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Homework Help: Train car Momentum questions

  1. Dec 4, 2007 #1
    i have no idea at all how to do these.
    someone help please

    1. On a hot summer afternoon, Keith and Nate are out fishing in their rowboat when they decide to jump into the water and go for a swim. Keith, whose mass is 65.0 kg, jumps straight off the front of the boat with a speed of 2.00 m\s relative to the boat, while Nate propels his 68.0-kg body simultaneously off the back of the boat at 4.00 m\s relative to the boat. If the 100.-kg boat is initially traveling forward at 3.00 m\s, what is its velocity after both boys jump?

    2. Lilly, whose mass is 45.0 kg, is ice skating with a constant speed of 7.00 m\s when she hits a rough patch of ice with a coefficient of friction of 0.0800. How long will it take before Lilly coasts to a stop?

    3. In a train yard, train cars are rolled down a long hill in order to link them up with other cars as shown. A car of mass 4000. kg starts to roll from rest at the top of the hill 5.0 m high, and inclined at an angle of 5.0o to the horizontal. The coefficient of rolling friction between the train and the track is 0.050. What velocity would the car have if it linked up with 3 identical cars sitting on flat ground at the bottom of the track? (Hint: The equation for rolling friction is just like the one for sliding friction.)
  2. jcsd
  3. Dec 4, 2007 #2


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    I moved your thread to the homework section of PF where homework questions belong.

    Re-read the rules of the homework section (which is especially there for you!), try to show some work, and we will help you...
  4. Dec 4, 2007 #3
    1. Conservation of momentum, the system considered is the two boys and boat initally all moving at 3.00 m/s afterwards the boys jump of, so now the boat has some final momentum. Pi-Pf=0

    2. In this case momentum is not conserved, and outside force is applied. So you want to find the net forces acted on her, from there find the accelleration, with the given intial velocity and position solve for time.

    Solving these first two problems should help in figuring out the third. The third problem is just asking you to integrate the first and second questions.
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