Block on inclined plane - pushing vs pulling

  1. When resolving all of the VERTICAL forces in a block that is being pushed up an incline with some incline amount theta, then when PUSHING with force P at an angle of alpha on the block, then the vertical component of force P is sin (alpha) * P in the downward direction (opposite of the Normal force); however, when pulling the block UP the incline with same alpha and same force P, then the vertical force is sin (alpha) * P upwards in the SAME direction as the normal force N. In both cases friction is in the direction of DOWN the incline. In both cases the Parallel and Perpendicular forces of gravity are the same. So, it would seem that the force P needed to push the block will be different than the force necessary to pull the block. Then seems counter-intuitive. Am I thinking correctly here?
     
  2. jcsd
  3. Nathanael

    Nathanael 1,132
    Homework Helper

    It only makes sense to speak of an angle relative to something.

    If the pushing and pulling are truly in the same direction, then there will be no difference at all.
     
  4. A.T.

    A.T. 5,290
    Gold Member

    I don't understand what the point of the incline is. You have the same thing on level ground: Pulling forward & up reduces the normal force, and thus friction. Pushing forward & down increases friction.
     
  5. Wes Tausend

    Wes Tausend 124
    Gold Member

    You could imagine that the object is pulled by a rope attached to the center of the forward face. Since the rope will be taut under tension, it acts just like a stiff wire would under tension. The most likely direction to pull something up an inclined plane is to pull parallel with the incline. In other words, the rope/wire aims up, the handhold having the same distance from the inclined slope as it does at the attachment point. So one is lifting as one also pulls forward in this case.

    Now imagine pushing the object up the slope with a stiff wire since the rope would collapse under compression. The wire is again attached to the center of a face, but the rear face this time. The most likely direction of the wire is again parallel with the incline. In this case one is pushing up (not down) as one pushes forward.

    The rope (or stiff wire) can be quite short, but the push/pull efforts are still both parallel with the slope, even until they are infinately short. The two cases of friction are now identical.

    Wes
    ...
     
  6. A.T.

    A.T. 5,290
    Gold Member

    OK, I misunderstood that it's about pulling/pushing at an angle to the surface. When parallel then both are the same.
     
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