Quick Question Cons. Energy, elastic collision, projectile

[0338jw]

Homework Statement

In a physics lab a cube slides down a frictionless incline and elastically strikes another cube at the bottom that is only half it's mass. If the incline is 30 cm high and the table is 90 cm off the floor where does each cube land?

Homework Equations

mgh=1/2mv^2
v=at+v_o
x=.5at^2 +v_o*t +x_0
conservation of momentum/energy

The Attempt at a Solution

I know that this problem has three parts, conservation of energy, collision and projectile but I had a question about the mass. so I simply use 2m and M or can i substitute any value in as long as it;s twice the value of the smaller block? I know how to solve all parts but this first part stumbled me a little bit. Thanks in advance, all help is appreciated!

 

[Doc Al]

Don't use numerical values. Call the masses m and m/2. (Or something equivalent.)

Since the actual values of the masses are not needed, the masses will drop out of any final calculation.

 

[0338jw]

well I used numerical values, and I still got the right answer, but thanks for the advice regardless! The big block lands .35m away and the smaller one flies 1.4m away from the table.

 

[Doc Al]

well I used numerical values, and I still got the right answer, but thanks for the advice regardless!

Of course you can plug in numbers, but it's much better--and easier!--to set the equations up symbolically and understand why the masses cancel out.