# Two object collision question impossible

1. Nov 17, 2008

### mrjoe2

1. The problem statement, all variables and given/known data
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

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

3. The attempt at a solution
first i tried the problem with the conservation of energy where the energy from the first block is transfered to the second (the Ek of 1st block is transfered to Ek second block). that didnt work. then i tried the conservation of momentum but you cant use it because you dont 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!

2. Nov 17, 2008

### Hootenanny

Staff Emeritus
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.

3. Nov 17, 2008

### 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!

4. Nov 17, 2008

### mrjoe2

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!

5. Nov 17, 2008

### Hootenanny

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