Gravitational Fields/ Orbits

Question 1

Gravitational field def

Answer 1

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

Question 2

Can gravitational fields be repulsive?

Answer 2

No

Question 3

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

Answer 3

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

Question 4

What do the components of the gravitational field strength equation mean

Answer 4

Question 5

What does the gravitational field strength equation show

Answer 5

• 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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

Question 8

Why are gravitational fields non-uniform

Answer 8

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

Question 9

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

Answer 9

The gravitational field strength g is the same throughout

Question 10

When is an object regarded as a point mass

Answer 10

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

Question 11

What is Newton’s law of gravitation

Answer 11

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

Question 12

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

Answer 12

Question 13

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

Answer 13

Question 14

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

Answer 14

Question 15

Is gravitational field strength vector or scalar

Answer 15

vector

Question 16

Describe the direction of gravitational field strength

Answer 16

• 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

Question 17

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

Answer 17

Question 18

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

Answer 18

0

Question 19

Gravitational potential definition

Answer 19

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

Question 20

What is the symbol and unit of gravitational potential

Answer 20

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

Question 21

Why is gravitational potential always a negative value

Answer 21

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

Question 22

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

Answer 22

because the gravitational potential increases with distance from a mass

Question 23

What do the components of the gravitational potential equation mean

Answer 23

Question 24

Is gravitational potential vector or scalar

Answer 24

Vector

Question 25

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

Answer 25

Question 26

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

Answer 26

Question 27

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

Answer 27

Question 28

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

Answer 28

Question 29

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

Answer 29

Question 30

What is the work done in moving a mass equal to

Answer 30

the change in gravitational potential energy

Question 31

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

Answer 31

Question 32

What is this equation only used for

Answer 32

- 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.

Question 33

What do equipotential lines/ surfaces join

Answer 33

points that have the same gravitational potential

Question 34

What are the rules of equipotential lines/surfaces

Answer 34

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)

Question 35

What are equipotential lines like in radial/ uniform fields

Answer 35

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

Question 36

Is work done as an object moves along an equipotential line

Answer 36

No

Question 37

Draw the equipotential lines in a radial field

Answer 37

Question 38

Draw the equipotential lines in a uniform field

Answer 38

Question 39

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

Answer 39

Question 40

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

Answer 40

Question 41

What is Kepler's third law

Answer 41

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

Question 42

How can Kepler's third law be summarised

Answer 42

Question 43

How can an orbiting satellite's total energy be calculated

Answer 43

Total energy = Kinetic energy + Gravitational potential energy

Question 44

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

Answer 44

- 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

Question 45

Escape velocity def

Answer 45

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)

Question 46

When does an object reach escape velocity

Answer 46

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

Question 47

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

Answer 47

Question 48

What is the derive version of the escape velocity equation

Answer 48

Question 49

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

Answer 49

- 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

Question 50

What is a synchronous orbit

Answer 50

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

Question 51

What do synchronous orbits usually refer to

Answer 51

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

Question 52

What is a geosynchronous orbit

Answer 52

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

Question 53

Describe the key features of a geosynchronous orbit

Answer 53

- 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

Question 54

What are geosynchronous orbits used for and how does this work

Answer 54

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

Question 55

What are low orbits

Answer 55

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

Question 56

What are low orbits useful for

Answer 56

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

Question 57

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Question 59

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Question 61

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Question 62

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