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AQA A Level Physics > Gravitational Fields > Flashcards

Gravitational Fields Flashcards

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1
Q

What is a force field

A

A region in which a body experiences a non-contact force

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2
Q

How are force fields represented

A

As vectors

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3
Q

How do force fields arise

A

From the interactions of mass, static charge and between moving charges

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4
Q

What are the similarities between gravitational and electrostatic forces

A

Both have inverse square force laws that have many common features such as :

use of field lines
use of potential concepts

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5
Q

What are the differences between gravitational and electrostatic forces

A

Masses always attract but charges may attract or repel

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6
Q

What is gravity

A

A universal attractive force acting between all matter

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7
Q

gravitational field strength at a point =

A

force due to gravity (weight) / mass

F=mg

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8
Q

what does the equation g = F/m show

A

The larger the mass of an object, the greater its pull on another object

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9
Q

What factors affect the gravitational field strength at the surface of a planet

A

The radius/diameter of a planet

The mass of the planet

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10
Q

What is the direction of a gravitational field represented by

A

Gravitational field lines

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11
Q

In what direction do gravitational field lines around a point mass act

A

Radially inwards

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12
Q

How are gravitational field lines of a uniform field represented

A

By equally spaced parallel lines

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13
Q

What are radial/non-uniform fields

A

When the gravitational field strength is different depending on how far you are from the centre

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14
Q

What is a uniform sphere

A

One where its mass is distributed evenly

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15
Q

How do the gravitational field lines around a uniform sphere and a point mass compare

A

They are identical

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16
Q

How do the directions of the field lines differ for radial and uniform fields

A

Radial fields : Towards the centre of the sphere or point charge

Uniform fields : Towards the surface of the object

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17
Q

What is the mass of a uniform sphere considered to be at its centre

A

A point mass at its centre

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18
Q

Newtons law of Gravitation

A

The gravitational force between 2 point masses is proportional to the product of the masses and inversely proportional to the square of their separation

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19
Q

Gravitational force between two masses =

A

Newtons gravitational constant x point mass(1) x point mass (2) / distance between the centres of the 2 masses ^2

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20
Q

gravitational field strength in a radial field =

A

G x M / r^2

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21
Q

What would the graph of g against r look like

A

When r < R (the radius of the planet) g is directly proportional to r

When r > R, g is inversely proportional to r^2

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22
Q

What is GPE

A

The energy an object has when lifted off the ground

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23
Q

What is the GPE on the surface of the Earth

A

0

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24
Q

What is GPE outside Earth’s surface

A

The energy an object possesses due to its position in a gravitational field

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25
Q

What is gravitational potential at a point also know as

A

the gravitational potential energy per unit mass at that point

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26
Q

Definition of gravitiational potential energy per unit mass at that point

A

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

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27
Q

Why is gravitational potential always a negative value

A

It is defined as 0 at infinity

Since the gravitational force is attractive, work must be done on a mass to reach infinity

28
Q

Why do two points at different distances from a mass have different gravitational potentials

A

The gravitational potential increases with distance from a mass

29
Q

gravitational potential difference =

A

final gravitational potential - initial gravitational potential

30
Q

Gravitational potential, V =

A
  • GM / r
31
Q

Why is the gravitational potential always negative near an isolated mass such as a planet

A

The potential when r is at infinity is defined as 0

Work must be done to move a mass away from a planet

32
Q

What happens to gravitational potential when a mass is closer to a planet

A

Becomes smaller (more negative)

33
Q

inverse square law for g

A

g is directly proportional to 1/r^2

34
Q

is gravitational potential scalar or vector quantity

A

Scalar, unlike gravitational field strength which is a vector

35
Q

Are gravitational forces attractive or repulsive and what does this mean as r decreases

A

Always attractive.
This means that as r decreases, positive work is done by the mass when moving from infinity to that point

36
Q

What happens to gravitational potential when a mass is closer to a planet

A

Gravitational potential becomes smaller and more negative

37
Q

What happens to gravitational potential when a mass moves away from a planet

A

Its gravitational potential becomes larger and less negative until it reaches 0 at infinity

38
Q

equation relating V and g

A

g = - change in gravitational potential (V) / distance from the centre of a point mass (r)

39
Q

Key characteristics of the graph of V against r for a planet

A

Values for V are all negative
As r increases, V against r follows a -1/r relation
Gradient is g at that point
Gradient has shallow increase as r increases
(graph bends to the right and is always under x axis)

40
Q

Key characteristics of the graph of g against r for a planet

A

Values of g all positive
As r increases, g against r follows a 1/r^2 relation - inverse square law
Area under the graph is equal to the change in gravitational potential
Graph has steep decline as r increases
Looks like a ski slope

41
Q

work done in moving a mass against the force of gravity =

A

Mass x change in gravitational potential
∆W = m∆V

42
Q

Change in work done =

A

Change in GPE

43
Q

Change in GPE for an object at a distance from a larger mass to another distance

A

GMm/r1- GMm/r2
​

44
Q

GPE when V = 0

A

GPE = 0

45
Q

Why does the equation for change in GPE for an object at a distance from a larger mass to another distance not involve g

A

g varies for different planets and is no longer a constant outside the surface of a planet

46
Q

Characteristics of equipotential lines

A

Join together points with same gravitational potential
Perpendicular to gravitational field lines in both radial and uniform fields
Represented by dotted lines
NOT VECTORS SO HAVE NO DIRECTION

47
Q

Equipotential lines in a radial field

A

Concentric circles around the planet
Further apart further away from the planet

48
Q

Equipotential lines in a uniform field (near Earth’s surface)

A

Horizontal straight lines
Parallel
Equally spaced

49
Q

Work done moving along an equipotential line or surface

A

No work is done moving along it only between equipotential lines or surfaces as there is no change in gravitational potential along an equipotential line

50
Q

What is the centripetal force needed by a planet to stay in orbit

A

The gravitational force between the Sun and the planet

51
Q

Kepler’s third law

A

For planets or satellites in a circular orbit about the same central body, the square of the time period is proportional to the cube of the radius of the orbit.

T^2 is proportional to r^3

52
Q

Derivation of Kepler’s third law

A
  1. GMm/r^2 = mv^2 /r
  2. v^2 = GM/r
  3. v = 2pi x r / T
  4. v^2 = (2pi x r / T)^2 = GM/r
  5. T^2 = 4pi^2r^3 / GM
53
Q

How can we graphically show the relationship between T and r

A

using log graphs

53
Q

Total energy of an orbiting satellite =

A

KE + GPE

Total energy is always constant

54
Q

What happens to the satellite’s KE and GPE if orbital radius decreases

A

KE increases and GPE decreases

55
Q

What happens to the satellite’s KE and GPE if orbital radius increases

A

KE decreases and GPE increases

56
Q

What happens to the satellite when the radius of the orbit is smaller

A

Larger gravitational force on it
Higher speed
Higher KE
Lower GPE
Shorter orbital timer period

57
Q

Escape velocity

A

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

58
Q

What happens to an objects kinetic energy when it reaches escape velocity

A

it has been transferred to gravitational potential energy

1/2 x m x v^2 = GMm / r

59
Q

escape velocity =

A

root ( 2GM /r)

60
Q

Why do rockets launched from Earth’s surface not need to achieve escape velocity to reach their orbit around Earth

A

They are continuously given energy through fuel and thrust to help them move.
Less energy is needed to achieve orbit than to escape from Earth’s gravitational field

61
Q

Synchronous orbit

A

When an orbiting body has a time period equal to that of the body being orbited and in the same direction of rotation as that body

62
Q

Geosynchronous orbit

A

When the plane of the orbit is directly above the equator.
Always orbits at the same point above Earth’s surface
Orbital time period equal to 24 hours
Moves west to east

63
Q

What are geostationary satellites used for

A

Telecommunication transmissions
Television broadcast

64
Q

Low orbits

A

When the altitude of the satellite is closer to the Earth’s surface

65
Q

Example of low orbits

A

Polar orbit where the satellite orbits around the north and south poles

66
Q

Uses of low orbits

A

Weather
Military applications
This is because they can take high quality photos of the Earth’s surface

AQA A Level Physics (15 decks)

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