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Homework Help: Find total force resisting motion.

  1. Sep 3, 2010 #1
    A boy and his bicycle have a combined mass of 45kg. When traveling at 20km/h on a level road, he ceases to pedal and finds his speed reduced to 16km/h in 10 seconds. What is the total force resisting motion?

    Thanks for reading! It should be easy for me but I just can't figure it out. If someone could please provide me with the calculations or formula that would be great!

    I know the answer is 5 N, But have no idea how to get it.
  2. jcsd
  3. Sep 3, 2010 #2


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    How about you start by posting some relevant formulas.
    For example, there is something about a change in velocity with the time given, and something with a force... surely you can find some formulas in which they occur?
  4. Sep 3, 2010 #3
    Thank you for the reply.
    In fact I have found the answer, I had the right formula all along. I forgot to input brackets into my calculator, so I was getting the wrong answer. -_-

    But here's my working out if anyone has a similar problem.
    First I changed Km/h to m/s 20km/h = 5.55 16km/h = 4.44

    I then used the equation: Final velocity = Initial velocity + Acceleration x time
    Transformed that into :
    Final velocity - Initial velocity / time = Acceleration

    which is (4.44 - 5.55) / 10 = -.111 (negative acceleration)

    Acceleration is also equal to Force\Mass. And that Force is then
    .111 x 45 = 4.995 Newton! keep in mind the boy stopped peddling, thus the only force left was a resistant force. Hope that made sense.
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