# 1 dimensional elastic collision

1. Nov 1, 2008

### IniquiTrance

A 0.415-kg hockey puck, moving east with a speed of 2.65 m/s, has a head-on collision with a 0.910-kg puck initially at rest.
Assuming a perfectly elastic collision, what will be the velocity of each object after the collision?
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The solution is:

v ' (lighter puck) , v' (heavier puck) = -0.990, 1.66 m/s, respectively.

My question is, wouldn't the lighter puck be expected to have a higher velocity magnitude than the heavier puck following the collision?

2. Nov 2, 2008

### phyzmatix

For perfectly elastic collisions, kinetic energy is conserved.

$$K_i=K_1+K_2$$

3. Nov 2, 2008

### IniquiTrance

Ok, i made that assumption when solving the problem.

I'm asking conceptually, wouldn't the heavier object be expected to move with a smaller velocity magnitude?

4. Nov 2, 2008

### phyzmatix

That will only be the case if the initial kinetic energy is divided equally between the two objects, i.e. when after the collision

$$\frac{1}{2}m_1v_1^2=\frac{1}{2}m_2v_2^2$$

so that

$$\frac{m_1}{m_2}=\frac{v_2^2}{v_1^2}$$

5. Nov 2, 2008

### IniquiTrance

Ah gotcha. Thanks for the help!