1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Basic Physics Ramp & Friction Question

  1. Nov 7, 2014 #1
    A 90 kg box is pushed by a horizontal force F at constant speed up a ramp inclined at 28°, as shown. Determine the magnitude of the applied force.
    a. when the ramp is frictionless.
    b) when the coefficient of kinetic friction is 0.18.

    Fnet = ma
    Fg= mg

    a) With upwards and right as positive and downwards and left as negative
    i) After drawing a diagram, I broke the Fnet into its x and y components.
    I determined Ffrictionx= Ffriction(cos28), Ffrictiony= Ffriction(sin28). I will not include these in the horizontal and vertical components as it is a frictionless surface.

    Fnet y = ma
    Fnormal - Fg = ma
    Fn= m(a-g)
    The object is not accelerating in the horizontal direction so a=0
    Fn= m (-g)
    Fn = 90kg (-(-9.8m/s^2))= 882N.
    So the vertical force is 882N in the upward direction.

    Fnet x= ma
    Fpush= ma

    I don't know what the acceleration is in the horizontal direction so I'm not sure where to go from here.

    b). Again, break into horizontal and vertical components with up/right as positive, down/left as negative.
    Fnet y = ma
    Fpush - Ffrictionx = ma
    Fp - Ff(Cos28) = ma
    Fp - (cos28) (uk)(Fn)= ma
    Fp - (cos28(uk)(-mg))= ma
    Fp= ma +140.2
    ... I'm not sure if I'm on the right track here, or if I am, where to go from here.
  2. jcsd
  3. Nov 7, 2014 #2


    User Avatar
    2017 Award

    Staff: Mentor

    The speed is constant, what do you expect?
    No. Why should it? There is more than one force involved.

    I don't understand your approach at (b), but try to solve (a) first.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted