April 17, 2014

Homework Help: Physics Work and Energy, Momentum

Posted by Kelly on Sunday, May 13, 2007 at 4:35pm.

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??????????????

Answer this Question

First Name:
School Subject:

Related Questions

physics - An object rests on the surface of planet y. Its mass is unknown. Find...
College Physics - Suppose a planet is discovered that has the same total mass as...
PHYSICS - A 5.0-kilogram sphere, starting from rest, falls freely 22 meters in 3...
earth - A 5.0-kilogram sphere, starting from rest, falls freely 22 meters in 3.0...
physics - acceleration due to gravity questions when an object is dropped from a...
physics help 3!! ** - Based on the following data about planet X (which orbits ...
Physics - 1. Planet X orbits the sun with a mean orbital radius that is 6200 ...
phyics-universal gravity - A hypothetical planet has a radius 3.3 times that of ...
earth - a space traveller determines the radius of a planet to be half of earth...
Physics-Gravity - A 17.0-kg satellite has a circular orbit with a period of 2.31...