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How would you find force in this problem?

  1. Oct 31, 2012 #1
    A worker uses a ramp to slide a refrigerator onto the back of the truck. The ramp is 5.0m long, and raises the refrigerator 1.5m above the ground. The amount of work done by the worker is 1800J. What is the magnitude of the force the worker must exert to load the refrigerator?

    Ok so the first thing I did was calculate the angle by doing tangent^-1 (1.5/5.0) and got 17 degrees. Then I multiplied cos(17) and and 5.0 and divided it by 1800.
    When I looked on the answer sheet it said the force was 3.6X10^2. I got 376.4. What did I do wrong?! I figured you calculate the angle since he is pushing the fridge at that degree up the ramp. Isn't the formula W=Fd(cosθ)? The answer sheet said I shouldn't have the degree, why?
     
  2. jcsd
  3. Oct 31, 2012 #2
    tangent is opposite over adjacent, not opposite over hypotenuse (that's sine)

    the ramp is forming the hypotenuse of the right triangle
     
  4. Oct 31, 2012 #3
    ok, but shouldn't it have some sort of angle since the worker is pushing it up. I used tangent to find the angle.
     
  5. Nov 1, 2012 #4

    ehild

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    5cos(17) is the horizontal component of the displacement which is along the slope at angle 17° with respect to the horizontal. If you divide it by 1800 you can not get a 376.

    The worker pushes the block along the slope. He needs the smallest force if the force is parallel with the slope. The displacement is 5 m along the slope. If the force and displacement are parallel, theta is zero.

    ehild
     
  6. Nov 1, 2012 #5
    So when do I know to use an angle?
     
  7. Nov 1, 2012 #6

    ehild

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    When you calculate work you need to use the angle between displacement and force. In this case, the angle is zero.

    ehild
     
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