# 5.4 Gravitational Flashcards

What causes gravitational fields

Objects having mass

How a spherical mass can be modelled

As a point mass

How do gravitational field lines work

They point in the direction that a mass would move and the closer together they are the stronger the field

Gravitational field strength

The force in newtons per unit mass

Equation for gravitational field strength in terms of forces (g=)

g=F/m

How are gravitational fields related to electric fields

They are both a form of a field giving rise to a force

What is the force between two masses proportional to

The product of the masses

The inverse square of the separation

Equation for gravitational field strength in terms of distance

g = -GM/r^2

Why do we not have to consider changing gravitational field strength when considering problems on earths surface

The change in distance is so small that the field can be considered uniform at the surface (this also applies to direction since we can consider the surface flat)

Relation between acceleration of free fall and gravitational field strength

They have the same numerical value (9.81) but are different things with different units

Keplers first law

Planets move around the Sun in an ellipse, with the Sun at one focus of the ellipse

Keplers second law

A line joining the Sun to a planet will sweep out equal areas in equal times

Keplers third law

T^2 ∝ r^3

where T is the period and r is the mean distance

How centripetal force relates to gravity

The centripetal force on a planet is provided by the gravitational force between it and the Sun

How to derive the constant for keplers third law

Equation centripetal force to the gravitational force