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Physics help? Topic: kinetic energy?

  1. Sep 29, 2014 #1
    A block of mass 10 kg is pulled 5 m upwards along a rough surface at an angle of 20 deg, an external force of 100 N and vi = 1.5 m/s. The coefficient of friction is 0.4. Calculate:

    1) The work done by gravity
    2) The increase in internal energy between the block and the surface as a result of friction
    3) Work done by external force
    4) The change in kinetic energy of the block
    5) The speed of the block after moving 5 m.

    Thanks a lot!
     
  2. jcsd
  3. Sep 29, 2014 #2
    Please show your work first, and then we will be able to guide you.
     
  4. Oct 2, 2014 #3
    For 1), I calculated the work done by gravity by musing the formula Fd, so Wg = mgsintheta x d, so Wg = 167.6 J.

    I don't know how to do 2)

    For 3), I multiplied the external force by cos 20 and then by 5 m = 469.8 J

    For 4), the change in kinetic energy equals the work done by external force minus the work done by friction.
    The frictional force is the coefficient of friction multiplied by the normal force (N = mg - F sin theta).
    So the frictional force is 25.5 N
    Change in kinetic energy = 469.8 J - (25.5 x 5) J = 342.4 J

    For 5), change in kinetic energy = 1/2 x m(vf^2 - vi^2)
    And I got vf = 8.36 m/s
     
  5. Oct 2, 2014 #4

    BvU

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    Hello sugar and welcome to PF. Good thing you re-posted with at least something. Next time use the template. Read the guidelines to find out why that's reall really useful.

    2) might have something to do with the work the friction force 'does'.
    3) Does the external force pull along the slope or does it pull horizontally ? Or vertically ?
    4) Not correct. you forget another energy change. And, again, which way is F pulling in your N calculation ?

    And: make a drawing !
     
    Last edited: Oct 2, 2014
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