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Friction Force question

  1. Oct 29, 2003 #1
    The problem starts with two masses, m=16kg and M=88kg. m is pushed against the middle of M with a static friction coefficient of .38, and M is on a frictionless floor. The first part of question asked to find the force necessary to prevent m from falling. After drawing the free body diagram it is determined that N=mg/static friction coefficient. Thus, to prevent the block m from falling there needs to be a normal force of 413 newtons on block m. The next question asks how much force is needed on block M, on the other side that block m is on, to prevent m from falling down block M. So, instead of applying the force to m to keep it up, the force will be applied to the larger block M to keep the smaller block on the other side from falling.

    I cannot figure out where to start my calculations?
  2. jcsd
  3. Oct 29, 2003 #2


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    If you are going to ask people to help you, at least let us know what you symbols mean! What is N? I would gues the force you are asked for in the first part of the question but it would have nice of you to tell us that!
    Yes, that looks pretty good.

    No, that's not the way I would interpret the question. Since M is sitting on a frictionless surface, pressing m against it with 413 N of force will cause it to accelerate away from m- m will fall. You need the force on the other side of M as well as the force pressing m agains M. In fact, they have to be the same in order to keep M and m together! I would answer 413 N for this one also.

    A more interesting problem would be if they gave you a coefficient of static friction for the surface M is on as well!
  4. Oct 29, 2003 #3
    The force applied to mass (m) is taken away and a force is applied to the larger mass (M). What I'm trying to determine is the force needed to be applied to mass (M) with no force on mass (m), to keep the mass (m) held against mass (M). For example: if you hold a book against a refrigerator with some force to keep the book up against the refrigerator, then remove that force and apply a force directly to the refrigerator on the other side of the book, how much force is needed to push the refrigerator and keep the book held against the refrigerator?
  5. Oct 29, 2003 #4


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    In the "refrigerator" example the only reason the book doesn't fall is that there is enough friction between the refrigerator and the floor to keep the refrigerator from moving. In your problem, we were specifically given that " M is on a frictionless floor". Just pressing m against M will make M slide away- it won't support m.

    And, by the way- when I pressed a book against my refrigerator, sure enough, the refrigerator didn't move and the book stayed there.
    But when I pressed on the other side of the refrigerator and let go of the book, guess what? The refrigerator still didn't move and the book fell!

    Either you are misinterpreting the problem or the problem is just wrong. Pressing mass m against mass M on a frictionless floor will NOT support m!
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