# Homework Help: 2 cube collision inquiry

1. Nov 16, 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.

2. Nov 16, 2008

### natives

At first use conservation of energy u=sqrroot(2gh) when the granite block slides down the ramp...Then use both conservation of momentum m1u1+m2u2=m1v1+m2v2 and conservation of energy m1sqr u1+m2sqr u2=m1sqr v1+m2sqr v2...If m1 is granite then u1 is initially zero..Sort out the two equations using the momentum equation to eliminate v2 in the energy equation...Yah,assume a perfectly elastic condition, they normally have a way of telling you if its not!

3. Nov 16, 2008

### mrjoe2

this doesnt work. did you try and get the correct answer when you used your very long process?

Last edited: Nov 16, 2008
4. Nov 17, 2008

### natives

Yes,it did work and its not a very long process..You'll get used to it once you attend to more questions!Show me what you have done!?

5. Nov 17, 2008

### natives

Yes,it did work and its not a very long process..You'll get used to it once you attend to more questions and be careful what value of g you use,refer to the book's tables!Show me what you have done!?

6. Nov 17, 2008

### mrjoe2

i set the total energy before the collision (mgh) equal to the kinetic energy of the initial cube and the now moving cube after collision. so

(.1)gh = .5(.1)v1'^2 + .5(.2)(1.5)^2

now i have two variables i need to solve for.... first of all, this method doesnt work!!!! if i put in conservation of momentum, then

.1(v1) = .1(v1') + .2(v2') now what!

7. Nov 17, 2008

### natives

Remember v1 is the velocity of the first cube whose initial kinetic energy .5m1v1^2 u substitute with m1gh...So even though you didnt know but u had let .5m1v1^2=m1gh..cant you make v1 a subject and substitute in your momentum equation?then u'd 2 simultaneous equations which am sure you can solve easily!

8. Nov 17, 2008

### natives

Remember v1 is the velocity of the first cube whose initial kinetic energy .5m1v1^2 you had substitute with m1gh...So even though you didnt know it but u had let .5m1v1^2=m1gh..cant you make v1 a subject out of this and substitute it in your momentum equation?then u'd 2 simultaneous equations which am sure you can solve easily!Also remember v2' was given as 150!

9. Nov 17, 2008

### natives

Remember v1 is the velocity of the first cube whose initial kinetic energy .5m1v1^2 you had substitute with m1gh...So even though you didnt know it but u had let .5m1v1^2=m1gh..cant you make v1 a subject out of this and substitute it in your momentum equation?then u'd 2 simultaneous equations which am sure you can solve easily!Also remember v2' was given as 150 cm/s!