Two object collision question impossible

[mrjoe2]

Homework Statement

a 100g granite cube slides down a 40 degree frctionless ramp. at the bottom, just as it exits onto a horizontal table, it collides with a 200g steel cube at rest. how high above the table should the granite cube be released to give the stell cube a speed of 150cm/s

Homework Equations

Ek=1/2mv^2
Eg=mgy
maybe Ptot=ptot'

The Attempt at a Solution

first i tried the problem with the conservation of energy where the energy from the first block is transferred to the second (the Ek of 1st block is transferred to Ek second block). that didnt work. then i tried the conservation of momentum but you can't use it because you don't know if it is a perfectly elastic collision of the cubes stick together. you cannot assume anything, so i think i need to utilize the conservation of momentum, but its not giving me the right answer! the answer should be 25.9cm and i keep getting 23cm. also, using the conservation of energy, the angle is irrelevant. try it to see if you get the correct answer. i really doubt you willlll! :P help!

 

[Hootenanny]

If one assumes that the collision is completely elastic and simply uses conservation of energy, where an angle is indeed irrelevant as you point out, then one should arrive at the correct answer.

 

[Sourabh N]

In most general cases, if nothing is said about the elastic porperty of collision, we assume it to be a perfectly elastic one!

 

[mrjoe2]

If one assumes that the collision is completely elastic and simply uses conservation of energy, where an angle is indeed irrelevant as you point out, then one should arrive at the correct answer.

i used the conservation of energy with an elastic collision and it did not work. you can try it for yourself, the "correct" answer is posted!

 

[Hootenanny]

i used the conservation of energy with an elastic collision and it did not work. you can try it for yourself, the "correct" answer is posted!

I did - and got the correct answer. Perhaps if you posted you're working we could point out where you're going wrong.