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Homework Help: The NET FORCE and a lawnmower

  1. Oct 21, 2007 #1
    [SOLVED] The NET FORCE and a lawnmower

    1. The problem statement, all variables and given/known data
    Well im working on a quesiton here and am finding trouble with one part. The answers are in brackets. The trouble im having is with part c, where i believe you have to find the accleration then the time to work out power which is equal to work over time.

    A person pulls on a 20 kg lawn mower with 25 N and moves it 4.5 m. The handle of the lawnmower in inclined at 15 degrees above the horizontal and the friction co-effiecient is 0.1 If vi=0 find
    a.)amount of work done by person (108.7)
    b.)amount of heat energy generated by friction (85.3)
    c.)rate at which the person does work (18.6 Watts)
    d.)the final speed of the lawnmower(1.53)

    2. Relevant equations

    p = w/t, F=ma ---> a =F/m

    3. The attempt at a solution
    Well part a is basicaly w= fcos15 * 4.5 m
    and b is finding out friction by using force diagrams and finding Fn
    but c i am not sure how to find the accel and consequently the time.

    wat is the net force in the system? please help. thank you.
  2. jcsd
  3. Oct 21, 2007 #2


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    The net force on the system in the direction of motion is the sum of EVERY force acting on the object in that direction or the negative of that direction. In this case, your forces to be summed will be any forces acting in the horizontal direction. Can you tell me which forces fit this description?
  4. Oct 21, 2007 #3
    well the forces wrking in that direction are friction, the person pulling and the gravity so i would think that the net force would be Fpull - mgsin15 - Ff

    but the answer i get just doesnt workout

  5. Oct 21, 2007 #4


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    OK, by my understanding, the problem does not say that the person is pulling the lawnmower up or down a hill. How can there be a component of gravity involved if the object is not moving along a slope?
    Last edited: Oct 21, 2007
  6. Oct 21, 2007 #5
    i think we have to assume its going up
  7. Oct 21, 2007 #6


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    I agree with G01. The lawn mower is not moving up an incline, the handle is at an angle of 15 degrees, and the force of the person pulling it is directed along that handle.
  8. Oct 21, 2007 #7


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    As hage pointed out. We do not have to assume this. Think of the man pulling at the angle of the handle, but the lawn mower still moving horizontally.
  9. Oct 23, 2007 #8
    Ya, you guys were right! Thank you so much!! Solved!!!
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