# Homework Help: Elastic collison of 2 moving object

1. Nov 5, 2008

### HelloMotto

a .900 kg object is moving east at 2.8 m/s.
a 1.30 kg object is moving west at 1.4 m/s.
If the collision is perfectly elastic find the velocities after the collision. (neglect friction)

I know momentum and kinetic energy is conserved but Im really clueless as to what to do.

2. Nov 5, 2008

### joeyar

Two simultaneous equations here may be useful, one for KE and one for momentum.

Let V1i be object 1's velocity before impact V2i be object 2's veloctiy before impact V1f be object 1's velocity after imapct and V2f be object 2's velocity after impact. m1 is first object's mass, m2 = object 2's mass

Then m1V1i + m2V2i = m1V1f + m2V2f.
m1V1i^2 + m2V2i^2 = m1V1f^2 + m2V2f^2

(Note how I have cancelled out the 'half' in K = 1/2 * mv^2 - in effect doubling every term in the 2nd equation, so it's simpler to work with but still doesn't effect the system of equations).

Substitute in the values you are given. Take moving east as moving in the positive direction (and therefore moving west in the negative direction).

3. Nov 5, 2008

### djeitnstine

You can just use Vi - Vf = -(V2i - V2f)

Is a shortcut my professor showed us instead of using quadratics by 1/2 mv^2