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Two blocks/friction question

  1. Nov 5, 2008 #1
    1. The problem statement, all variables and given/known data
    A block of mass 20kg sits on top of a block mass 50kg. The coeficent of static friction between the top block and the bottom block is .6, while the kinetic is .4. The bottom block is on a frictionless surface. The top block is pulled with a force of 200N. What is the acceleration of the bottom and top block?

    3. The attempt at a solution
    I started out making it as a system, so that the system has a mass of 70kg and only 20kg accounts for the friction. So the friction would be 120N while not moving, and 80N after it has began to move. So with that being said, it appears that the top block would have a friction of 6m/s^2 because it would be (200N-80N)/20kg (i am using 10 for g at the moment). But what is the prupose of the static friction? Does the bottom block get pulled by the top block at the rate of static friction? I am not sure why static is in this problem, but my only guess would be for that.
  2. jcsd
  3. Nov 5, 2008 #2


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    if the top block didn't move with respect to the bottom block, then you would have had to use a friction force less than or equal to the max available static friction force. Since you have shown it does move, forget about it.
    The top block does get pulled by the friction between the 2 blocks...is it static or kinetic?
  4. Nov 5, 2008 #3
    I am sorry i meant acceleration, just wasn't thinking i suppose. It seems to me that the bottom block would still get pulled by kinetic friciton because they are both moving, so the top block is moving away from the bottom block, but then why in the world would there be a static friction in the question?
  5. Nov 5, 2008 #4
    i have a problem like this with an object on top of another...I am trying to go about how to figure out the maximum static friction on the top object before it begins to slide i believe.
  6. Nov 5, 2008 #5
    The whizz,
    I am not sure i understand why you are trying to figure this out. I dont know why it would be in the question if it was not intended to be used, but the answer in the back of the book matches the (Fnormal*kinetic mu)/mass. This makes sense to me, but just what doesn't make sense is why information would be supplied in the question that had no relevance.
  7. Nov 5, 2008 #6
    well its a bit different of a question but very similar. I am trying to find the maximum force that can be exerted on the sled before the penguin begins to slide.

    And i believe that this is when the static friction is at its max on the top object before becoming kinetic friction.

  8. Nov 5, 2008 #7


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    It is relevant because the question wants you to understand when to use kinetic friction and when to use static friction. What if the pulling force was 80N instead of 200N. Or what if the pulling force was only 60N? What would be the accelerations of each block under these scenarios?
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