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Why does friction point the opposite as gravity?

  1. Dec 8, 2014 #1
    1. The problem statement, all variables and given/known data
    a 280-kg piano slides 4.3m down a 30 degree incline and is kept from accelerating by a man who is pushing back on it parallel to the incline.

    I know how to find the solution, but there's something that I don't understand about why it's that way. In order to find the force the man needs to exert to keep it from accelerating you subtract the friction force by the component of gravity. But I don't get why you subtract it rather than adding it. If it is not accelerating how do you determine which direction the friction force is acting? At first I thought it would be along the same direction of the horizontal component of gravity because it slides down that way, but instead friction is helping the man in this case. Why does it point against gravity and how are you able to tell? Thanks!
     
  2. jcsd
  3. Dec 8, 2014 #2

    Bandersnatch

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    Friction always acts against motion. Acceleration is irrelevant here.
    For example, if you throw a ball up in the air, its acceleration will always be the same - g pointing down, but the direction of velocity will change from up to down, and similarly the air resistance (friction) will change direction as well.
     
  4. Dec 9, 2014 #3

    haruspex

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    To be precise, it acts against relative motion of surfaces in contact. Thus, friction does not act to oppose the motion of a rolling ball on a level surface, since even if friction were to suddenly vanish the surfaces in contact would not move relative to each other. Similarly, friction does not oppose the forward movement of a driven car. It opposes the tendency for the wheels to skid, so it acts forwards, propelling the car.
     
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