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A Level Physics > Module 5: Chapter 18 - Gravitational Fields > Flashcards

Module 5: Chapter 18 - Gravitational Fields Flashcards

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1
Q

What is gravity?

A

The universal attraction between any 2 masses

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2
Q

What is gravitational force?

A

An attractive force

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3
Q

What is a gravitational field?

A

A field created around any object with mass, extending all the way to infinity, but diminishing as the distance from the centre of mass of the object increases

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4
Q

What is gravitational potential (u)?

A

The work done per unit mass to bring an object from infinity to a point in the gravitational field

Unit: J Kg⁻¹

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5
Q

What is graviational potential energy?

A

The capacity for doing work as a result of an objects position in a gravitational field

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6
Q

What is graviational field strength?

A

The gravitational force exerted per unit mass at a point within a graviational field

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7
Q

What is Newtons law of universal gravitation?

A

The graviational force between 2 bodies is directly proportional to the product of their masses and inversely proporitonal to the square of the separation

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8
Q

What is the equation for the gravitational force between 2 objects?

A

F = -GMm/r²

G= Universal Gravitational Constant, M =Mass 1, m =Mass 2, r =Separation

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9
Q

Why does the equation for the gravitational force between 2 objects contain a negative sign?

A

To indicate the force is attractive

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10
Q

What is the Universal Gravitational Constant (G)?

A

G = 6.67x10⁻¹¹ Nm²kg⁻²

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11
Q

What is a gravitational field line?

A

The line that indicates the direction of the gravitational force that would act on a test mass placed in the field. The closer the lines are together, the stronger the field

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12
Q

What is a test mass?

A

A mass that is small enough not to affect the shape of the field with its own gravity

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13
Q

What is the equation for gravitational field strength?

A

g = F/m

g = -GM/r²

M = mass of body generating field, m = mass of other body

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14
Q

What is a radial field?

A

A symmetrical field that diminishes with distance² from its centre, such as the gravitational field around a spherical mass or the electrical field around a spherical charged object

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15
Q

When is a gravitiational field a uniform field?

A

When you are very close to the surface of the object

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16
Q

What is the relationship between the Mass of a large body and the linear speed of an orbiting object?

A

M = v²r/G

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17
Q

What is Keplers first law of planetary motion

A

All orbits are elliptical in shape with the body being orbited at one of the 2 foci

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18
Q

What is a perihelion?

A

The point at which an orbiting body is closest to the sun

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19
Q

What is a perigee?

A

The point at which an orbiting body is closest to the earth

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20
Q

What is a periapsis?

A

The point at which an orbiting object is closest to the body it is orbiting

General term

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21
Q

What is an aphelion?

A

The point at which an orbiting body is furthest from the sun

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22
Q

What is an apogee?

A

The point at which an orbiting body is furthest from the earth

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23
Q

What is an apoapsis

A

The point at which an orbiting body is furthest from the body it is orbiting

General term

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24
Q

What are the properties of an ellipse?

A
  • An ellipse has 2 foci
  • The semi-major axis is the distance from the centre (half way between the foci) and the widest part of the ellipse
  • The semi-minor axis is the distance from the centre and the narrowest part of the ellipse
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25
Q

What is the eccentricity of an ellipse?

A

It is how long and narrow it is. The further the foci are from each other, the more eccentric it is.

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26
Q

What is keplers second law of planetary motion?

A

A line joining a planet and the sun sweeps out equal areas during equal intervals of time

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27
Q

When is a planet moving fastest?

A

At the perihelion

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28
Q

When is a planet moving slowest?

A

At the Aphelion

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29
Q

What is keplers third law?

A

The square of the orbital period T of a planet is directly proportional to the cube of its average distance r from the sun
T² ∝ r³

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30
Q

What is the orbital period?

A

The time taken for a satellite to make one complete circuit

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31
Q

What is the equation for keplers third law?

A

T² = r³ 4π²/GM

4π²/GM is constant for a given scenario

32
Q

Derive keplers third law

A

t = s/v
T = 2πr/v
T² = 4π²r²/v²

M = v²r/G
v² = GM/r

T² = 4π²r²/(GM/r)
T² = 4π²r³/GM
T² = r³ 4π²/GM

33
Q

What is the mass of the earth?

A

6x10²⁴ kg

34
Q

What is the orbital radius of a geostationary satellite?

Mass of earth = 6x10²⁴ kg

A

4.2x10⁷ m

35
Q

What is a satellite?

A

A body that orbits around another body in space

36
Q

What is an example of a natural satellite?

A

The moon

37
Q

What are examples of artificial satellites?

A
  • The ISS
  • The Hubble Space Telescope
38
Q

What does the orbit height depend on?

A

The speed

39
Q

In which direction is the velocity of an object compared to the gravitational force?

A

perpendicular

40
Q

What is the equation for orbit velocity?

A

v = root( GM/r )

41
Q

What is a polar orbit?

A
  • A low orbit passing around the earth over the north and south poles
  • Earth rotates underneath them as they orbit
  • Used for large-scale mapping and global weather monitoring
42
Q

What is a geostationary orbit?

A
  • An orbit which stays above the same point on earth
  • Speed of orbit matches the earth’s rotation, so orbit time is 24 hours
  • Used for communications, satellite TV, weather forecasting, intelligence, global positioning system (GPS)
43
Q

What does the graph for gravitational field strength against distance from the centre of a planet look like?

A
44
Q

What is the relationship between gravitational field strength and distance from the centre of a planet when outside the planet?

A

g ∝ 1/r²

45
Q

What is the relationship between gravitational field strength and distance from the centre of a planet when inside the planet?

A

g ∝ r

46
Q

What is the equation for gravitational potential difference?

A

ΔV = W/m

Gravitational Potential difference = work done / mass

47
Q

What is gravitational potential difference?

A

a change in gravitational potential

48
Q

Why can we use “Δ gravitational energy = mgΔh”?

A
  • This equation gives a change in gravitational potential energy, with sea level being given a value of 0. It is NOT the actual gravitational potential energy.
  • It can also only be used very close to the surface of the body as it assumes a constant gravitational field strength
49
Q

How can you get an absolute value for GPE?

A

You need a reference point. It is defined at being 0 at infinity and then measured from there

50
Q

What is the equation for gravitational potential energy?

A

Eₚ = -GMm/r

51
Q

Why is Eₚ always negative?

A

Because work must be done to move m towards infinity against the attraction of the other mass

52
Q

What are the 2 equations for gravitational potential?

A
  • V = Eₚ/m
  • V = -GM/r
53
Q

What are equipotential lines?

A

Lines along which gravitational potential is constant:
* No work is done by gravitational force when a mass is moved along an equipotential line
* Equipotential lines are always perpendicular to field lines

54
Q

How do you calculate the work done to move an object from one point in a gravitational field to another point?

A

Calculate the energy at both points and find the difference

55
Q

How much work must be done to move a spacecraft of mass 2000kg from an orbit of radius 6.6x10⁶m to one of 6.7x10⁶m

Mass of earth = 6x10²⁴kg

A

1.81x10⁹ J

56
Q

What is the work done to escape a planet’s gravity?

A

It is the work done to get from their current position to where their gravitational potential energy is 0J (it is the same as their energy at their initial position)

57
Q

A rocket has a mass of 200kg on a planet with a gravitational potential of -50MJkg⁻¹ at its surface:
a) what is the work done for it to escape the planets gravity?
b) what is the work done to move from a position with gravitational potential -30MJkg⁻¹ to a position with gravitational potential -20MJkg⁻¹

A

a) 1x10¹⁰ J
b) 2x10⁹ J

58
Q

What is the minimum work done to lift an astronaut of mass 80kg from the earths surface to 300km?

earths radius = 6400km, mass of earth = 6x10²⁴ kg

A

2.24x10⁸ J

59
Q

What is potential gradient?

A

The change in potential per metre at a point within a gravitational field. It is the same as gravitational field strength

60
Q

What is the equation for potential gradient?

A

potential gradient = ΔV/Δr

61
Q

Show how potential gradient is the same as gravitational field strength:

A

potential gradient = ΔV/Δr
= (-GM/r)/r
= -GM/r²
= - gravitational field strength

62
Q

Why is potential gradient the negative of gravitational field strength?

A

They act in different directions (potential gradient is away from the body)

63
Q

How do equipotential lines change with distance from the body?

A

The distance between the equipotential lines of regular intervals increases

64
Q

What is the intial velocity given to an object which removes it completely from the bodys gravitational field?

A

The escape velocity

65
Q

What is the intial velocity given to an object of 80kg to completely remove it from the earths gravitational field?

A

11185 ms⁻¹

66
Q

What does the graph of gravitational potential against distance from the earths centre look like?

A
67
Q

How do you calculate the escape velocity of an object?

A

Gravitational potential energy = kinetic energy
0.5mv² = GMm/r

68
Q

What is the escape velocity for a golf ball on the moon

mass of moon = 7.34x10²² kg, mass of golf ball = 46g, radius of moon = 1.64x10⁶m

A

2440ms⁻¹

69
Q

How can you determine the resultant gravitational force acting upon an object if ti is within multiple gravitational fields?

A

Calculate the force acting upon it from each body individually, then calculate the resultant force (in the same way as any other forces)

70
Q

Why can we only see comets for short periods of time?

A

Comet’s orbits are highly elliptical adn they can be clearly seen when they are closest to the sun, however they are travelling fastest at this point so quickly move away. They travel slowest furthest from the sun where they cannot be seen. Therefore they spend most their time where they cannot be seen

71
Q

Why can orbits be modelled as circles?

A

They usually have low eccentricity

72
Q

What does a graph of gravitational potential look like when travelling from the earth to the moon?

A
73
Q

Why does gravitational potential increase with distance from the body?

A

Because it is always negative, so increasing distance may decrease its magnitude but increases its value

74
Q

How does escape velocity vary for different objects?

A

It does not, the escape velocity will be the same for all objects on a given planet regardless of their mass

75
Q

What is escape velocity?

A

The minimum velocity at which an object has just enough energy to leave a specified gravitational field

A Level Physics (29 decks)

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