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Homework Help: Static friction

  1. Apr 26, 2017 #1
    1. The problem statement, all variables and given/known data
    A 50kg object begins at rest on a surface with a static coef of friction at 0.6. If you push horizontally on the object with a force of 27N, how quickly will it move across the surface?

    2. Relevant equations

    3. The attempt at a solution

    This is from Trivium Praxis test prep book. The solution given is 0.6*50=30N of static friction, so no, it will not move. In the very brief section explaining friction, it said that friction=coef * normal force, but they labelled it as capitol N. Shouldnt the normal force be 50g? So F=0.6 * 50g? I decided to try to pass the test this summer after taking physics 15+ years ago!
    Last edited by a moderator: Apr 26, 2017
  2. jcsd
  3. Apr 26, 2017 #2


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    Staff: Mentor

    There's likely a typo in the question. The object description was probably meant to give its weight as 50 Newtons, not mass in kilograms.
  4. Apr 26, 2017 #3
    Welcome to Physics Forums.
    Are you sure it is a 50 kg object, and not a 50 Newton object?
    But assuming it is a 50 kg object: If it is a horizontal surface, the normal force N will be equal to the weight of the object. The weight is mg = (50 kg)(9.8 m/s^2) = 490 N (where N is Newtons)
    So the normal force will be N = 490 N. And the maximum friction force will be f = μN = (0.6)(490) = 294 N. So if you push with 27 N, the friction force will also be 27 N. Thus the object will not move.
  5. Apr 26, 2017 #4
    Thanks. This is the 2nd error I think I've found and I've only done 12 questions in of the first section!
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