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Homework Help: Kinetic and static friction

  1. Oct 26, 2008 #1
    A crate of mass 150.0 kg lies at rest on a horizontal storage room floor. The
    coefficient of static friction between the crate and the floor is 0.70, and the
    coefficient of kinetic friction is 0.35. (You may assume g = 10.0 m/s2).

    a) Suppose someone pulls on a horizontal rope attached to the crate. They slowly
    increase the tension in the rope. Draw a free body diagram of the system.

    b) Suppose someone pulls on a horizontal rope attached to the crate with a force of
    200 N. Does the crate remain at rest or start moving? If it moves does it have
    constant speed or continue to accelerate? If it accelerates, what is its acceleration?
    Under these conditions what is the frictional force on the crate?

    c) Answer part (b) again if the pull on the rope is 1000 N.

    d) What force on the rope is needed to start the crate sliding?
  2. jcsd
  3. Oct 26, 2008 #2
    What part are you having trouble with?
  4. Oct 26, 2008 #3
    well i got part A now im stuck on part B
  5. Oct 26, 2008 #4
    Well, your going to have to see if the force of friction is less than or greater than the force that is being applied.
  6. Oct 26, 2008 #5
    ok i got that normal force is 1500 and then i multiplied that times the coefficient which is .7 which gave me 1050 but then im not sure is im suppose to subtract that from the 200N which is the force from the rope also i dont know how to do what you just ask
  7. Oct 26, 2008 #6
    Well basically you just look at what you have. You are pulling the crate at 200N while the force of friction is 1050. So is 200N greater than 1050N. No, so that box ain't going nowhere.
  8. Oct 26, 2008 #7
    oh so in order to move the box you will need a force greater than 1050 right???
  9. Oct 26, 2008 #8
    Yes sir. You need to make sure you understand the difference between kinetic friction and static friction. You need more force to get it moving then to keep it moving. Since the block is at rest you are correct in using the coefficient of static friction in your equation.

    You showed that you didn't have enough force to overcome the force of friction, however, had the block already been moving then you would have used the coefficient of kinetic friction and therefore would have (in the part c) had enough force to move the block.
  10. Oct 26, 2008 #9
    All makes sense now, Thank you so much!!!!
  11. Oct 26, 2008 #10
    Your welcome
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