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Homework Help: Baseball and medicine ball thrown with different KE/P/V

  1. Oct 18, 2012 #1
    1. The problem statement, all variables and given/known data

    Your physical education teacher throws a baseball to you at
    a certain speed and you catch it. The teacher is next going to throw you a
    medicine ball whose mass is ten times the mass of the baseball. You are given
    the following choices: You can have the medicine ball thrown with (a) the
    same speed as the baseball, (b) the same momentum, or (c) the same kinetic
    energy. Rank these choices from easiest to hardest to catch.

    2. Relevant equations

    p = mv
    KE = 1/2 m v^2

    3. The attempt at a solution

    i originally thought it would be a, c, b, but i know thats wrong now and i don't know how i can derive the right answers mathematically with variables
  2. jcsd
  3. Oct 18, 2012 #2
    Think about that answer. If you catch something 10 times heavier going at the same speed, it's kinetic energy and its momentum are both going to be multiplied by 10. So given that you have a choice between the same momentum, the same kinetic energy, or 10 times as much momentum and kinetic energy, I'd say the choice "a" is by far the worst option.

    In order to keep the same momentum and kinetic energy for the heavier ball, it's speed would have to be reduced, so b and c would make much more sense.
  4. Oct 18, 2012 #3
    ok i understand how a is the worst but how would you be able to tell out of b and c is easier/harder?
  5. Oct 18, 2012 #4
    I recommend plugging in some numbers to both formulas, say... .5 kg for the baseball and 5 kg for the medicine ball. Then make up a velocity for the baseball to be going. Solve for momentum and kinetic energy. Once you get momentum in kgm/s and kinetic energy in joules, then plug those in for the medicine ball with 10 times as much mass, and solve for the velocity the medicine ball would have to be going for the momentum/KE to be the same.
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