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Homework Help: Car drives down a ramp

  1. Jan 25, 2010 #1
    A car drives down a ramp onto the back of a moving lorry. Both are moving at high speed, the car slightly faster than the lorry.

    The car has mass 1250 kg and is moving at a speed of 28.0 m s–1. The lorry has mass 3500 kg and a speed of 25.5 m s–1. The length of the flat back of the lorry allows a braking distance of 5.0 m.

    By considering both momentum and energy show that the stunt is possible, provided a minimum force of about 600 N slows the car down. You should support your explanations with calculations.

    Treat the situation as one in which two objects join together

    I manage to get the answer of 576 N using the method told to use.

    However, I wanted to know, why doesn’t the answer work out if I do:

    1. find velocity using conservation of linear momentum (as would in the normal procedure)
    2. use constant acc equations to find the deceleration
    3. then do F=ma?

    This would give Acc = -9.73 and F = 12kN?
  2. jcsd
  3. Jan 25, 2010 #2

    Char. Limit

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    Gold Member

    Good question... I don't believe conservation of momentum really applies here, as the two objects aren't crashing into each other. But I might just be crazy. Now for my question: What the heck is a lorry?
  4. Jan 25, 2010 #3
    Yeah it doesn't, thats what i thought as well, but the questions states it.

    Its british for a truck lol...
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