Bullet fired at a block of wood

AI Thread Summary
A 19-g bullet is shot into a 5-kg block, causing the block to lift 5 mm. The impulse on the block due to the bullet's capture is calculated using the change in momentum, not energy conservation, as the bullet causes permanent deformation. The initial attempts at the solution incorrectly applied energy conservation principles and did not consider the time factor, which is deemed irrelevant for the impulse calculation. The correct approach focuses on the momentum change of the block after the bullet penetrates. The discussion emphasizes the need to apply conservation of momentum for accurate results.
Motorboar
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Homework Statement



An 19-g bullet is shot vertically into an 5-kg block. The block lifts upward 5 mm. The bullet penetrates the block in a time interval of 0.001 s. Assume the force on the bullet is constant during penetration. The impulse on the block due to capture of the bullet is closest to:

Homework Equations


Impulse = P_2 - P_1


The Attempt at a Solution


1/2(mbullet + mblock)v2^2 = (mbullet + mblock)gy
v2 = sqrt(2gy)
v1 = ((mbullet + mblock)/mbullet)*v2
Impulse = m2v2 - m1v1

Comes out wrong, and didn't use the time given. I modeled my solution after a ballistic pendulum problem. Any pointers?
 
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try to do it using conservation of momentum, instead of conservation of energy. total mechanical energy is not conserved here, as the block got permanent deformations by the bullet.
 
Motorboar said:

The Attempt at a Solution


1/2(mbullet + mblock)v2^2 = (mbullet + mblock)gy
v2 = sqrt(2gy)
This is good. Note that you are asked to find the impulse on the block, so what is the change in momentum of the block?

The time is irrelevant for this question. (But I suspect there are other parts to this problem.)
 

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