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Homework Help: Applying Newtons Laws to solve this

  1. Oct 2, 2006 #1
    Luggage is transported from one location to another in an airport by a conveyor belt. At a certain location, the belt moves down an incline that makes an angle of 2.5 degrees with the horizontal. Assume that with such a slight angle there is no slipping of the luggage. Determine the magnitude and direction of the frictional force by the belt on a box weighing 69 N when the box is on the inclined portion of the belt for the following situations:
    (a) The belt is stationary.
    (b) The belt has a speed of 0.65 m/s that is constant.
    (c) The belt has a speed of 0.65 m/s that is increasing at a rate of 0.20 m/s^2.
    (d) The belt has a speed of 0.65 m/s that is decreasing at a rate of 0.20 m/s^2.
    (e) The belt has a speed of 0.65 m/s that is increasing at a rate of 0.57 m/s^2.

    So, I was able to figure out the answer to a, and b is 3.00 N, just by doing 69 sin 2.5. So... what would I do for the other 3? c and d are similar, so if I can find one, I better be able to find the other. Still need to find d as well. Ideas? Thanks!
  2. jcsd
  3. Oct 2, 2006 #2


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    Since the box is not moving with respect to the belt, this is a case of static friction.

    The direction of the friction force opposes the inertial force of the mass of the box. In 'c', the box is acceleration, and in 'd' it is decelerating, in addition to going down an incline.

    The solution to 'e' is similar do that of 'c'.
  4. Oct 7, 2006 #3
    I know, but I need to solve this problem. What do I do?
  5. Oct 7, 2006 #4
    The frictional force due to gravity is always the same and acting up the plane of the slope. When the belt is moving at constant velocity it is not causing a force to act on the box so you are correct for a) and b). Now if the belt is being accelerated or deccelerated another frictional force is causing the box to stay in the same position relative to the belt. Think about what direction the friction must be acting to cause the box to stay in the same place in relation to the belt. If you then look at all of the forces and work out the mass you should see that you have the correct acceleration.
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