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Physics Work and Energy, Momentum

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Let g be the acceleration due to gravity at the surface of a planet of radius R. Which of the following is a dimensionally correct formula for the minimum kinetic energy K that a projectile of mass m must have at the planet's surface if the projectile is to escape from the planet's gravitational field?
A) K=sqt.(gR)
B) K=mgR
C) K=mg/R
D) K=m*sqt.(g/R)
E K=gR

(I thought the answer should be K=mgR/2, but the key is B)... )

The sum of all the extenral forces on a system of particles is zero. Which of the following must be true of the system?
A)the total mechanical energy is constant
B)total potential energy is constant
C)total kinetic energy is constant
D)total linear momentum is constant
E) it is in static equilibrium

I am so confused of these kind of question. How can I do these kind of problem? And when I see a problem, how can I figure out which method to use, momentum conservation or energy conservation, is easier? Is any characteristics?


Any helps??????????????

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