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Homework Help: Impulse and momentum in an inelastic collision

  1. Mar 21, 2012 #1
    Please check my work for the first question. The second and third questions, I'm not 100% sure how to solve them.
    A ssume that a cement truck with a mass of 10,000 kg hits you while you were stopped at a traffic light, and that you are driving a Smart car with a mass of 750 kg.

    1. If the truck was traveling at 15 mph and the crash makes you stick together (you and the truck have the same final velocity), what is your final velocity? Ignore friction with the road.

    Momentum is conserved so Pinitial = Pfinal
    15mi/h(1609m/mi)(hr/3600s) = 6.68 m/s
    Pinitial = (10,000kg)(6.68 m/s) + 750kg*0 = 66,800
    Pfinal = (10,000kg + 750kg)vfinal = 66,800 --> vfinal = 6.21 m/s

    2. If the crash impact took 0.2 s, what was your average acceleration? Convert it to units of g, the acceleration of gravity.

    Since F = delta p/delta t, and delta p = 66,800
    For the car,
    F = 66,800 / 0.2 s = 333,400 = ma --> 333,400/750 = a = 445.28 ms^-2.

    3. Solve the previous problem assuming you are in the cement truck instead of the Smart car. In which vehicle would you rather ride out the crash? Why?
    For the truck,
    F = 66,800 / 0.2 s = 333,400 = ma --> 333,400/10,000= a = 33.4 ms^-2.
    So it would be more preferable to be in the truck because the truck accelerates less?

    I'm really not sure whether to use 66800 for the truck's delta p since p doesn't change for the truck. does it??? please help!
    Last edited: Mar 21, 2012
  2. jcsd
  3. Mar 21, 2012 #2
    Delta p means the change in momentum, not the total momentum. You need to calculate how much the momentum changed for either vehicle. Calculate the p before impact and after impact for either vehicle.
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