Gravitational Fields

Question 1

State 3 similarities between gravitational and electric fields

Answer 1

• both follow inverse square law
• both use field lines to be represented
• both have equipotential surfaces

Question 2

State 2 differences between gravitational and electric fields

Answer 2

• in gravitational fields, the force is always attractive / in electric fields the force can be attractive or repulsive
• electric force acts on charges / gravitational force acts on mass

Question 3

Define newtons law of gravitation

Answer 3

The magnitude of the gravitational force between two objects is directly proportional to the product of their masses and inversely proportial to the square of the distance between them

Question 4

For two objects where is the distance between them measured between

Answer 4

Between the two centres of masses

Question 5

What are the two types of gravitational field

Answer 5

• uniform field
• radial field

Question 6

Describe a uniform field

Answer 6

Exerts the same gravitational force on a mass everywhere in the field

Question 7

Describe uniform field lines

Answer 7

Parallel to each other and equally spaced

Question 8

Describe a radial field

Answer 8

The force exerted depends on the position of the object in the field
As distance increases, force decreases

Question 9

Define gravitational field strength

Answer 9

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

Question 10

What is the formula for gravitational field strength in a uniform field

Answer 10

g = F / m

Question 11

What is the formula for gravitational field strength for a radial field

Answer 11

g = G M / r^2

Question 12

Define gravitational potential at a point

Answer 12

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

Question 13

What is the value of gravitational potential at infinity

Answer 13

0

Question 14

Why is gravitational potential always negative

Answer 14

Gravitational potential at infinity is 0, and to move an object to a given point energy is released as gravitational potential energy

Question 15

Define gravitational potential difference

Answer 15

The energy needed to move a unit mass between 2 points

Question 16

Describe the potential on an equipotential surface

Answer 16

It’s constant everywhere

Question 17

Explain graivtional potential difference and work done when moving along an equipotential surface

Answer 17

When moving along an equipotential surface, the gravitational potential difference is zero as all points have equal potential so no work is done

Question 18

Describe the relationship between gravitational potential and distance between the centres of two objects

Answer 18

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

Question 19

How do you find gravitational field strength at a certain distance ( using a graph)

Answer 19

On graph of G against r, draw a tangent to the curve at that distance and calculate gradient, then multiply gradient by -1

Question 20

What is the area under a graph of g against r

Answer 20

The change in gravitational potential difference

Question 21

Define Keplers 3rd law

Answer 21

The square of the orbital period is directly proportional to the cube of the radius

T^2 = k R^3

Question 22

Write the proof for keplers 3rd law

Question 23

Give the equation for the total energy of a satellite

Answer 23

Total energy of satellite = kinetic energy + potential energy

Question 24

How could you describe the total energy of a satellite

Answer 24

It’s always constant

Question 25

Define the escape velocity of an object

Answer 25

The minimum velocity it must travel at to escape the gravitational field at the surface of the mass

Question 26

Describe the energy of an object at escape velocity

Answer 26

Kinetic energy = gravitational potential energy

Question 27

What is the formula for escape velocity

Answer 27

V = (2GM / r )^1/2

Question 28

What is the relationship between escape velocity and mass

Answer 28

Escape velocity is independent of mass of object

Question 29

What is a synchronous orbit

Answer 29

One where the orbital period of the satellite is equal to the rotational period of the object its orbiting

Question 30

What is the orbital period of a geostationary satellite

Answer 30

24 horus

Question 31

Explain the movement of a geostationary orbit in relation to the earth

Answer 31

It’s always above the same point because they orbit directly above the equator

Question 32

Give a use for a geostationary satellite

Answer 32

Sending TV and telephone signals

Question 33

Why are geostationary satellites good for sending TV and telephone signals

Answer 33

It’s always above the same point on the earth so you don’t have to alter the plane of an aerial or transmitter

Question 34

What is the formula for time period

Answer 34

4pi^2 T^2 = ——— X r^3 GM

Question 35

Why do low orbital satellites have much smaller orbital periods than geostationary satellites

Answer 35

They have lower orbits so travel faster

Question 36

Give a difference between low orbital satellites and geostationary satellites

Answer 36

Low orbital satellites have a much smaller orbital period

Question 37

Give 3 uses of a low orbital satellite

Answer 37

- monitoring the weather - making scientific observations about places which are unreachable - military applications

Question 38

Why are low orbital satellites good for what they’re used for

Answer 38

They require less powerful transmitters as they have smaller orbital periods