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Homework Help: Multiple questions, please help, test friday

  1. Dec 7, 2011 #1
    Hi. I have a test on friday, and I was given a sheet to review. However, many of these questions I have no clue how to answer, as my teacher never went over them. I'd really appreciate a step by step walkthrough on how to do the problems, as I don't want the answers, I want to know how to understand it.

    1) A box slides down a 35° ramp with an acceleration of 1.14 m/s2. Determine the coefficient of kinetic friction between the box and the ramp.

    2) A 4.10 kg block is pushed along the ceiling with an constant applied force of F = 78.5 N that acts at an angle θ = 59° with the horizontal, as in the figure below. The block accelerates to the right at 5.90 m/s2. Determine the coefficient of kinetic friction between the block and the ceiling.

    3) A clerk moves a box of cans down an aisle by pulling on a strap attached to the box. The clerk pulls with a force of 176.0 N at an angle of 23° with the horizontal. The box has a mass of 36 kg, and the coefficient of kinetic friction between the box and the floor is 0.45. Find the acceleration of the box.

    4) A 880 N crate is being pulled across a level floor by a force F of 385 N at an angle of 27° above the horizontal. The coefficient of kinetic friction between the crate and the floor is 0.25. Find the magnitude of the acceleration of the crate.

    5) A block with a mass 3.9 kg is held in equilibrium on an incline of angle θ = 30° by a horizontal force, F. Find the magnitude of normal force, and the magnitude of F

    6) A 5.6 kg bucket of water is raised from a well by a rope. If the upward acceleration of the bucket is 3.7 m/s2, find the force exerted by the rope on the bucket of water.

    7) A 2.88 kg block starts from rest at the top of a 30° incline and accelerates uniformly down the incline, moving 1.86 m in 1.50 s. Find the magnitude of the acceleration of the block. Find the coefficient of kinetic friction between the block and the incline. Find the magnitude of the frictional force acting on the block. Find the speed of the block after it has slid a distance 1.86 m.

    8)The coefficient of static friction between the 3.49 kg crate and the θ = 33° incline shown below is 0.300. What is the magnitude of the minimum force, F, that must be applied to the crate perpendicularly to the incline to prevent the crate from sliding down the incline?

    Thank you so much for helping!
  2. jcsd
  3. Dec 7, 2011 #2


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    Welcome to PF,

    I doubt that, but okay.

    Where's the template for homework help posts that you were supposed to use? What have you done so far on these problems? You must show us your attempts in order to receive homework help. It's the rules. We are happy to help you learn on this site, but we won't do your homework for you. It would NOT help you in the slightest if we did.

    The template that you deleted exists for a reason: it forces you to provide information that forms the basis for a strategy for solving the problem (list the givens and unknowns, and any relevant equations or physical principles).

    Remember that thing called Newton's Second Law? If you know the acceleration, then you know the net force. From this (and a free body diagram) you can start working out all the individual forces that act on the object, including the frictional force. (The net force is just the total force -- it's the sum of all the individual forces acting).

    My advice is the same as for the first problem. Draw a free body diagram in order to take an inventory of all the forces that are acting on the object.

    This is just a variation of the above two. You know all the individual forces that act on the body, and hence you can find the NET force, and from that, the acceleration. Draw a free body diagram (are you sensing a theme here?).

    This is basically the same as the previous problem.

    If the object is in equlibrium (i.e. stable and not moving or about to start moving), then what do Newton's laws say about the net force on this object?

    All the rest of the problems are just variations of the above. The whole worksheet is just testing whether you understand Newton's laws and how to apply them to physical situations. Let's just tackle them one at a time.
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