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Kinetic Energy?

  1. Oct 16, 2011 #1
    Hi, this is my first time using this forum, so i have no idea if this is even the correct place to be asking for help. I have a problem with my High School Higher Physics Homework. The question is as follow :
    1. The problem statement, all variables and given/known data During one run, a car and passengers of mass 800kg are released from rest at point A, a height of 20m above the ground. The car travels a distance of 120m along the track until it reaches point C, a height of 15m above the ground. A constant frictional force of 250N acts between the car and the track as the car moves from A to C. (A diagram of the track is included, see attachments). Find the Kinetic Energy of the Car on reaching point C.

    2. Relevant equations In the previous question i have been asked to calculate the work done against friction in moving from A to C, and i calculated this to be 30,000 N (W=fd W=120 * 250) (Not sure if this is correct though?) I think Ep=mgh is maybe relevent and obviously to work out the kinetic energy Ek=1/2mv^2 will be used.

    3. The attempt at a solution I genuinlly have no idea where to start on this question ? I am thinking that it will maybe have something to do with Potential energy loss, and the energy lost due to friction but i am completly confuzed. Can anyone please help me ?

    Attached Files:

    Last edited: Oct 16, 2011
  2. jcsd
  3. Oct 16, 2011 #2

    Doc Al

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    Staff: Mentor

    Think in terms of mechanical energy. You start out with some total mechanical energy at A, but you end up with less at C due to work done against friction. Set up an equation expressing this.
  4. Oct 16, 2011 #3
    So would 30,000N previously worked out be the Energy 'lost to the surroundings', and also, if I am working out the kinetic energy, i assume i am not supposed to be working out the speed and using the kinetic energy equation, so how do i work it out? Sorry I'm really confuzed.
  5. Oct 16, 2011 #4

    Doc Al

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    Staff: Mentor

    Total mechanical energy = PE + KE

    So what's the total mechanical energy at point A?
    Then you can figure out what it is at point C. And use that to solve for the KE at point C.
  6. Oct 16, 2011 #5
    Ohhhhhhhh, thanks very much, I forgot that Total mechanical energy = PE + KE, Thanks for the help :)
  7. Oct 16, 2011 #6
    Just realised i should have written down 30,000J instead of 30,000N as it is energy but that's just a units error, but once again, thanks.
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