# Homework Help: Collisions in One and Two Dimensions

1. Mar 22, 2006

### helen3743

Ok. I need help in how to even just start this problem.

"Starting with an initial speed of 5.00 m/s at a height of 0.280 m, the 1.75 kg ball swings downward and strikes the 4.75 kg ball that is at rest.

a) Using the principle of conservation of mechanical energy, find the speed of the 1.75 kg ball just before impact.
b) Assuming that the collision is elastic, find the velocities (magnitude and direction) of both balls just after the collision.
c) How high does each ball swing after the collision, ignoring air resistance?"

I know that the formula I will be using is:
0.5mvf^2 + mghf = 0.5mvo^2 + mgho

But I'm confused which m's will be for the first ball and second ball.
I tried to do this:
0.5m1vf1^2 + m2ghf = 0.5m1vo1^2 + m2gho
but then i feel like that doesn't make sense. Why is ho & hf only multiplied by m2 and not m1.... I don't know.

And for question a) how do you find the answer right before impact? I know how to find it after impact, and before impact, but how do you find it right before impact?

I need help... thanks!

2. Mar 23, 2006

### andrevdh

a) The idea is to totally ignore the existence of the ball waiting at the bottom for the ball that is swinging downwards. We concentrate on the ball swinging downwards and apply the principle of conservation of energy to it. That is its mechanical energy at the top will be equal to its mechanical energy at the bottom
$$E_{top}=E_{bottom}$$
each of these two energies may consist of a potential and kinetic energy component of the swinging ball.

Last edited: Mar 23, 2006
3. Mar 23, 2006

### DaMastaofFisix

How's the second two parts coming along, sshow us what you've deon and we'll point you in the right direction with it all, and btw...This problem isn't technically a two dimensional collision problem, but that is of no concern to us now.