Momentum of a bullet problem

  • Thread starter KD
  • Start date
  • #1
KD
27
0

Homework Statement



A 7.45g bullet has vi = 353 m/s. It strikes the 0.725 block of a ballistic pendulum and passes completely through the block. If the block rises through a distance h = 12.1 cm, what was the velocity of the bullet as it emerged from the block?


Homework Equations


Energy conservation and momentum

The Attempt at a Solution


First I solved for the velocity of the block when the bullet hit it. I used conservation of energy 1/2mv2 = mgh. I got the velocity as 2.41 m/s. Then I used conservation of momentum.
.00745(353)=.725(2.41) + .00745vf.
I solved for vf and got 118.5 m/s. The answer is supposed to be 203 m/s. I have tried combining the masses with the 2.41 velocity, but it still doesn't work out.
Can some one help me out? Thanks.
 

Answers and Replies

  • #2
cristo
Staff Emeritus
Science Advisor
8,107
73

Homework Statement



A 7.45g bullet has vi = 353 m/s. It strikes the 0.725 block of a ballistic pendulum and passes completely through the block. If the block rises through a distance h = 12.1 cm, what was the velocity of the bullet as it emerged from the block?


Homework Equations


Energy conservation and momentum

The Attempt at a Solution


First I solved for the velocity of the block when the bullet hit it. I used conservation of energy 1/2mv2 = mgh. I got the velocity as 2.41 m/s. Then I used conservation of momentum.
.00745(353)=.725(2.41) + .00745vf.
I solved for vf and got 118.5 m/s. The answer is supposed to be 203 m/s. I have tried combining the masses with the 2.41 velocity, but it still doesn't work out.
Can some one help me out? Thanks.
The number in bold that you have calculated (2.41) is actually the square of the velocity (you forgot to take the square root) Take sqrt(2.41) and plug this into your momentum conservation equation, and you should get the correct answer.
 
  • #3
KD
27
0
Oh, good catch, thanks!
 

Related Threads on Momentum of a bullet problem

  • Last Post
Replies
6
Views
6K
  • Last Post
Replies
6
Views
1K
  • Last Post
Replies
21
Views
2K
  • Last Post
Replies
7
Views
7K
  • Last Post
Replies
13
Views
337
Replies
5
Views
5K
  • Last Post
Replies
1
Views
1K
Replies
3
Views
1K
  • Last Post
Replies
13
Views
2K
  • Last Post
Replies
3
Views
2K
Top