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

Bullet shot into a block

  • Thread starter oadeyemi
  • Start date
9
0
1. Homework Statement

A bullet of mass m is fired into a block of mass M initially at rest on a frictionless table of height h. The bullet remains m in the block, and after impact the block lands a distance d from the bottom of the table. Determine the initial speed of the bullet.

2. Homework Equations

I used (m +M)gh for the potential energy and set that equal to the kinetic energy ½(m + M)v₂² and found that v₂ = √(2gh).

And since it was a completely inelastic collision (right?) I used mv₁ = (m + M)v₂ → v₁ = (m + M)v₂ * 1/m

Is this right?


3. The Attempt at a Solution

initial velocity = √(2gh)*((m + M)/m)
 
Last edited:

Answers and Replies

114
0
Do you have an attempt at the problem? If not, tell me your thought process for the problem.
 
9
0
Do you have an attempt at the problem? If not, tell me your thought process for the problem.
I used (m +M)gh for the potential energy and set that equal to the kinetic energy ½(m + M)v₂² and found that v₂ = √(2gh).

And since it was a completely inelastic collision (right?) I used mv₁ = (m + M)v₂ → v₁ = (m + M)v₂ * 1/m

Is this right?
 
114
0
I used mv₁ = (m + M)v₂ → v₁ = (m + M)v₂ * 1/m
Something about this chunk doesn't seem right to me for some reason. Everything else is correct, from my understanding. Is this a formula you received from your text, or is this something that you derived yourself?

Check out the Wiki article, I think I see where you're going with this, but I see a slight error in your formula: http://en.wikipedia.org/wiki/Inelastic_collision
 
9
0
Something about this chunk doesn't seem right to me for some reason. Everything else is correct, from my understanding. Is this a formula you received from your text, or is this something that you derived yourself?

Check out the Wiki article, I think I see where you're going with this, but I see a slight error in your formula: http://en.wikipedia.org/wiki/Inelastic_collision
It's from my textbook
 
Borek
Mentor
28,136
2,646
I think the formula here is OK, wiki describes different situation (initial velocity of M not being zero) and uses different symbols, hence the confusion.

Problems are with the other part - v2 that you calculated from the energy conservation is a vertical component of the speed when block with a bullet hits the ground. That's not initial HORIZONTAL speed of the body.
 
9
0
I think the formula here is OK, wiki describes different situation (initial velocity of M not being zero) and uses different symbols, hence the confusion.

Problems are with the other part - v2 that you calculated from the energy conservation is a vertical component of the speed when block with a bullet hits the ground. That's not initial HORIZONTAL speed of the body.
Elaborate please
 
Borek
Mentor
28,136
2,646
Once the bodies (m+M) left the table they behave just like something thrown norizontally - they maintain horizontal speed, but they start to fall down, till they hit the ground at distance d from the table border.
 

Related Threads for: Bullet shot into a block

  • Last Post
Replies
2
Views
6K
  • Last Post
Replies
3
Views
11K
Replies
6
Views
6K
  • Last Post
Replies
1
Views
944
  • Last Post
Replies
10
Views
2K
Replies
3
Views
2K
Replies
4
Views
6K
Replies
2
Views
3K
Top