Homework Help: physics

Posted by elisabeth on Friday, February 19, 2010 at 11:13pm.

When a lead bullet hits a solid target at high speed, its entire kinetic energy is converted into heat. For simplicity, assume that all the heat is concentrated in the bullet rather than the target.
If the bullet's initial temp. is 20` C, how fast should it be moving before the collision in order to reach the lead's melting temp 327.3` C and have 53% of its mass melted after the collision? Lead's specific heat is 128 J/kg x K and it's latent heat of fusion is 24500 J/kg.

answers is in m/s?????

physics - drwls, Friday, February 19, 2010 at 11:19pm
Set the bullet's kinetic energy
(1/2) MV^2
equal to the amount of heat energy required to do that amount of heating and melting. You do not need to know the mass M; it will cancel out later.

Both the kinetic energy and the energy needed to melt 53% of the lead are proportional to M
physics - elisabeth, Saturday, February 20, 2010 at 12:10am
i'm still confused.... :(
physics - drwls, Saturday, February 20, 2010 at 2:18am
Try to follow the steps.
(1) compute the energy (per mass) required to raise the lead tempertaure and melt 53% of it.

(2) Set that energy equal to the kinetic energy (per mass), and solve the resulting equation for the velocity required.

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