Dynamics - Another collision question

[Chantry09]

Homework Statement

http://img291.imageshack.us/img291/7829/collisionok5.jpg

Homework Equations

Conservation of momentum
Conservation of Kinetic Energy
GPE
KE

The Attempt at a Solution

Ive done the first part of the question. With the second part, i thought about it this way:

If the ram is seen to have zero velocity just after impact then all of the rams kinetic energy must have been transphered to the pile, no?

In which case, having worked out the gravitational potential energy in the ram initially to be 5886J. The kinetic energy in the pile must be:

5886 = 0.5 x 300 x V^2
39.24 = V^2
6.26 = V

However apparently the answer *should* be 7.23 m/s.

Can someone tell me where i have gone wrong?

James

 

[Doc Al]

Your mistake is in assuming that KE is conserved.

 

[Chantry09]

Ok. I worked out that the block would be traveling at 5.42m/s when it strikes the pile (using V^2= U^2 + 2as).

Now i know original momentum of the ram = 400 x 5.42 = 2168
Now if momentum is conservered: 2168 = 300 x V
V = 7.23 m/s

Oh, that's right...whoops. Thank you.