Physics... momentum and kinetic energy?

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Maite C

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Hi, I've been trying to do these problems in Mastering Physics but I can't seem to get the correct answer, please any help would be greatly appreciated
1) A cardinal (Richmondena cardinalis) of mass 3.90×10−2 and a baseball of mass 0.140 have the same kinetic energy. What is the ratio of the cardinal's magnitude of momentum to the magnitude of the baseball's momentum?
2) A man weighing 750 and a woman weighing 450 have the same momentum. What is the ratio of the man's kinetic energy to that of the woman ?
 
The units would be nice, but are not necessary as we will work this by ratios.

KE = 1/2 mV^2 = 1/2 Mv^2 = ke; where m = 3.9E-2 mass units and M = .140 mass units. m/M = (v/V)^2 and v/V = sqrt(m/M)

mV = P and Mv = p, the respective momenta.

1) mV V = P V = p v = Mv v from the KE. P/p = v/V = sqrt(m/M) = sqrt(3.9E-2/.140) = ? the ratio you're looking for. You can do the math. Note, the units, whatever they are, cancel out; which is why I assert they are not needed.

2) Mv = mV and v/V = m/M ke/KE = 1/2 Mv^2/1/2 mV^2 = p v/P V = v/V = m/M = 450/750 = ? since p = P You can do the math; again, units are not germane.

NOTE: When comparing like characteristic (e.g., p to P, or KE to ke) the best approach is to use ratios like P/p or ke/KE. When you do that, you'll find a lot of factors will cancel out.
 
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