Gravitational Fields

Question 1

What is a force field

Answer 1

An area in which an object experiences a non-contact force

Question 2

How are force fields represented

Answer 2

• Vectors
• Diagrams containing field lines, the distance between field lines represents the strength of the force exerted by the field in that region

Question 3

How are force fields formed

Answer 3

• Gravitational fields - formed during the interaction of masses
• Electric fields - formed during the interaction of charges

Question 4

What are the similarities and differences between gravitational fields and electric fields

Answer 4

Similarities -

• Forces both follow an inverse-square law
• Use field lines to be represented
• Both have equipotential surfaces

Differences -

• In gravitational fields, the force exerted is always attractive, while in electric fields the force can be either repulsive or attractive
• Electric force acts on charge, while gravitational force acts on mass

Question 5

What is gravity

Answer 5

The universal attractive force which acts between all matter and is always attractive

Question 6

What is Newton’s law of Gravitation (The Inverse Square Law)

Answer 6

Newton’s Law of Gravitation assumes that the gravitational force between two masses is:

• Always an attractive force
• Directly proportional to the product of the masses
• Inversely proportional to the sqaure of the distance between them

Question 7

What is the Equation for Gravitational Force

Answer 7

F = G x M1 x M2 / r²

Gravitational Force = Gravitational Constant x Mass 1 x Mass 2 / Distance²

Question 8

What are the two types of gravitational field

Answer 8

• Uniform fields - exerts the same gravitational force on a mass everywhere in the field (shown by the parallel and equally spaced field lines)
• Radial fields – the force excreted depends on the position of the object in the field ( as an object moves further away from the center, the magnitude of force would decrease because the distance between field lines decreases)

Question 9

What is gravitational field strength (g)

Answer 9

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

Question 10

What are the equations for gravitational field strength

Answer 10

g = F / m

Gravitatioanl Field Strength = Force / Mass

or

g = GM / r² (For radial fields only)

Gravitational Field Strength = Gravitational Constant x Mass / Distance²

Question 11

What is Gravitational Potential

Answer 11

The work done per unit mass when moving an object from infinity to that point

Question 12

Why is Gravitational Potential always negative

Answer 12

• Gravitational potential at infinity is zero
• As an object moves from infinity to a point, energy is released as the gravitational potential energy is reduced
• Therefore gravitational potential is always negative

Question 13

What is the Equation for Gravitational Potential

Answer 13

V = − GM / r (For a radial field)

Gravitational Potential = - Gravitational Constant x Mass / Distance

Question 14

What is the Equation for Work Done

Answer 14

Work done = mΔV

Work Done = Mass x Graviational Potential

Question 15

What is an Equipotential Surface

Answer 15

• A surface where the potential is constant everywhere
• As these points all have equal potential, the gravitational potential difference is zero when moving along the surface, so no work is done when moving along an equipotential surface

Question 16

Describe a graph of Gravitational Potential against Distance

Answer 16

Gravitational potential is inversely proportional to the distance between the centres of the two objects

Question 17

Describe the graph of Gravitational Field Strength against Distance

Answer 17

You can find the gravitational potential difference by finding the area under the curve.

Question 18

What is Kepler’s Third Law

Answer 18

The square of the orbital period (T)of an object is directly proportional to the cube of its radius (r)

T² ∝ r³

Question 19

What is the escape velocity of an object

Answer 19

• The minimum velocity it must travel at, in order to escape the gravitational field at the surface of a mass
• This is the velocity at which the object’s kinetic energy is equal to the magnitude of its gravitational potential energy
• GMm/r = 1/2mv²

Question 20

State two reasons why rockets launched from the Earth’s surface do not need to achieve escape velocity to reach their orbit

Answer 20

Energy is continually added in flight through fuel and less energy is needed to achieve orbit than to escape from the Earth’s gravitational field

Question 21

How do you calculate the total energy of an orbiting satellite

Answer 21

Kinetic Energy + Potential Energy

Question 22

What is represented by gravitational field lines

Answer 22

Gravitational Field lines show the direction of force on a mass

Question 23

What is a Synchronous Orbit

Answer 23

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

Question 24

What is a Geostationary satellite

Answer 24

A satellite that has an orbital period of 24 hours so they always stay above the same point on the Earth (the equator)

Question 25

What are Low Satellites orbits used for

Answer 25

• Low satellites have lower orbits in comparison to geostationary satellites, therefore they travel much faster meaning their orbital periods are much smaller
• Therefore these satellites can potentially orbit across the entire Earth’s surfac
• The satellites are useful for monitoring the weather, making scientific observations about places which are unreachable and military applications
• They can also be used for communications but because they travel so quickly, many satellites must work together to allow constant coverage for a certain region