Solving for Impact Velocity: A Bullet of Ice at 0°C

[noobatphysics]

Homework Statement

A Bullet of ice at a temperature of 0 degrees celieus is fired into a target
Assuming that all the impact energy is retained by the ice
what impact velocity is needed to melt 68% of the ice

Homework Equations

1/2mv^2
Q = ml

The Attempt at a Solution

i got..
v^2 = 668000
i doubt that is the correct answer
can some show me the steps to work this out

 

[ideasrule]

Why didn't you use the 68%? The kinetic energy turns into the energy needed to melt 0.68m, not m.

 

[noobatphysics]

um I am still puzzled
LOL
can u show me the steps?

 

[ideasrule]

I thought you used:

1/2mv^2
Q = ml

and equated them to get:

1/2mv^2=ml
v^2=2l

The statement 1/2mv^2=ml means "all the kinetic energy turns into the energy needed to melt the mass". However, only 68% of the mass melts, so the kinetic energy has to equal 0.68ml.

 

[noobatphysics]

oh i see...

i got... 674 m/sec

doesn't seem right

Ek = 334800 x .68?