# Looking for deltaK, change in total kinetic energy

On a frictionless horizontal air table, puck A (with mass 0.253 kg) is moving toward puck B (with mass 0.373 kg), which is initially at rest. After the collision, puck A has velocity 0.124 m/s to the left, and puck B has velocity 0.650 m/s to the right.

Calculate DeltaK, the change in the total kinetic energy of the system that occurs during the collision.

- I got ΔK = .007 but was not right
-puck A with speed 0.834 is 0.5*0.253*(0.834)2 . Total energy of the two pucks afterwards are 0.5*0.253*(0.124)2 +0.5*0.373*(0.650)2

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rock.freak667
Homework Helper
What you did should be right, but your change should be negative though.

yea...-0.007 was my first answer which was wrong also.

edit...i think it is wrong because it is supposed to be in J.

That is in joules. Is this a web assignment? Perhaps you don't have enough significant figures.

yea i realized it is in joules after looking again. At first I put in -.007 yesterday and now I put in -0.007 and it says:
"Not quite. Check through your calculations; you may have made a rounding error or used the wrong number of significant figures." which 4 sig figures is right. so im not sure and usually if I am around the answer it will round it for me.

Yeah, you didn't use enough significant figures. Your answer has one sig-fig when your given values have three.