Newton's law of gravitation

Which of the following statements about Newton’s law of gravitation is correct?

Newton’s gravitational law explains

• the origin of gravitational forces.

• why a falling satellite burns up when it enters the Earth’s atmosphere

• why projectiles maintain a uniform horizontal speed.

• how various factors affect the gravitational force between two particles.

Two planets X and Y are in concentric circular orbits about a star S. The radius of the orbit of X is R and the radius of orbit of Y is 2R .

The gravitational force between X and Y is F when angle SXY is 90°, as shown in the diagram.

What is the gravitational force between X and Y when they are nearest to each other?

• 2F

• 3F

• 4F

• 5F

Masses of M and 2M exert a gravitational force F on each other when the distance between their centres is r. What is the gravitational force between masses of 2M and 4M when the distance between their centres is 4r?

• 0.25F

• 0.50F

• 0.75F

• 1.00F