Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Looking for deltaK, change in total kinetic energy

  1. Mar 31, 2008 #1
    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
  2. jcsd
  3. Mar 31, 2008 #2


    User Avatar
    Homework Helper

    What you did should be right, but your change should be negative though.
  4. Mar 31, 2008 #3
    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.
  5. Mar 31, 2008 #4
    That is in joules. Is this a web assignment? Perhaps you don't have enough significant figures.
  6. Mar 31, 2008 #5
    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.
  7. Mar 31, 2008 #6
    Yeah, you didn't use enough significant figures. Your answer has one sig-fig when your given values have three.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook