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Few questions on mass and velocity

  1. Oct 16, 2009 #1
    1. The problem statement, all variables and given/known data

    A 3.0-kg block is traveling to the right (the +x direction) at 5.0 m/s and a second 5.0-kg block is traveling to the right at 3.0 m/s.

    1. Find the total kinetic energy of the two blocks, in Joules, to two significant digits.

    2. Find the velocity of the center of mass of the two-block system, to one decimal place.

    3. Find the velocity, including sign, of the first block relative to the center of mass, to one decimal place.

    4. Find the velocity, including sign, of the second block relative to the center of mass, to one decimal place.

    5. Find the kinetic energy of the blocks relative to the center of mass, in Joules, rounded to a whole number.



    2. Relevant equations



    3. The attempt at a solution\

    I got 1 and 2, how to do the rest?

    1. 60 J
    2. 3.8 m/s ? Are they right?
     
  2. jcsd
  3. Oct 17, 2009 #2

    Doc Al

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    Staff: Mentor

    Yes, those are correct.

    To solve the others you must view things from a frame of reference moving along with the center of mass.

    Example: If you were riding a bike at 10 mph and I was driving a car at 60 mph, both in the same direction, then relative to you I would be going at 60 - 10 = 50 mph. Relative to me, you'd be going at 10 - 60 = -50 mph.
     
  4. Oct 17, 2009 #3
    So

    3. 5.0 - 3.8 = +1.2 m/s?
    4. 3 - 3.8 = -0.8 m/s?
    5. 4 J ? (rounded)

    Are they right?
     
  5. Oct 17, 2009 #4

    Doc Al

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    Staff: Mentor

    Looks good.
     
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