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Real world problem, need help with KE problem

  1. Dec 2, 2013 #1
    What forum should I post this in, my friend died in a horrible car accident, some of us just want more answers I guess...

    Object A is in straight line motion
    Object B is static

    Object A impacts Object B

    Object A moves straight forward 60 m and rests
    Object B moves straight forward 76 m and rests

    Object A has a mass of 1,500 kg
    Object B has a mass of 1,500 kg

    What was the velocity of Object A when it impacted Object B?

    To account for some rolling resistances, Object A was a car that rolled and tore up large patches of sod, Object B was a rectangular boulder that rolled ~20 times. I guesstimated 124 mph (see below), but I can't believe it (I feel like I missed a bunch of things or didn't draw the problem right).
  2. jcsd
  3. Dec 2, 2013 #2


    Staff: Mentor

    Are you sure the boulder is the same weight as the car. from your description it seems it is lighter.

    On a pool table, when the cue ball strikes a stationary ball, the cue ball stops and the stationary ball continues with the same velocity unless the cue ball has some additional spin. In your case, after the collision both objects are moving forward.

    I think you can solve it via conservation of momentum.
    Last edited: Dec 2, 2013
  4. Dec 2, 2013 #3
    The boulder is about the volume of a fluffy lazy boy chair (minus the back). My landscape friend estimated that weighed 1.5-2 tons. Imagine literally, a car running directly into this boulder, transferring enough energy to launch the rock 62 meters (I just measured my stride, 737 mm x 84 strides), and keep enough energy to move / drag / roll the car 45 meters. I don't even know why I am doing this, I guess this is what a nerd does when his friend dies.
  5. Dec 2, 2013 #4


    Staff: Mentor

    What answer do you hope to get from this? I'm thinking that any calculation will be far off from reality. Are you trying to determine speed? is there some negligence involved?
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