# Elastic Collision with a Spring Constant

1. Aug 5, 2012

### Drstevebrule

1. The problem statement, all variables and given/known data

A pinball with mass 80g is struck by a lever with mass 80g in a pinball machine in an elastic collision. The lever was pulled back 2 cm by a spring with spring constant k=1.4N/cm. What is the speed of the ball just after collision?

2. Relevant equations
F=kx

1/2kx^2=1/2mv^2

M1V1 = M2V2

3. The attempt at a solution

1/2 (2.8)(4)=5.6 5.6=1/2 (80) v^2 <-- trying to find velocity of lever just before contact

5.6/40 = v^2 V=.37

now this is where i get even more stuck, the masses of the ball and the lever are equal and they cancel out in the equation M1V1 = M2V2, which would leave the velocities as being equal. However, the answer choices are as follows:

A) 1.1 m/s
B) 1.4 m/s
C) 11 m/s
D) 14 m/s

If someone can shed some light on how to solve this problem it would be greatly appreciated.

2. Aug 5, 2012

### pgardn

First thing I would look at is units. g to kg

centimeters to meters, I would change both and see how it works out.

3. Aug 7, 2012

### Drstevebrule

i tried that, and i am still stumped