A-level Physics (Advancing Physics)/Gravitational Potential

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Gravitational potential is the gravitational potential energy given to objects per unit mass:

V_{grav} = \frac{E_{grav}}{m} = \frac{-\frac{GMm}{r}}{m} = \frac{-GM}{r}

For short changes in distance Δh (Δh << r), gravitational potential energy is given by:

E_{grav} \approx mg\Delta h,

where

g = \frac{GM}{r^2}

So, for short changes of distance, gravitational potential is equal to gΔh. Gravitational potential is measured in Jkg-1.

Equipotentials[edit]

On a field diagram, lines can be drawn which, like contours on a map, show all the points which have the same gravitational potential. These lines are known as equipotentials. Equipotentials are always perpendicular to field lines, and get closer together as the field strength increases, and the density of field lines increases. Over short distances, equipotentials are evenly spaced.

Summary of Gravity[edit]

You should now know (if you did the gravity section in the right order) about four attributes of gravitational fields: force, field strength, potential energy and potential. These can be summarised by the following table:

Force

F_{grav} = \frac{GMm}{r^2}

integrate

with respect to r

Potential Energy

E_{grav} = \frac{-GMm}{r}

per. unit mass
Field Strength

g = \frac{GM}{r^2}

integrate

with respect to r

Potential

V_{grav} = \frac{-GM}{r}

Questions[edit]

G = 6.67 x 10-11 m3kg-1s-2

g = 9.81 ms-2

1. What is the gravitational potential at the Earth's surface? (mass of Earth = 5.97 x 1024 kg,radius of Earth = 6371 km)

2. Taking the Earth's surface as Vgrav = 0, what is the gravitational potential 2m above the ground?

3. A 0.2kg firework reaches a gravitational potential relative to the ground of 500Jkg-1. If the firework is 30% efficient, how much energy was expended to get there?

4. Express gravitational potential in terms of gravitational force.

5. Draw the equipotentials and field lines surrounding the Earth.

Worked Solutions