Momentum question, seems easy, y can't I get it

[lala56]

"A 10.0 g marble slides to the left with a velocity of magnitude 0.600 m/s on the frictionless, horizontal surface of an icy, New York sidewalk and has a head-on elastic collision with a larger 25.0 g marble sliding to the right with a velocity of magnitude 0.250 m/s (Fig. 8.35)."

I need to "Calculate the change in kinetic energy (that is, the kinetic energy after the collision minus the kinetic energy before the collision) for each marble."

So, I will start with the 25g marble going right, which hits the 10g marble and revserses direction to go left

I correctly found the velocity of the 25g marble after the collision, it is -.24.
Now, to find the change in kinetic energy, i did this:

v1=intial velocity
K2-K1=.5mv^2 - .5mv1^2

.5(.025)(-.24)^2 - .5(.025)(.25)^2

isn't this how one is supposed to find change in kinetic energy? Then why is it incorrect? did i go wrong with the positive negative sign?

this one should be negative right?

thankyou

 

[Doc Al]

Your method is correct, but since initial and final speeds are so close I would suggest using a more accurate value for the final speed than -.24 m/s when calculating the change in KE. You can always round it off later.