Gravitational fields

Define gravitaional field strength

The gravitational force exerted per unit mass on a small object placed at that point within the field.

What are Newton’s laws of gravitation (in words)

The force bewtween two point masses is:

• Directly proportional to the product of the masses, F ∝ Mm

• Inversely proportional to the square of their separation, F ∝ 1/r²

What are Newton’s laws of gravitation (equation)

F = -GMm/r²

A minus sign is required to show that gravitational force is an attractive force.

Kepler’s 1st law

Each planet moves in an elliptical orbit with the sun at one of the two foci

Kepler’s 2nd law

A line segment joining a planet and the sun sweeps out equal areas during equal intervals of time

Kepler’s 3rd law

The square of the orbital period of a planet, T, is directly proportional to the cube of its average distance, r, from the sun (can also be applied to satellite orbits) i.e. T² ∝ r³ or T²/r³ = k