Gravitational Forces And Fields

Question 1

What is Newtons Law of Gravitation?

Answer 1

Any two point masses attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of their separation.

Question 2

Formula for gravitational force (of two point masses)

Answer 2

F = GMm / r²
OR
F = Gm₁m₂ / r²

(Where r is measured from the center)

Question 3

Twice as far away (from the planet) gives…

Answer 3

…..¹/₄ the force

Question 4

What is gravitational field strength at a point?

Answer 4

The gravitational force exerted per unit mass on a small object placed at a point.

Question 5

Equation for graviatational field strength

Answer 5

g = GM / r²

Question 6

Gravitational field strength at a point depends on:

Answer 6

• the mass M of the body causing the field.

* the distance r from the center.

Question 7

What is the zero of g.p.e?

Answer 7

This is a mass at infinity that is at an infinite distance from all other masses.
NB: g.p.e is always negative

Question 8

Whys is g.p.e always negative?

Answer 8

This is because as a mass is brought towards another mass it’s g.p.e decreases. Since g.p.e is zero at infinity then at anywhere else the g.p.e and potential will be less than zero i.e negative.

Question 9

Formula for speed to achieve a circular orbit

Answer 9

v = √(GM / r)

NB: the mass of he satellite does not affect the speed in a particular orbit.

Question 10

Formula for orbital period

Answer 10

T² = (4π² / GM) r³

Therefore, T² is proportional to r³

Question 11

What is a geostationary orbit?

Answer 11

This is a satellite that is positioned so that, as it orbits, the earth rotates below it at the same rate. The satellite remains above a fixed point above the earths equator.