Calculating Change in Kinetic Energy for a Collision on a Frictionless Air Table

[Elissa]

Homework Statement

On a frictionless, horizontal air table, puck A (with mass 0.250 kg ) is moving toward puck B (with mass 0.360 kg ), that is initially at rest. After the collision, puck A has a velocity of 0.125 m/s to the left, and puck B has velocity 0.655 m/s to the right.

I already solved for the puck's initial velocity, which is 0.818m/s. Now I have to find the change in the total kinetic energy of the system that occurs during the collision.

Homework Equations

KE=(1/2)mv^2

The Attempt at a Solution

KE(initial)=(1/2)(0.250kg)(0.818m/s)^2=0.0805J
KE(final)=(1/2)(0.250kg)(0.125m/s)^2+(1/2)(0.360kg)(0.655m/s)^2=0.07917
ΔK=0.0805J-0.07917J=0.00487J.
I tried this both negative and positive, but both are wrong. I don't know what I did wrong.

 

[Orodruin]

I already solved for the puck's initial velocity, which is 0.818m/s.

Please show your work.

 

[Elissa]

Please show your work.

mv=m1v1+m2v2
v=(-(0.125m/s)(0.250kg)+(0.655m/s)(0.360kg))/0.250kg=0.818m/s. This is definitely right because I already submitted the answer and got it correct.

Should the velocity of Puck A be negative when solving for kinetic energy?

 

[Orodruin]

Is whatever program you are feeding the answers into sensitive to the number of significant digits?

 

[Orodruin]

KE(initial)=(1/2)(0.250kg)(0.818m/s)^2=0.0805J

This is not numerically correct.