Potential Energy converted to kinetic energy

[kelly0107]

Homework Statement

A spring gun is loaded with a 500 gram projectile (position is given in centimeters and time is given in seconds) The spring is massless and therefore has no kinetic energy. How much potential energy is converted to kinetic energy when the ball is at -4 cm.

Velocity=2.69 cm/s
Time=.72 seconds

Homework Equations

K=1/2 mv squared

The Attempt at a Solution

1/2 (500g)(7.236 cm squared/seconds squared)

3618 grams/cm squared/seconds squared

Now I don't know how to set up the conversion to meters and kilograms. Can someone please help?

The other question I have is... How much potential energy is converted to kinetic energy in the spring gun? Would the answer be zero because the spring is massless?

 

[G01]

I think it would be much easier to convert to kilograms and meters before you multiplied everything out. That way your answer for the kinetic energy would already come out in Joules. This way, you only need to remember the conversions for the basic units and all other, more complicated units will already be converted once you get to them!

If the spring is taken to have 0 mass, then assume that it has no kinetic energy. Therefore, none of the potential energy is lost to the spring itself. (Obviously, in a real spring with mass, energy is lost to the spring.)