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Homework Help: Forces - Newton's Laws Problem

  1. Oct 11, 2013 #1
    1. The problem statement, all variables and given/known data

    A 1200 kg car travels at 50 km/h and experiences air resistance of 5000 N and road friction of 2200 N. If the wheels push with a force of 7500 N, what is the car's acceleration?

    2. Relevant equations

    a = ƩF/m

    3. The attempt at a solution

    I got ƩF = 8590 N [E52°S] then plugged it into the equation,
    but it did not work to the correct answer of 0.250m/s^2.

    I'm thinking I probably had to make use of the 50 km/h variable somewhere, which I didn't.
    When and where would that apply...?
  2. jcsd
  3. Oct 11, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    How did you find that sum? Realize that the forces point in different directions. Where did that angle come from?
  4. Oct 11, 2013 #3
    how did you get ƩF=8590 [E52°S]?

    To me it seems you interpreted it wrong. Its a one dimensional problem, but by the looks of your answer it is a vector result.

    car is being pushed forward by 7500 N...........backwards by air resistance of 5000 N........again backwards by road friction of 2200 N.


    try again you should arrive at the right answer.

    P.S. - No offense, but please consider revising the key points of the chapter associated with Forces.

    Edit : Doc Al got there first!!!
  5. Oct 11, 2013 #4
    ohh, okay i don't know why i thought air friction was in the direction north.
    but thank you and yes, i just started this unit yesterday, i will make sure to revise!
  6. Oct 11, 2013 #5
    great, glad you appreciate the help!!!:smile:
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