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A Level Physics > Gravitational Fields/ Orbits > Flashcards

Gravitational Fields/ Orbits Flashcards

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1
Q

Gravitational field def

A

A region of space where a mass experiences a force due to the gravitational attraction of another mass

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2
Q

Can gravitational fields be repulsive?

A

No

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3
Q

What is the relationship between the size of a gravitational field and mass

A
  • There is a greater gravitational force around objects with a large mass
  • There is a smaller gravitational force around objects with a small mass
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4
Q

What do the components of the gravitational field strength equation mean

A
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5
Q

What does the gravitational field strength equation show

A
  • The larger the mass of an object, the greater its pull on another object
  • On planets with a large value of g, the gravitational force per unit mass is greater than on planets with a smaller value of g
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6
Q

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

A
  • The radius (or diameter) of the planet
  • The mass (or density) of the planet
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7
Q

Draw the gravitational field lines for a point mass and a uniform gravitational field

A
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8
Q

Why are gravitational fields non-uniform

A

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

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9
Q

Why are parallel field lines on the Earth’s surface considered uniform

A

The gravitational field strength g is the same throughout

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10
Q

When is an object regarded as a point mass

A

When a body covers a very large distance as compared to its size, so, to study its motion, its size or dimensions can be neglected

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11
Q

What is Newton’s law of gravitation

A

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

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12
Q

What do the components of Newton’s law of gravitation equation mean

A
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13
Q

Describe the inverse square law within Newton’s law of gravitation equation

A
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14
Q

Describe the gravitational field strength (in a radial field) equation

A
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15
Q

Is gravitational field strength vector or scalar

A

vector

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16
Q

Describe the direction of gravitational field strength

A
  • The direction of g is always towards the centre of the body creating the gravitational field
  • This is the same direction as the gravitational field lines
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17
Q

Draw and describe the graph of a the distance away from a planet, against its gravitational field strength

A
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18
Q

What is the G.P.E at the surface of the earth assumed to be (when objects are near the earth’s surface)

A

0

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19
Q

Gravitational potential definition

A

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

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20
Q

What is the symbol and unit of gravitational potential

A

The symbol is V and is measured in J kg-1

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21
Q

Why is gravitational potential always a negative value

A

Because:
- It is defined as zero at infinity
- Since the gravitational force is attractive, work must be done on a mass to reach infinity
- This means that the gravitational potential is negative on the surface of a mass (such as a planet), and increases with distance from that mass (becomes less negative)
- Work has to be done against the gravitational pull of the planet to take a unit mass away from the planet
- The gravitational potential at a point depends on the mass of the object producing the gravitational field and the distance the point is from that mass

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22
Q

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

A

because the gravitational potential increases with distance from a mass

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23
Q

What do the components of the gravitational potential equation mean

A
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24
Q

Is gravitational potential vector or scalar

A

Vector

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25
Q

What do the components of the g, V, r equation mean

A
26
Q

Draw and describe the key features of the graph of V against r

A
27
Q

Draw and describe the key features of the graph of g against r

A
28
Q

What is the equation for gravitational field strength that is not on the formula sheet and what does it mean

A
29
Q

What do the components of the work done in moving a mass equation mean

A
30
Q

What is the work done in moving a mass equal to

A

the change in gravitational potential energy

31
Q

How can ‘the change in G.P.E, or work done, for an object of mass m at a distance r1 from the centre of a larger mass M, to a distance of r2 further away’ be written in an equation

A
32
Q

What is this equation only used for

A
  • for an object lifted in a uniform gravitational field (close to the Earth’s surface)
  • The new equation for G.P.E will not include g, because this varies for different planets and is no longer a constant (decreases by 1/r2) outside the surface of a planet.
33
Q

What do equipotential lines/ surfaces join

A

points that have the same gravitational potential

34
Q

What are the rules of equipotential lines/surfaces

A

They are always:
- Perpendicular to the gravitational field lines in both radial and uniform fields
- Represented by dotted lines (unlike field lines, which are solid lines with arrows)

35
Q

What are equipotential lines like in radial/ uniform fields

A

In a radial field (eg. a planet), the equipotential lines:
- Are concentric circles around the planet
-Become further apart further away from the planet

In a uniform field (eg. near the Earth’s surface), the equipotential lines are:
- Horizontal straight lines
- Parallel
- Equally spaced

36
Q

Is work done as an object moves along an equipotential line

A

No

37
Q

Draw the equipotential lines in a radial field

A
38
Q

Draw the equipotential lines in a uniform field

A
39
Q

What are the components of the linear speed in orbit equation, which is not on the formula sheet

A
40
Q

What is the equation relating the time period T and orbital radius r, which is not on the formula sheet

A
41
Q

What is 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

42
Q

How can Kepler’s third law be summarised

A
43
Q

How can an orbiting satellite’s total energy be calculated

A

Total energy = Kinetic energy + Gravitational potential energy

44
Q

What happens to the KE and GPE of a satellite if there is a change in orbital radius

A
  • If the orbital radius of a satellite decreases its KE increases and its GPE decreases
  • If the orbital radius of a satellite increases its KE decreases and its GPE increases
45
Q

Escape velocity def

A

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

(It is the same for all masses in the same gravitational field ie. the escape velocity of a rocket is the same as a tennis ball on Earth)

46
Q

When does an object reach escape velocity

A

when all its kinetic energy has been transferred to gravitational potential energy

47
Q

What is the equation used for calculating an object’s escape velocity

A
48
Q

What is the derive version of the escape velocity equation

A
49
Q

Why do rockets launched from the Earth’s surface not need to achieve escape velocity

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
50
Q

What is a 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

51
Q

What do synchronous orbits usually refer to

A

Satellites (the orbiting body) around planets (the body being orbited)

52
Q

What is a geosynchronous orbit

A

When the plane of a synchronous orbit is directly above the equator

53
Q

Describe the key features of a geosynchronous orbit

A
  • Remains directly above the equator
  • Is in the plane of the equator
  • Always orbits at the same point above the Earth’s surface
  • Moves from west to east (same direction as the Earth spins)
  • Has an orbital time period equal to Earth’s rotational period of 24 hours
54
Q

What are geosynchronous orbits used for and how does this work

A

Geostationary satellites are used for telecommunication transmissions (e.g. radio) and television broadcast:
- A base station on Earth sends the TV signal up to the satellite where it is amplified and broadcasted back to the ground to the desired locations
- The satellite receiver dishes on the surface must point towards the same point in the sky
- Since the geostationary orbits of the satellites are fixed, the receiver dishes can be fixed too

55
Q

What are low orbits

A

Some satellites are in low orbits, which means their altitude is closer to the Earth’s surface

56
Q

What are low orbits useful for

A

Low orbits are useful for taking high-quality photographs of the Earth’s surface. This could be used for:
- Weather
- Military applications

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62
Q
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A Level Physics (33 decks)

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