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Static friction

  1. Feb 25, 2010 #1
    How can we find the horizontal force required to move an object with the coefficient of static friction and the mass??

    I just need a formula please, thankyou
  2. jcsd
  3. Feb 25, 2010 #2
    Fr = μN

    Fr - resistive force of friction
    μ - coefficient of friction
    N - normal force (gravity/applied pressure)
  4. Feb 25, 2010 #3
    a) The friction force only acts in the opposite direction to a horizontal force on an object. The only forces acting on the block described are gravity and the normal force from the table, both of which are in the vertical direction. Therefore, there is no friction force acting on the block.

    b) If a horizontal force F acted on the block, the force due to static friction would then begin to act upon the block. This would happen until the force F became greater then the opposing friction force, which would result in the blocks motion. Problem b would be solved by finding the maximum force due to static friction. This is equal to the coefficient of static friction multiplyed by the normal force: μs*Fn.

    c) The static friction force equals the force F acting upon the block until F rises greater then the value of the maximum static friction force. So, if half the force needed to overcome the static friction force were applied, 1\2 SF (SF equaling static friction force max), then the static friction force would be an equal and opposite force 1\2 SF as well. This would be written as 1/2*m*g*μs, were m is the mass of the block, g is the acceleration due to gravity: 9.81m\s2, and μs is the coefficient of static friction. (the normal force is equal to the mass of the block multiplied by the acceleration due to gravity)

    d) The sum of the horizontal (vertical and horizontal forces are calculated separately) forces equals the mass times the horizontal acceleration of an object as stated in Newtons second law. The two forces acting upon the block are a force F and the kinetic friction force (the kinetic friction force is the friction force acting upon moving bodies, where as the static friction force is upon stationary bodies.) The equation is written F-Kf=m*a, where Kf is the force due to kinetic friction.(Note: the reason the kinetic friction force is subtracted from the horizontal force acting on the block instead of vise versa, is because the movment of the block is in the direction of the force F.) Solving for the acceleration of the block, the mass m is moved to the other side of the equation like this (F-Kf)\m=a or (F-m*a*μs)\m=a. (the kinetic friction force is equal to the coefficient of kinetic friction multiplied by the normal force)

    Hope this is what your looking for.
  5. Feb 26, 2010 #4
    Ok so ... lets say I have a 6kg object, I know the coefficient of static force is 0.42, and I want to know what force is required to move the object ...

    so I start by finding the normal force of the object by multiplying the mass and force of gravity
    which would be: Fn= 6kg X 9.81m/s2= 58.86
    Fn= 58.86N

    and now I can find the needed force? μf= F/Fn
    0.42= F/58.86
    F= 0.42×58.86
    so a 24.72N force is needed to move the object?
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