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Homework Help: Calculate the magnitude of the average net force on the car

  1. Oct 7, 2008 #1

    A 1270-kg car coasts on a horizontal road with a speed of 18.1 m/s. After crossing an unpaved, sandy stretch of road 28.4 m long, its speed decreases to 11.3 m/s. Was the net work done on the car positive, negative, or zero?

    b. Calculate the magnitude of the average net force on the car in the sandy section.

    2. Relevant equations
    W = F(delta r)(cos theta)

    3. The attempt at a solution
    I started looking at it. I understand work but for some reason I am stuck here.
  2. jcsd
  3. Oct 8, 2008 #2
    Well look at it this way, initially you consider all of its energy to be kinetic. Once it begins to cross the sand, some of it's kinetic energy is transcribed into work:

    [tex] \frac{1}{2}mv_1^2 = \frac{1}{2}mv_2^2 + F\Delta d [/tex]

    Mass remains constant, and we are told that [tex] v_2 [/tex] decreases, that implies that work; [tex] F\Delta d [/tex] is positive (or else the equation would not be balanced).

    For part b) you just need to solve the equation listed above. You have everything except F.
  4. Oct 8, 2008 #3
    Alright, I got it. Thanks.