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Dynamics and F=ma, where a is 0

  1. Mar 11, 2012 #1
    1. The problem statement, all variables and given/known data

    A 24kg box is placed on a rough slope. Determine the magnitude of the frictional force exerted by the slope on the box if the box is moving downward at a constant velocity of 9.0m/s. The slope incline is 40°

    2. Relevant equations

    F=ma

    3. The attempt at a solution

    F = 24 x 0
    a = 0 because speed is constant.
    Resultant force = Initial force - frictional force
     
    Last edited: Mar 11, 2012
  2. jcsd
  3. Mar 11, 2012 #2
    What is that attempt? You seem to have learned things in way too hurry. First draw the FBD of the block. See what are the forces acting on it. How can it's acceleration be put to zero etc.,
     
  4. Mar 11, 2012 #3

    vela

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    Start with a free-body diagram. I encourage you to explain what you're doing instead of trying to make us guess what you reasoning is.
     
  5. Mar 11, 2012 #4
    Well, as stated in the title, I used F=ma, so the working I came up with was F=24x0, with 24 being the mass of the box and 0 for the acceleration since the speed is constant.
     
  6. Mar 11, 2012 #5
    Haha sorry, I'm still very new to Physics, as I'm only 13. Our teacher just started teaching us, but he's far from thorough so we're struggling...

    For the FBD, I drew
    1) weight acting vertically downwards
    2) friction acting opposite the direction of motion
    3) a force acting perpendicular to the incline of the slope
    4) the direction of motion

    Thanks for offering your help! :)
     
  7. Mar 11, 2012 #6

    vela

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    What do you mean by "initial force"?

    I figured that's what you probably did, but it's better to hear you actually say it. Students can quite often come up with the correct answer for the wrong reasons.

    Choose a set of axes and resolve the forces into x and y components. Then set up two equations:
    \begin{align*}
    \sum F_x &= ma_x \\
    \sum F_y &= ma_y
    \end{align*} where, as you noted, you have ##a_x = a_y = 0## since the box moves at constant speed.
     
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