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Problem on Work

  1. Jul 21, 2011 #1
    1. The problem statement, all variables and given/known data
    A girl uses a 3.0 m long ramp to push her 110 kg motorbike up to a trailer, the floor of which
    is 1.2 m above the ground. How much work is done on the motorbike?

    2. Relevant equations
    W=FD where W is work, F is force and D is distance travelled.

    3. The attempt at a solution
    Assuming that 3.0m long ramp means a base length of 3.0 meters, then the hypotenuse of the ramp must be approximately 3.2meters. Thus, the bike must travel 3.2 meters in a diagonal direction. What I cannot find is the force which will push this bike up the ramp.
  2. jcsd
  3. Jul 21, 2011 #2
    Think of the free body diagram for this. Apart the force the girl exerts on the bike (and I'm assuming there's no friction, since it isn't included in the problem) what are the forces acting on the bicycle?
  4. Jul 21, 2011 #3
    Gravity. Which on an inclined plane like this, can be divided into two components; perpendicular and parallel. Calculating the degree of the ramp I find 22 degrees by arctan1.2/3.0. Using cos22 x 1078 I find the perpendicular component of gravity,1000N, which is the same as the normal force exerted by the ramp on the bike. The parallel force is 404N when I use sin22 x 1078, and it is the amount of downward (relative to ramp) force acting on the bike if nothing supports it.
    The problem is, given these components, I still cannot find work because the applied force of the girl isn't given.

    Thanks for your reply.
  5. Jul 21, 2011 #4
    Exactly. The weight of the bike, the normal force, and the force the girl exerts. These forces are equal, if the girl pushes the bike up with constant velocity.
    Take a look at the attached free body diagram, write a couple of equations (with the x and y components), and the force the girl exerts should be pretty clear.
    Let me know if you have any questions :)

    Attached Files:

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