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A-Level Physics > Gravitational Fields > Flashcards

Gravitational Fields Flashcards

1
Q

What is a gravitational field?

A

A special type of field in which any object with mass experiences a gravitational force

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2
Q

What are gravitational fields set up around?

A

Any objects with a mass

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3
Q

What are gravitational fields always?

A

Attractive

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4
Q

Where are the lines of a gravitational field always directed?

A

Towards the centre of mass of the body

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5
Q

What do gravitational fields map?

A

How a gravitational force would be exerted on a mass

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6
Q

Where do all gravitational fields extend to?

A

Infinity

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7
Q

What can a uniform sphere be considered as?

A

A point mass

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8
Q

What are the field lines like around a point mass?

A

Radial

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9
Q

Are radial fields uniform or non-uniform fields?

A

Non-uniform; the GF strength is different depending on how far object is from the centre of mass

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10
Q

What is a uniform gravitational field?

A

Where the field strength is the same at all points

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11
Q

How is a uniform field represented?

A

Equally spaced parallel gravitational field lines

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12
Q

What are the factors of GF lines?

A
  • They do not cross
  • The arrows on them show the direction of the field
  • They always point to the centre of mass
  • A stronger field is represented by field lines that are closer together
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13
Q

What is gravitational field strength?

A

The force per unit mass at a given point

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14
Q

What is the unit of gravitational field strength?

A

N kg^-1

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15
Q

What is the equation for gravitational field strength?

A

g = F/ m

g = gravitational field strength, N kg^-1
F = force, N
m = mass, kg

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16
Q

What are the factors that affect GF strength on the surface of a planet?

A
  • Radius of the planet
  • The mass or density of the planet
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17
Q

What is the value of g on earth?

A

9.81 N kg^-1

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18
Q

What do gravitational fields give a rise to?

A

(Gravitational) force - it acts on any object with a mass

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19
Q

What is Newtons Law of Gravitation?

A

The gravitational force between 2 masses, M & m, is proportional to the product of their masses and inversely proportional to the square of their separation

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20
Q

What are the proportionality equations of the gravitational force?

A

F ∝ Mm
F ∝ 1/ r^2

so F ∝ Mm/ r^2

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21
Q

What is Newtons Law of Gravitation equation?

A

F = -GMm/ r^2

F = gravitational forces between the 2 masses, N
G = Newtons gravitational constant
M = mass 1, kg
m = mass 2, kg
r = seperation, m

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22
Q

What does the negative sign in Newtons Law of Gravitation equation represent?

A

That the force is attractive

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23
Q

What happens to the GF lines in a radial field?

A

They get further apart from each other

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24
Q

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

A

g = -GM/ r^2

g = GF strength, N kg^-2
G = Newtons gravitational constant
M = mass of an object, kg
r = distance from the centre of mass, m

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25
Q

What is the value of Newtons Gravitational constant?

A

6.67 x 10^-11 Nm^2 kg^-1

26
Q

What is the graph for g against r?

A
27
Q

What can the GF field near the earths surface be described to be?

A

Uniform; meaning the GF strength is constant at every point near the earths surface

28
Q

What is Kepler’s First Law?

A

The orbit of a planet is an ellipse; with the sun at one of 2 foci

29
Q

What is an ellipse?

A

An elongated circle with 2 foci

30
Q

What is eccentricity?

A

A measure of how elongated the circle is

31
Q

What is aphelion?

A

Furthest point from the sun

32
Q

What is perihelion?

A

Closest point to the sun

33
Q

What is Kepler’s Second Law?

A

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

34
Q

What does this show?

A

Area x = area y when time AB = time CD

35
Q

When does the sun move faster?

A

When it is closer to the sun

36
Q

When does the sun move fastest?

A

Perihelion

37
Q

When does the sun move slowest?

A

Aphelion

38
Q

What is Kepler’s Third Law?

A

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

T^2 ∝ r^3

39
Q

How do you relate the gravitational force equation and the centripetal force equation?

A

Typically, most planets and satellites have a near circular orbit. Therefore, the gravitational force between the sun and another planet provides the centripetal force needed to stay in orbit

F = F centripetal

Gmm/ r^2 = mv^2/ r
v^2 = GM/ r

v = sqr (GM/ r)

40
Q

What does equation, v = sqr (GM/ r), indicate?

A

All satellites, whatever their mass, will travel at the same speed in a particular orbital radius

41
Q

Assuming a planet or satellite is travelling in circular motion when in orbit, what equations can be used?

A

v = 2πr/ T

v^2 = GM/ r

so T^2 = (4π^2 / GM) r^3

42
Q

What can Kepler’s Third Law be applied to?

A

Any body is orbit about some larger body ; this means it can be applied to other systems not just the planets in out solar system orbiting the sun

43
Q

What is a geostationary satellite?

A

A satellite which:
- Remains directly above the equator
- Is in the plane of the equator
- Always orbits at the same point above the Earths surface
- Moves from west to east
- Has an orbital time period equal to the earths rotational period of 24 hours

44
Q

What are the uses of satellites?

A
  • Telecommunications transmissions eg. radio
  • Television broadcast
  • Military surveillance
  • Scientific research and analysis
  • Weather and climate analysis
  • Global positioning
45
Q

What is gravitational potential?

A

The work done per unit of mass in bringing a mass from infinity to a defined point

46
Q

What is infinity?

A

A point so far from the object producing the field that its strength equals 0

47
Q

When is gravitational potential at a maximum?

A

At infinity, where its value is 0 J kg^-1

48
Q

Why are gravitational potential values negative?

A

Because all masses are attracted to each other, and gravitational potential considers the energy for 1 mass to move to another

49
Q

When would gravitational potential be positive?

A

When work done is needed to move a point towards infinity

50
Q

What does the gravitational potential at any point is a radial field depend on?

A
  • Distance r from point mass producing the field
  • The mass M of the point mass
51
Q

What is gravitational potential proportional to?

A

Mass of the point mass

52
Q

What is gravitational potential inversely proportional to?

A

The distance from the point mass

53
Q

What is the equation for gravitational potential?

A

Vg = -GM/ r

Vg = gravitational potential
G = gravitational constant
M = mass of body causing field
r = distance from centre of mass to point in the field

54
Q

What is the equation for change in potential?

A

ΔVg =.GM ((1/ initial r) - (1/ final r) )

55
Q

What does a gravitational force-distance graph look like?

A
56
Q

What does the area under a gravitational force-distance graph equal?

A

Work done

57
Q

What are the equation for GPE and work done?

A

E = mVg

E = -GMm/ r

ΔW = mΔVg

ΔGPE = GMm ((1/ initial r) - (1/ final r))

ΔW = work done/energy transferred

58
Q

What is gravitational potential energy?

A

The work done to move the mass from infinity to a point in a gravitational field

59
Q

What happens when an object moves away from the surface of the earth?

A

It gains GPE because it gains gravitational potential

60
Q

What is escape velocity?

A

The minimum speed that will allow an object to escape a gravitational field with no further energy input

61
Q

What is the key feature of escape velocity??

A

It is the same for all masses in the same gravitational field

62
Q

What is the equation for escape velocity?

A

v = sqr( 2GM/ r)

A-Level Physics (23 decks)

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