Static/Kinetic Friction Problem

  1. 1. The problem statement, all variables and given/known data
    "A block that weighs 50N is initially at rest on a rough horizontal surface. A 20N horizontal force is then applied to the block. If the coefficients of friction (static = 0.6, kinetic = 0.3), what is the magnitude of the frictional force on the block?"

    2. Relevant equations
    Frictional force = coefficient of kinetic friction x normal force
    Max static frictional force = coefficient of static friction x normal force

    3. The attempt at a solution
    Once again I'm worried this problem is misleadingly easy:

    Max static frictional force = coefficient of static friction x normal force
    Max static frictional force = 0.6 x (50N - 20N)
    Max static frictional force = 0.6 x 30N
    Max static frictional force = 18N

    Is this method correct?
     
  2. jcsd
  3. LowlyPion

    LowlyPion 5,332
    Homework Helper

    No.

    You need to draw a force diagram on the block.

    What are the normal forces?

    What is the horizontal forces.

    If the block weighs 50N what is the maximum static frictional force it can resist?
     
  4. I think I just realized my mistake..

    The normal force of the block is the given 50N, and the horizontal force applied is 20N, so
    Max static frictional force = 0.6 x 50N
    Max static frictional force = 30N

    The max static frictional force exceeds the force applied horizontally (30N > 20N)
     
  5. LowlyPion

    LowlyPion 5,332
    Homework Helper

    I think you are on to something.

    Good Luck.
     
  6. So if the max static frictional force is 30N, and you are only applying 20N of force to it, won't that make the magnitude of frictional force 20N (since the block won't move anyway)?
     
  7. LowlyPion

    LowlyPion 5,332
    Homework Helper

    Yes indeed.
     
  8. Thanks I appreciate it!
     
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?