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Work done on a block by net force

  1. Jan 7, 2010 #1
    1. You drag a(n) 16.9 kg steamer trunk over a
    rough surface by a constant force of 177 N
    acting at an angle of 34◦ above the horizontal.
    You move the trunk over a distance of 59.3 m
    in a straight line, and the coefficient of kinetic
    friction is 0.186.
    The acceleration of gravity is 9.8 m/s2 .How much is the work done on the block by
    the net force?
    Answer in units of J.

    u= coefficiant of finetic friction
    @= theta
    Fnet= [sqrt (mgsin@^(2)+177^(2))] -uFncos@

    W= Fnet (d)
    W= ? * (59.3)

    I have trouble figuring out how to calculate Fnet.
     
  2. jcsd
  3. Jan 7, 2010 #2
    I would start by drawing a diagram of the situation. Draw the 4 forces in, i.e. weight W, reaction force R from the floor, friction F' and the force we are pulling with F.

    Since the trunk isn't moving the vertical direction, we may equate the 'downwards' forces with the 'upwards' forces, i.e.

    W=R+Fsin(@)

    Also we know F'=uR, where u is the coeff of kinetic friction, so we may rearrange the above to find the frictional force in terms of the F, W and u.

    It is this frictional force that we do work against, so the work is F' times s, the distance through which we move it trunk.
     
  4. Jan 7, 2010 #3
    is reaction force the same as normal force?
     
  5. Jan 7, 2010 #4
    Yes it is in this case. It is pointing vertically upwards.
     
  6. Jan 7, 2010 #5

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi kt7477! Welcome to PF! :smile:

    Technically, the reaction force is the total force of the block on the ground, or vice versa.

    So it's the normal force plus the friction force.

    It's best just to call the normal force "the normal force". :wink:

    (sorry if that seems unnecessarily complicated :smile:)
     
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