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Homework Help: Elementary - braking force of a trolley

  1. Oct 19, 2012 #1
    1. The problem statement, all variables and given/known data

    A trolley weighing 16 000kg is moving along a horizontal surface with a velocity od 6m/s. What braking force would cause it to brake on a distance of 10m and how long it would take?

    2. Relevant equations

    a = F/m = ΔV/t

    3. The attempt at a solution

    So ΔV = 6m/s, m=16 000kg, s = 10m and F = mΔV/Δt. We know neither F nor t so how could I solve this? It's just one equation but two unknown variables. Could you please help?
  2. jcsd
  3. Oct 19, 2012 #2
    Do you know the relationships between the work done by a force acting on a body and its energy?
  4. Oct 19, 2012 #3
    I know Newton's laws of motion, that's why I have the a=F/m, but what does that give me?
  5. Oct 19, 2012 #4


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    If a, acceleration, is constant, then v= at+ v0 and d= (1/2)at^2+ v0t.

    You know that v0= 6 m/s and that d= 10m . Solve the quadratic equation (1/2)at^2+ 6t- 10= 0 for t, with parameter a. The final velocity was 0 so at+ 6= 0. Put the result you got for t into that and solve the resulting equation for a.
  6. Oct 19, 2012 #5
    Rather have a look at the chapter on work and energy.
  7. Oct 19, 2012 #6
    Do you know what the answer should be?
  8. Oct 20, 2012 #7
    HallsofIvy - OK, thank you. So we get t = (2 (sqrt(5 a+9)-3))/a (the other one is negative so doesn't matter), we plug it into the latter and get (2 (sqrt(5 a+9)-3) so (sqrt(5 a+9)=3 but then... a would have to be 0, which it's not, is it? Where did I make a mistake?
  9. Oct 20, 2012 #8
    Could someone please tell me where did I make a mistake? :(
  10. Oct 21, 2012 #9
    No matter what I do to the expression, it never outputs anything other than 0 - why? Do I lack some variable?
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