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Friction problem on a incline

  1. Mar 23, 2016 #1
    1. The problem statement, all variables and given/known data
    A 20kg box has a static coefficient of friction of 0.28. If a force of 200N is applied at an angle of 45deg, is the box able to move. Explain why once the static friction is overcome, the frictional force decreases and the box moves easier.

    2. Relevant equations
    F=m*a
    Ff=μFN


    3. The attempt at a solution
    I dont fully understand this concept. I made a diagram with FN going straight down on the box (perpendicular to the ramp), and Fg going directly down. I know how to calculate the frictional force but I am confused on the solution to the problem. Am i supposed to make a right triangle and use trig to find the hypotenuse and the bottom side?

    The right triangle I made consisted of
    FN=200N and Fg= (9.8m/s*20kg). Im pretty sure the Fg is wrong because the hypotenuse of the right triangle would be smaller than one of the other sides. I feel likes I have the general gyst on how to set up on visualizing it, but I cant execute using the variables at hand
     
  2. jcsd
  3. Mar 23, 2016 #2

    Doc Al

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    Staff: Mentor

    Realize that the applied force affects the normal force between box and surface. You need to calculate that normal force. Start by identifying all the forces acting on the box in a free body diagram.
     
  4. Mar 23, 2016 #3
    https://gyazo.com/373eea5a0624f1562ed194fd5074fd02
    https://gyazo.com/373eea5a0624f1562ed194fd5074fd02

    Is the normal force not 200N? or am I supposed to factor in the mass of 20kg?
     
  5. Mar 23, 2016 #4

    Doc Al

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    Staff: Mentor

    Does that diagram relate to this problem? You didn't mention an incline in your first post.

    Yes, the weight of the mass will affect the normal force.
     
  6. Mar 23, 2016 #5

    Doc Al

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    Staff: Mentor

    I guess you did mention a ramp, but your diagram shows 40 degrees. And you state in post 1 that the applied force is at a 45 degree angle.

    Can you restate the problem?
     
  7. Mar 23, 2016 #6

    Doc Al

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    Also... why in the world is this thread titled "momentum and collision" problem! :)
     
  8. Mar 23, 2016 #7
    I was initially going to put a different problem but I forgot to change the title!

    The diagram angle was a mistake, It was 45deg. Not 40deg.

    Is the normal force 200N? or am I mistaken in the way that i'm labeling variables and calculating them?
     
  9. Mar 23, 2016 #8

    Doc Al

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    No worries. (Give me a new title and I'll change it.)

    No. The applied force is 200 N. You also must consider the component of the box's weight normal to the surface.
     
  10. Mar 23, 2016 #9

    New Title: Friction problem

    Is the normal force the mass of the box*gravity so 20kg(9.8m/s^2)=196N?
    and if so, is that the force perpendicular to the ramp in the diagram?
     
  11. Mar 23, 2016 #10

    Doc Al

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    No, that's the weight of the box. It acts vertically down.

    The 200 N force shown in your diagram is the applied force.

    Four forces act on the box, but three have components perpendicular to the ramp. (What are they?) What must be the net force on the box perpendicular to the ramp? Use that to solve for the normal force.
     
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