7.2. Gravitational Fields: Paper 2: Physics A Level AQA

What is gravity?

Gravity is the universal attractive force which acts between all matter.

What is G?

The universal gravitational constant.

Approximately 6.67 x 10¯¹¹ m³ • kg¯¹ • s¯²

What can field lines tell you about a field?

The direction of the field and the strength of the field depending on the density of the field lines.

What is 𝘨?

𝘨 is the force per unit area in a uniform field. In a radial field the magnitude of 𝘨 is the the proportionality constant at that point between force and mass.

Iet 𝘨 = G•M/r²

What is gravitational potential?

The potential energy per kilogram, at any point in the field. 0 potential is defined at infinity, hence at a point close to a mass the potential of an object would be negative.

What is the work done by moving a mass in a field?

Mass x change in potential

What is the gravitational potential difference?

Gravitational potential difference is the difference in the gravitational potentials of two points in a gravitational field.

What is an equipotential surface?

A surface in which every point on the surface has the same potential.

How much work is done when you move 1 km in any direction on an equipotential?

No work is done when moving across equipotentials, as the potential at each point is the same.

Why is gravitational potential a negative value?

Work needs to be done to move an object from the inside the field to outside the field. Since outside the field’s potential is defined as 0 then the potential inside the field must be negative.

How is the orbital period related to the radius of a circular orbit?

T² ∝ R³

What equations could one use to find the speed of an orbiting satellite?

The orbiting object (mass m) is in circular motion, so we would use F = ma with F = GMm/r^2 rearranged to a = v^2 / r = ω^2/r. This can be sold to find the speed (v), angular speed (ω), the radius of the orbit or using T = 2π/ω its period.

Compare the PE and KE of a lower orbit to a higher one.

A lower orbit (smaller m) has less potential energy and more kinetic energy than a higher orbit (bigger r).

What is the period of a geosynchronous orbit?

Geosynchronous orbits have a period of one day.