Gravitational fields

Question 1

What are the two types of gravitational fields?

Answer 1

Uniform and radial

Question 2

What does gravity act on?

Answer 2

Any objects which have mass.

Question 3

What is F in Newton’s law of gravitation?

Answer 3

The gravitational force between two masses

Question 4

What is the symbol for gravitational field strength and what is it?

Answer 4

g
The force per unit mass exerted by a gravitational field on an object.

Question 5

What is gravitational potential (V)?

Answer 5

The work done per unit mass against the gravitational force to move an object from infinity to a given point.

Question 6

What is the gravitational potential at infinity?

Answer 6

Zero, so it is always negative

Question 7

What is gravitational potential difference?

Answer 7

The energy needed to move a unit mass between two points.

Question 8

What are equipotentials?

Answer 8

Equipotentials are surfaces which are created by joining points of equal potential together, therefore the potential on an equipotential surface is constant everywhere.

Question 9

What is the gravitational potential difference when moving along an equipotential?

Answer 9

Zero, so no work is done.

Question 10

How are equipotentials situated?

Answer 10

Perpendicular to field lines

Question 11

What does the graph for V against R look like?

Answer 11

Negative inverse proportion graph

Question 12

How can you get g (gravitational field strength) from a V/r graph?

Answer 12

Drawing a tangent to the curve at the necessary distance and then multiplying by -1.

Question 13

What does a g/r graph look like?

Answer 13

Inverse proportion

Question 14

How can you get gravitational potential difference from a g/r graph?

Answer 14

Area under the curve.

Question 15

What is Kepler’s third law?

Answer 15

The square of the orbital period (T) is directly proportional to the cube of the radius (r):
T^2 is proportional to r^3

Question 16

Derive Kepler’s third law

Answer 16

1 - set centripetal force equal to gravitational force
2 - rearrange to make v^2 the subject
3 - set that equal to the circular motion v equation ( v = 2 pi r / T)
4 - rearrange to make T^2 the subject

Question 17

What is the equation for the total energy of a satellite?

Answer 17

kinetic energy + potential energy

Question 18

What happens if the height of a satellite is decreased?

Answer 18

Its gravitational potential energy decreases, however it will travel at a higher speed meaning kinetic energy increases - therefore total energy is always kept constant.

Question 19

What is escape velocity?

Answer 19

The minimum velocity an object needs to travel at in order to escape the gravitational field at the surface of a mass. It is the velocity at which the object’s kinetic energy is equal to the magnitude of its gravitational potential energy.

Question 20

What is the formula for escape velocity?

Answer 20

the square root of 2GM/r
Independent of the mass of the object.

Question 21

What is a synchronous orbit?

Answer 21

Where the orbital period of the satellite is equal to the rotational period of the object that it is orbiting.

Question 22

What is the orbital period of geostationary satellites?

Answer 22

Geostationary satellites follow a geosynchronous orbit, meaning that their orbital period is 24 hours.

Question 23

Why do geostationary satellites always stay above the same point on the Earth?

Answer 23

Because they orbit directly above the equator.

Question 24

What are geostationary satellites useful for?

Answer 24

Communications (sending TV and telephone signals) because they are always above the same point on the Earth so you don’t have to alter the plane of an aerial or transmitter.

Question 25

How do the orbital periods of low-orbit satellites compare to that of geostationary satellites?

Answer 25

They are significantly lower orbits in comparison to geostationary satellites, therefore they travel much faster, so their orbital periods are much smaller.

Question 26

What are low-orbit satellites useful for?

Answer 26

Monitoring the weather, making scientific observations about places which are unreachable, and military applications.
Because they are smaller orbital periods, they require less powerful transmitters and can potentially orbit across the entire Earth’s surface.