Homework Help: Physics

Posted by Fred on Thursday, October 14, 2010 at 12:34am.

A bullet with a mass of 55 grams moving with a speed of 100 m/s slams into and wedges in a 1.9 kilogram block of wood supported by a string initially at rest. Which one, conservation of energy or conservation of momentum, can be applied to this situation? Why?

I think it's conservation of momentum because kinetic energy is not conserved in inelastic collisions. Correct?

Physics - Fred, Thursday, October 14, 2010 at 12:35am
Also, how high does the block swing after being hit by the bullet?
Physics - Fred, Thursday, October 14, 2010 at 12:54am
Actually, you can use the conservation of momentum first to find the initial velocity of the bullet-block system, and then you can plug that velocity into 1/2mv^2=mgh and solve for the height that the block swings, which gives a value of 0.403 m.

Correct?
Physics - drwls, Thursday, October 14, 2010 at 12:58am
Yes, that is exactly how to do the problem.

You are also correct that mechanical energy is not conserved.
Physics - Fred, Thursday, October 14, 2010 at 1:04am
Eureka!

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