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Homework Help: How do i calculate the necessary force on a hill

  1. Dec 21, 2016 #1
    1. The problem statement, all variables and given/known data
    A box is on a 10 degree hill what is the force i have to pull to make the box move? and what force do i need to make the box move 20 meters

    mass of the box: 12.000
    Coefficient of friction: 0.08
    speed 1 m/s

    2. Relevant equations

    Ffriction= μ * Fnormal

    F= m*a

    3. The attempt at a solution
    Fnormal = Fgravtity* cos 80

    Fn = (12,000* 10) * sin 80
    Fn= 118177
    Fn =118kN

    Ffriction= μ * Fnormal

    Ffriction= 0.08* 118
    Ff= 9,45 kN

    The total force to make the box move = 9,45 +120 * cos 80 = 30 kN

    Is this correct?

    F = m * a
    I wanna be at 1 m/s in 4s

    F = 10.000 * 0,25
    F= 2500N.

    Do I have to pull 30kN to make the box move and then pull 2,5 kN or do i have to pull 32,5kN?

    And when I achieve the 1 m/s what is the necessary force to make it a constante speed?

    Last edited: Dec 21, 2016
  2. jcsd
  3. Dec 21, 2016 #2


    User Avatar

    Staff: Mentor

    Special characters like "μ" are available in the menu that you can invoke by clicking on the ##\Sigma## icon in the edit window's top bar:


    so no need to try to insert them as images from other sites. Such images often appear as broken links.

    Where does the 80° angle come from? You didn't provide a drawing (or free body diagram) for the scenario that identifies the angles, so this introduction of 80° after saying that the hill is a "10 degree hill" is puzzling. Which angle is it?

    Also, What is the difference between Fnormal and Fn above? One uses the cosine of 80° and the other the sine.
    That will depend on sorting out what the 80° angle is. You should draw and post a free body diagram.
    Where did this come from? It wasn't in the problem statement. You shouldn't introduce new information during the solution attempt. Helpers need to see the complete problem statement ahead of the any solution attempt so that they can verify your approach and steps.
    What's the net force acting on the box in each case? What does Newton's 2nd law tell you about the motion of the box?
    What are the conditions for constant speed? Again, look to Newton's 2nd law.
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