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Calculating velocity given mass, force, kinetic friction & angle (crate diagram)

  1. Oct 18, 2014 #1
    1. The problem statement, all variables and given/known data
    The 40 kg crate shown in the Figure below rests on a horizontal plane for which the coefficient of kinetic friction is mk = 0.3. If the crate is subjected to a 350 N towing force as shown, determine the speed of the crate 3 seconds after starting from rest. + given angle of 30 degrees

    http://i.share.pho.to/49e78c92_o.png


    2. Relevant equations
    F=ma
    where F is force (N)
    m is mass (kg)
    a is acceleration (m/s^2)

    v=a/t
    v is the final velocity
    a is the (average) acceleration
    t is the time the body accelerates (sec)

    combing them gives:

    v(final) = Ft/m

    3. The attempt at a solution

    v(final) = 350N(3sec)/(40kg)
    = 26.25m/s?

    but it is wrong, the answer is v = 17.8m/s


    Notes:
    1.
    I don't know how they got an answer of 17.8m/s...I didn't use the kinetic friction value (0.3) and angle given (30 degrees)...

    2. Also when questions say 'speed', do they mean that you should find 'velocity'? In this case are they similar? Even though velocity is just speed with a direction?

    3. What goes through your mind before solving these types of problems? Are there steps? I.e. step 1. identify what is given , step 2. identify what is missing

    I'm not really strong with the area of physics, especially statics and dynamics, so a simple explanation with all the steps would be great

    Thank you in advance for your help!
     
  2. jcsd
  3. Oct 18, 2014 #2

    BiGyElLoWhAt

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    Gold Member

    If you divide acceleration by a time interval, what does that give you?
    Hint: what are the units of position, velocity, and acceleration? What are the units of a/t?
     
  4. Oct 18, 2014 #3

    BiGyElLoWhAt

    User Avatar
    Gold Member

    Also, F does not equal ma.

    ##\Sigma \vec{F} = m\vec{a}##
    You didn't do the summation part of it. Draw a free body diagram if you need to see the forces acting on the box.
     
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