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Homework Help: Acceleration after a Colission

  1. Mar 13, 2008 #1
    Hey guys, Here's the questions.. (It's a long one!)

    (A) Assume 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 Volkswagen with a mass of 750 kg. 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.
    (B) If the crash impact took 0.2 s, what was your average acceleration? Convert it to units of g, the acceleration of gravity.
    (C) Solve the previous problem assuming you are in the cement truck instead of the VW. In which vehicle would you rather ride out the crash? Why?
    (D)How much kinetic energy was lost during the collision? What percentage of the initial kinetic energy of the truck was lost during the collision?

    For relevant equations... I guess x=1/2at^2? or v=d/t? And KE= 1/2mv^2

    And my sucky attempt....

    (A)The final velocity of the VW would be 30 mph in the direction the truck was moving, assuming that wikipedia is right in saying that in an elastic collision when m1 is much greater than m2, m2 will move at twice the initial velocity of m1 in the opposite direction m2 was originally going.... or at least, I hope thats right!

    (B)30 mph = 13.4112 m/s
    13.4112/0.2 = 67.05600 m/s^2
    67.05600m/s^2 / 9.81 m/s^2 = 6.85 g
    For this one I'm not really sure that's right. Would that be average velocity, and not acceleration? and would that be for the VW, or for the driver? the question asked for *your* acceleration...

    (C) Now for this one I'm really confused. I get the that trunk would start to have a negative acceleration because of the impact, but I'm not really sure how to calculate what exactly it would be. As for the second part, I know that I'd rather be driving the truck rather than the VW because the great mass makes it safer.

    (D) For D... wow, at this point I'm *really* lost, exspecially if the last two are wrong/not solved. I guess for you could use the KE equation and plug in the new v to get the KE lost, but since I don't know how to find everything... :(

    Thanks in advance for your help! Sorry if it doesn't make sense, I can totally clarify things if need be.
    Last edited: Mar 13, 2008
  2. jcsd
  3. Mar 13, 2008 #2

    Doc Al

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    Staff: Mentor

    It's not an elastic collision, it's a perfectly inelastic collision (the vehicles stick together).

    Hint: What's conserved?
  4. Mar 13, 2008 #3
    It seems that you know what to do, but that you just missed the part of the question that says the car and the truck stick together after the crash. So they MUST be travelling the same speed after the collision. In your answer for A (30mph) you put that after the truck hits the car, it suddenly doubles in speed!