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

Linear momentum problem (ballistic pendulum)

  • #1

Homework Statement


A ballistic pendulum is a device that may be used to measure the muzzle speed of a bullet. It is composed of a wooden block suspended from a horizontal support by cords attached at each end. A bullet is shot into the block, and as a result of the perfectly inelastic impact, the block swings upward. Consider a bullet (mass m) with velocity v as it enters the block (mass M). The length of the cords supporting the block each have length L. The maximum height to which the block swings upward after impact is denoted by y, and the maximum horizontal displacement is denoted by x.

a) In terms of m, M, g and y, determine the speed of the bullet.

Homework Equations


mgh=1/2 m v2

The Attempt at a Solution


(1/2)(m)v2 = (m + M)(g)(y)
v =sqroot { [(2)(g)(m + M)(y)]/m }

The real solution states that one must find the initial velocity of the bullet-block system, and then use this equation. Why can I not just use the initial velocity of the bullet? The real final answer is what I have, except the (m+M)/m is outside the sqroot.
 

Answers and Replies

  • #2
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
13,487
5,979

Homework Statement


A ballistic pendulum is a device that may be used to measure the muzzle speed of a bullet. It is composed of a wooden block suspended from a horizontal support by cords attached at each end. A bullet is shot into the block, and as a result of the perfectly inelastic impact, the block swings upward. Consider a bullet (mass m) with velocity v as it enters the block (mass M). The length of the cords supporting the block each have length L. The maximum height to which the block swings upward after impact is denoted by y, and the maximum horizontal displacement is denoted by x.

a) In terms of m, M, g and y, determine the speed of the bullet.

Homework Equations


mgh=1/2 m v2

The Attempt at a Solution


(1/2)(m)v2 = (m + M)(g)(y)
v =sqroot { [(2)(g)(m + M)(y)]/m }

The real solution states that one must find the initial velocity of the bullet-block system, and then use this equation. Why can I not just use the initial velocity of the bullet? The real final answer is what I have, except the (m+M)/m is outside the sqroot.
What can you say about energy in an inelastic collision?
 
  • Like
Likes Dennis Heerlein
  • #3
What can you say about energy in an inelastic collision?
:( I stared at that for 30 minutes, thinking I was using conservation of momentum and not conservation of energy. Thank you for reading that block of text and helping me though! Much appreaciated
 

Related Threads on Linear momentum problem (ballistic pendulum)

Replies
1
Views
1K
Replies
2
Views
629
  • Last Post
Replies
2
Views
7K
Replies
3
Views
4K
  • Last Post
Replies
3
Views
7K
  • Last Post
Replies
1
Views
3K
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
2
Views
8K
  • Last Post
Replies
8
Views
4K
Replies
2
Views
7K
Top