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: Finding the applied force at an angle?

  1. Oct 26, 2014 #1
    1. The problem statement, all variables and given/known data
    A 2000 kg car is driving up a 20° grade at a speed of 85 km/h. What force must be applied to the car to accelerate it up to 100 km/h in 10s?

    m= 2000 kg
    θ= 20°
    Vo= 85 km/hr = 23.6 m/s
    Vf = 100 km/hr = 27.8 m/s
    t = 10 s

    2. Relevant equations
    F = ma (Fx = max)
    Fx= Fcosθ
    Vox = Vo cosθ
    Voy = Vo sinθ
    Vx = Vox + axt
    Vx= Vox +axt

    3. The attempt at a solution
    I found these using the cos or sin equations above:
    Vox = 22.1 m/s
    Voy = 8.07 m/s
    Vx = 26.1 m/s
    Vy = 9.51 m/s

    Using the last equation, I found ax = 0.4 m/s2

    Fx= 800 N

    I then solved for F using the second equation by dividing Fx by cosθ, making F= 851 N. I think I did something wrong when I solved for a, and my guess is that I couldn't use 10s for that since it was the incline, when I used it for the x values. I'm stuck on how to fix it, or on how to solve the problem a different way. These are practice problems that I have the answers to, but no solutions, so I know that F=7.5 kN.

    I tried working backward and got that a=3.75 m/s2 and ax=3.52 m/s2. Using F=ma and the cos, I got Fx = 7040 N, which gets me the right answer however I do it. This makes me fairly certain that I'm doing something wrong with getting the acceleration.

    Thanks in advance!
  2. jcsd
  3. Oct 26, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    you are making the problem complex by not choosing the x axis parallel to the incline and the y axis perpendicular to the incline. You are also not noting that it is the net force that produces the acceleration , not just the applied force, regardless of how you choose your axes. Draw a free body diagram.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted