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Homework Help: How to Find Force of Friction and Acceleration?

  1. Feb 10, 2016 #1
    1. The problem statement, all variables and given/known data
    Hey, guys I'm having trouble solving this problem, I'm not sure what to do first
    This is the info it gives:
    A stationary 15 kg object is located on a table near the surface of the earth. The coefficient of static friction between the surfaces is 0.40 and the coefficient of kinetic friction is 0.25.
    This is the problem:
    A horizontal force of 20 N is applied to the object.
    A)Determine the force of friction.
    B)Determine the acceleration of the object.

    2. Relevant equations

    3. The attempt at a solution
    For this part A of this problem I wasn't sure if this was right but what I did was I first found the weight which is 15x9.81=147.15 I then got confused since the problem gives coefficient of static and kinetic since I thought the object was moving I used the formula F=uN and used the coefficient of kinetic friction so .25 times the weigh 147.15 and then I got 36.7875 I'm not sure if this is the right answer for part b to find the force of friction.

    For part B of this problem I completely don't know what to do I thought that 20N was a force I can plug in for F=MA so I thought I can divide 20 by 15 which is the weight to find acceleration
  2. jcsd
  3. Feb 10, 2016 #2


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    When a force is applied to an object that is in resting contact with a surface, the first question is whether it will move relative to the surface. For that you must use the static coefficient. The static coefficient tells you the maximum frictional force available. The actual frictional force will never exceed the net of the other applied forces.
    If the static frictional force is insufficient to withstand the net applied force, then the object will move and the frictional force should be calculated using the kinetic coefficient.
  4. Feb 11, 2016 #3
    I get part A of my problem but now how do I find the acceleration of the object?
  5. Feb 11, 2016 #4


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    I don't think you do. If you did, you would be able to answer part B easily.
    As I wrote in post #2, your first step must be to determine whether the object moves at all. Please post your working for that.
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