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Homework Help: Momentum and Energy.

  1. Dec 8, 2009 #1
    part (1 of 3 ). A 15 metric ton train moves toward the south at 70 m/s.
    At what speed must it travel to have five times its original momentum? A metric ton
    is 1000 kg.
    Answer in units of m/s.

    and

    Part (2 of 3 ). At what speed must it travel to have a momentum of 5 × 105 kg · m/s?
    Answer in units of m/s.

    and

    Part (3 of 3). If there were a speed limit for this train as it traveled through a city, but not a weight limit, what mass must be added to the train to slow it down to 10 m/s while at the same time keeping the momentum the same as in the second part?
    Answer in units of kg.

    THANK YOU!!
     
  2. jcsd
  3. Dec 8, 2009 #2
    Part 1: p = mv

    So pi= 15000kg*70m/s = 1,005,000 Ns

    pf = 5*pi = 5,250,000 Ns

    Since we cant change the train's mass, we have to change its speed: 5,250,000 = 15000*v, v= 350 m/s
     
  4. Dec 8, 2009 #3
    Part 2: 5 x 10^5kg*m/s

    So if p = mv, then 5x10^5kg*m/s = 15000*v

    so v = 33.3 m/s
     
  5. Dec 8, 2009 #4
    Part 3:
    so
    p = 5 x 10^5 Ns
    v = 10 m/s

    p = mv, m = p/v; m = 50000kg final weight. Train starts out at 15000kg, so you must add 35000kg to have a total weight of 50000kg
     
  6. Dec 8, 2009 #5
    wow!! thank you soo much!! this makes so much more sense now!!!!!
     
  7. Dec 8, 2009 #6
    sure thing!
     
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