constant ) acceleration ( m/s/s )

You have mass ( 0.035 kg ) and force ( 4,500 N ), so, transpose equation for acceleration :
acceleration = force / mass
acceleration = 128,571.43 m/s/s

With the acceleration (a) and distance (s), you can use the classic equation :
v ² = u ² + ( 2 * a * s )
u = initial velocity which = 0, so drop it, then transpose for v ( final velocity )
So :
v = square root ( 2 * a * s )
v = 243.193 m/s

Now find the KE from :
KE = ½ * mass * velocity ²
KE = 1,035 Joules

10. Aug 18, 2014

### BvU

I'm still wondering if Bob had an empty toolbag to begin with, or if there was something he could use to work out this exercise...

11. Aug 18, 2014

### lolbob07

Umm yeah Sorry. I didn't realise they were 3 columns that we could fill in. Because with my second question I asked on another thread I realised. :/