Section 7.1 Gravitational Fields

Question 1

What is a test mass?

Answer 1

A mass that has a negligible affect on the field

Question 2

Define gravitational field strength

Answer 2

The force per unit mass

Question 3

Show how acceleration is equal to the gravitation field strength when a mass is in free fall

Answer 3

a = F/m
a = gm/m
a = g

Question 4

What is the unit of g?

Answer 4

N/kg

Question 5

What does gravity affect?

Answer 5

Masses

Question 6

What is a field line?

Answer 6

The path that a mass would take when placed in a gravitational field

Question 7

What is a uniform field?

Answer 7

A field where the field strength is constant

Question 8

What is a radial field?

Answer 8

Field strength is proportional to 1/r²

Question 9

What is gravitational potential energy?

Answer 9

The work done moving a mass from infinity to a distance r away from the centre of the field

Question 10

Why is gravitational potential energy negative?

Answer 10

because the potential energy at infinity is 0

Question 11

What is gravitational potential?

Answer 11

The work done moving a mass from infinity to a distance r away from the centre of the field

Question 12

What is an equipotential?

Answer 12

A surface of constant potential

Question 13

True or false equipotentials are at right angles to field lines?

Answer 13

True

Question 14

What is the eqaution for force in a radial field?

Answer 14

F = -G.M.m/r²

Question 15

What is the equation for gravitational field strength in a radial field?

Answer 15

g = -G.M/r²

Question 16

What is the equation for work done in a radial field?

Answer 16

W= -G.M.m/r

Question 17

What is the formula for gravitational potential?

Answer 17

V = -G.m/r

Question 18

What is the relationship between Force and work done?

Answer 18

Work done is the area under a force vs r graph

Question 19

What is the relationship between gravitational field strength and gravitational potential?

Answer 19

Potential is the area under a field strength vs r graph

Question 20

What is Keplers 3rd law?

Answer 20

• T² α r³
• square of the orbital period is directly proportional to the cube of the radius

Question 21

What is the formula for orbital velocity?

Answer 21

v= √G.M/r

Question 22

How is formula for orbital velocity derrived?

Answer 22

By equating force in radial field to centripetal force

Question 23

What is the formula that shows the formula for keplers 3rd law

Answer 23

T² =(4π/G.M).r³

Question 24

What is newtons law of gravitation?

Answer 24

• Gravity acts on any objects which have mass and is always attractive.
• force between two masses is directly proportional to the product of the masses
• inversely proportional to the square of the distance between them
• Modelled on point masses

Question 25

How is the formula for keplers 3rd law derived?

Answer 25

Equating force ina radial field and centripetal force

Question 26

What is the eqaution for escape velocity?

Answer 26

v = √ 2GM/r

Question 27

What is escape velocity?

Answer 27

the minimum velocity it must travel at, in order to escape the gravitational field at the surface of a mass

Question 28

How is the formula for escape velocity derrived?

Answer 28

Equating work done formula in a radial field and kinetic energy

Question 29

What is the total energy of a satelite?

Answer 29

Total energy of a satellite = kinetic energy + potential energy

Question 30

What is the formula for total energy of a satelite?

Answer 30

E = -GMm/2r

Question 31

What is a synchronous orbit?

Answer 31

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

Question 32

What is geostationary satellite?

Answer 32

they always stay above the same point on the Earth

Question 33

features of a geostationary satelelite?

Answer 33

Orbital period matches Earth’s rotational period exactly  Satellite maintains same position relative to Earth  Only one particular orbit radius is possible  Travels west to east above Equator (in same direction as Earth’s rotation)  Orbit height is greater than polar orbit satellite  Speed is less than that of polar orbiting satellite  Scans a restricted (and fixed) area of the Earth’s surface only  Applications: telecommunications generally, cable and satellite TV, radio, digital information, etc.  Satellite is in continuous contact with transmitting/receiving aerial  Aerial can be in a fixed position