# The Physical World I

Spring 2005
MWF 11:30

Instructor: Dr. Bruce Law, CW 327, Tel: 532-1618.

# E7. This can never happen. Although the mass of the Earth is much larger than the mass of the moon the rocks are at a much, much closer distance (r) to the moon than they are to the Earth. Hence as F = GMm/r2 the force between the rocks and the moon is much, much larger than the force between the rocks and the Earth.

## E11. The weight of the Earth in the gravitational field of the apple is 1N also, according to Newton`s third law.

E12. The Earth and Moon attract each other with the same force according to Newton's third law.

E14. The force of gravity is give by F=GMm/r2. Therefore if the radius of the Earth (r) is increased, the force of gravity (and hence your weight) will decrease. Similarly, if the radius of the Earth is shrunk (r smaller) then your weight would increase.

E17. F=GMm/r2, the mass of Juptier (M) is 300 time as large as that of Earth, however, the radius (r) of Juptier is much, much larger than that of Earth – this is the reason why an object on the surface of Juptier scarcely weighs three times as much as on Earth.

# PROBLEMS OF Home Work 9

P3. F=GMm/r2 where now r increases by a factor of 4, therefore, r2 increases by a factor of 4x4 = 16 and therefore the force F must decrease by a factor of 16 because you are dividing by r2. As a consequence the value of g at this distance would be 9.8/16 = 0.6m/s2.