• Support PF! Buy your school textbooks, materials and every day products Here!

Finding velocity and momentum of a bullet and recoiling gun

  • #1
2
0

Homework Statement


A bullet leaves the barrel of a gun with a kinetic energy of 90 J. The barrel of the gun is 50 cm long. The gun has a mass of 4 kg, the bullet 10 g. (a) Find the bullets final velocity. (b) Find the bullets final momentum. (c) Find the momentum of the recoiling gun. (d) Find the kinetic energy of the recoiling gun.

Homework Equations


KE = 1/2mv^2
p = mv

The Attempt at a Solution


(a) Since I know the kinetic energy of the bullet as it leaves the barrel, I attempted to find the velocity with this information.

90 J = (1/2)(.01 kg)v^2
which gives v = 134.16 m/s

(b) To find momentum of the bullet I plugged this value for v into p= mv, this gives the result p = 1.3416 kg m/s.

for (c) I used conservation of momentum, since the initial momentum of both the bullet and gun are zero.
0 = (1.3416 kg m/s) + (4 kg)v
solving this give v = -.3354 m/s

(d) KE =(1/2)mv^2 so the kinetic energy of the gun is KE = (1/2)(4 kg)(-.3354 m/s)^2 = .225 J

I'm not entirely sure if I solved this correctly. My main problem is that it gives the length of the barrel of the gun and I did not use this in the solution. Is this unnecessary information? or did I do this incorrectly? I thought that the 50 cm could be used to find the work done on the gun, but 50 cm is how far the bullet moved, not how far the gun moved.
 

Answers and Replies

  • #2
gneill
Mentor
20,792
2,770
Welcome to Physics Forums!

Your calculations look good. The barrel length is irrelevant as you have surmised. You might want to round your results to better reflect the number of significant figures provided in the given data.

In future, please retain the formatting template headings when you put together your post. I've re-inserted the headers for you this time.
 
  • #3
2
0
Thank you very much! I planned to round my final results but just kept them that way in case I needed to use them in further calculations. Thanks again for the input!
 

Related Threads on Finding velocity and momentum of a bullet and recoiling gun

Replies
1
Views
3K
Replies
1
Views
5K
Replies
10
Views
3K
Replies
11
Views
2K
  • Last Post
Replies
14
Views
58K
Replies
11
Views
3K
Replies
6
Views
6K
Replies
1
Views
2K
Replies
1
Views
2K
Replies
3
Views
3K
Top