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Homework Help: Conservation of momentum

  1. Jan 14, 2010 #1
    Question: A car of mass 1250kg moving at 28m/s drives up a ramp onto the back of a moving lorry of mass 3500kg and speed 25.5m/s.

    The back of the lorry allows a braking distance of 5m. Show that this stunt is possible with a min force of about 600N.

    My solution: I get the answer by first working out the velocity of their combined mass using conservation of momentum, v=26.2 m/s.

    Then doing Initial KE = Final KE + Work Done
    which gives the force of just under 600N

    My question was, why can this not be done using the equations of constant acceleration. After working out that v=26.2m/s, why cant we use v^2 = u^2 + 2as to get 'a', and sub this into F=ma? I tried this and it gives a wrong answer.
  2. jcsd
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