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Direction of Frictional Force

  1. Mar 6, 2016 #1
    1. The problem statement, all variables and given/known data
    This is not a specific problem, but is it true that if you added all of the forces acting on an object except for friction, that the direction of friction would be opposite the direction of that net force(technically, it's the net force except friction)? For example, If you knew that an object had the force of gravity, normal force, applied force and the force of friction acting on it, could you add the force of gravity, normal force, and applied force vectors and say that the direction of that sum is opposite the direction of friction?

    I know that friction is opposite the direction of motion, but I was just confused because I just did a question where you had to find the direction of the net force excluding friction in order to find the direction of the frictional force.

    2. Relevant equations
    Fnet=sum of all forces.

    3. The attempt at a solution
    I think that this statement is true, but I just want someone to confirm.
     
  2. jcsd
  3. Mar 6, 2016 #2
    You need to specify a bit more the conditions that the object is subject to. For example, if the object is stationary on a flat horizontal surface with no external force that has a component parallel to the floor, then no friction force exists, but the object still feels the gravitational force. If the object is under the influence of some external force that has a component parallel to the ground, and the floor has some coefficient of friction (static for a stationary object kinetic for a moving) then yes, you can find the frictional force which is simply the normal force times the appropriate coefficient of friction.
     
  4. Mar 6, 2016 #3
    What is an object was parallel to the floor with a force being applied to it, and it was in motion?
     
  5. Mar 6, 2016 #4

    Simon Bridge

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    So you want to know if it is true that the net friction force always points opposite to the sum of all the other forces?
    To test the statement, you need to look for counter-examples.

    Consider: an object, initially stationary, sits on a rug on a smooth horizontal surface, and someone quickly pulls the rug out from under it.
    There is friction between the rug and the object: what are the non-friction forces on the object and which way do they point?
    Which direction does the friction point?
     
  6. Mar 6, 2016 #5
    Non frictional forces: gravity, normal force

    Friction: Points in the direction that the rug is pulled.

    I assume the object doesn't move at all if the rug is pulled quick enough.
     
  7. Mar 6, 2016 #6

    Simon Bridge

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    Well, it moves a bit ... in the direction the rug moves. It doesn't matter for the question: lets say the rug moves slowly enough that the object just rides on the rug.

    So did you work out the sum of the non-friction forces on the object?
     
  8. Mar 6, 2016 #7
    Wouldn't Fnet just be 0N, if there was a normal force. If no normal force, Fnet=Fg
     
  9. Mar 6, 2016 #8

    Simon Bridge

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    That is correct - so how does the direction of the net non-friction force compare to the direction of the friction force?
    (Remember your original question?)
     
  10. Mar 6, 2016 #9
    They're not the opposite directions,
     
  11. Mar 7, 2016 #10

    Simon Bridge

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    ... so what is the answer to your question, posed in post #1?
    It is good science, having posed a question and investigated it to a result, to also record a conclusion based on the evidence.

    (Can you find an example of the net non-friction forces pointing in the same direction as the friction? How about pointing perpendicular?)
     
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