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Homework Help: Find the frictional force acting on the block

  1. Sep 30, 2004 #1
    After seeing some of the problems that you are asking help on, i feel really stupid, since mine seem so simple compared to yours. but here it is anyway

    I am really having trouble with Friction here, how exactly do u get the coefficient of kinetic friction? I have 2 problems here:

    1)A box slides down a 31.5° ramp with an acceleration of 1.28 m/s2. Determine the coefficient of kinetic friction between the box and the ramp.
    Answer: ??????

    Any hints? or something? i know some of the formulas, but i dont really know what to do. tried many things, and it gives me a wrong answer

    2)A 3.20 kg block starts from rest at the top of a 30.0° incline and slides 2.20 m down the incline in 1.50 s.

    (a) Find the acceleration of the block.
    Answer: 1.96 m/s^2
    (b) Find the coefficient of kinetic friction between the block and the incline.
    Answer: ????????????
    (c) Find the frictional force acting on the block.
    Answer: ??????????
    (d) Find the speed of the block after it has slid a distance of 2.20 m.
    Answer: 2.94 m/s

    I did find 2 of the answers in #2 but again, friction is killin me here

    Now I also have a normal dynamics/kinematics problem which i though i knew, but webassign keeps marking it wrong. here it is:

    3) A block with a mass of 20 kg is held in equilibrium on an incline of angle = 30.0° by the horizontal force, F, as shown in Figure 4-31. Find the magnitude of F.
    Answer: ?????
    Find the normal force exerted by the incline on the block. (Disregard friction.)
    Answer: ?????

    Figure 4-31 is just a ramp (a right triangle) with a cube above it, and the F arrow is horizontally poiting to the right above the cube.

    in this one I drew my free body diagram, all pretty and stuff, and calculated the Fn to be 169.9 and 170, i well, drew the vectors down Fg and Fy, connected them to form a triangle, applied

    COS(30)= Adjacent Angle/hypotenuse --> COS(30)=Y/((-9.81)(20))

    (acceleration)(mass) = F

    i got Fgy (Fy poiting down) =-169.9, so the Natural force should be 169.9 right? but the thing says im wrong.. :cry:

    This assigment is due today at 7am, if anybody can help me, please anwer this threat, PM me, or IM me to aim(cronowarrior007) or yahoo (coolkegcomputer) or MSN (coolcomputer7@hotmail.com).

    Thanks so much
  2. jcsd
  3. Sep 30, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    Dearly Missed

    Welcome to PF, Graif!
    Write Newton's 2.law out in the two components:
    Tangential to the ramp and normal to the ramp.
    Identify the terms you know, and remember the relation between frictional force and normal force.
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