Chapter 18 - Gravitational Fields

Question 1

What is a field?

Answer 1

A field is a region of space within bodies experience a force

Question 2

What is a gravitational field?

Answer 2

Gravitational fields exist around bodies which have mass and exert a force on other bodies with mass

Question 3

What is an electrostatic field?

Answer 3

Electrostatic fields exist around bodies which are charged and stationary, and exert a force on other charged bodies

Question 4

What is a magnetic field?

Answer 4

Magnetic fields exist around bodies which are magnetic and exert a force on other magnetic bodies

Question 5

What are the properties of field lines?

Answer 5

Field lines density determine the strength of the field
Fields are vectors and have a direction
Fields lines never cross
When bodies interact, the field densities and patterns change
Fields are uniform or radial

Question 6

What is the definition of ‘g’?

Answer 6

The force per unit of mass exerted upon a test mass

Question 7

What is the formula for gravitational field strength?

Answer 7

g = F/m

Gravitational Field Strength = Force / Mass

Question 8

What does gravitational field strength depend on on a planet?

Answer 8

Mass
Radius

Question 9

What is the relationship between gravitational field strength and mass?

Answer 9

‘g’ is directly proportional to mass

Question 10

What is the relationship between gravitational field strength and displacement?

Answer 10

‘g’ is inversely proportional to the square of displacement

Question 11

What is Newton’s law of gravitation used for?

Answer 11

Determines the force of gravity acting between any two bodies

Question 12

What does Newton’s law of gravitation assume about objects?

Answer 12

Point objects
Negligible size/volume such that all mass is in a single point in space

Question 13

What does Newton’s law of gravitation assume about gravitational force between bodies?

Answer 13

Always an attractive force
Proportional to mass of each body
Inversely proportional to square of distance between bodies

Question 14

What is the formula for gravitational force?

Answer 14

F = GMm / r²

Gravitational force = (Gravitational field constant)(Larger mass)(Smaller mass) / (Distance between centre of mass of bodies)²

Question 15

What is the value of the gravitational field constant?

Answer 15

6.67 x 10^-11 Nm²kg^-2

Question 16

What can be assumed about the strength of gravity close to earth?

Answer 16

Practically uniform

Question 17

What forces act on a satellite orbiting earth?

Answer 17

Only force is the gravitational attraction
Its always falling towards earth but speed keeps it at same height

Question 18

What is the formula for the speed of a mass in stable orbit?

Answer 18

v = (GM/r)^1/2

Question 19

Why do all orbiting satellites at a given height travel around at the same speed?

Answer 19

They are above the atmosphere, so no air resistance slows them down, so mass is irrelevant and speed is constant

Question 20

What are the modern uses of satellites?

Answer 20

Communications
Military uses
Scientific research
Weather and climate
Global positioning

Question 21

What are the different types of satellites?

Answer 21

Polar
Low
Geostationary

Question 22

What are polar orbiting satellites?

Answer 22

Polar orbits circle the poles and give a complete view of the Earth as the Earth rotates beneath the satellite
Used for mapping and reconnaissance

Question 23

What are low earth orbit satellites?

Answer 23

Satellites in this orbit take a short time to orbit Earth, less than 2 hours

Question 24

What are geostationary satellites?

Answer 24

Geostationary satellites are in an orbit so that it remains above the same point of Earth whilst the Earth rotates

Question 25

What are the requirements for a geostationary satellite?

Answer 25

Be in orbit above the Earth's equator Rotate in the same direction as Earth's rotation Have orbital period of 24 hours

Question 26

What is Kepler's third law?

Answer 26

The square of the orbital period 'T' of a planet is directly proportional to the cube of average distance 'r' from the Sun T²/r³=k

Question 27

What is Kepler's first law?

Answer 27

The orbit of a planet is an ellipse with the Sun at one of the two foci

Question 28

What is the shape of a planet's orbit?

Answer 28

Planet orbits have a low eccentricity, and so orbits are modelled as circles despite the aphelion and perihelion

Question 29

What is Kepler's second law?

Answer 29

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

Question 30

What does Kepler's second law mean on a diagram?

Answer 30

The area formed between the triangle and the two points between a planets orbit in the same time interval are equal

Question 31

Why does speed vary in a planet's elliptical orbit?

Answer 31

At the perihelion, the centripetal force is stronger, so velocity needs to be greater At the aphelion, centripetal force is smaller, so velocity can be less

Question 32

What is the graph formed by T² and r³?

Answer 32

The gradient is a constant and equal to 4pi²/GM

Question 33

What is gravitational potential defined as?

Answer 33

At a point in a gravitational field is the work done per unit mass to move an object to that point from infinity

Question 34

Why is gravitational potential negative?

Answer 34

It takes energy to move objects apart Gravitational potential is a maximum at r = 0 so it is negative below

Question 35

What does gravitational potential depend on?

Answer 35

The distance 'r' from the point mass producing the gravitational field The mass 'M' of the point mass

Question 36

What are the relationships of gravitational potential to mass?

Answer 36

V_g is directly proportional to mass

Question 37

What are the relationships of gravitational potential to radius?

Answer 37

V_g is inversely proportional to radius

Question 38

What is the formula for gravitational potential?

Answer 38

V_g = - GM / r (J/kg)

Question 39

What does the graph of 'V_g' and 'r' look like?

Answer 39

They are inversely proportional, and tends toward 0 and 'r' approaches infinity

Question 40

What does the graph of 'V_g' and '1/r' look like?

Answer 40

A straight line with a gradient equal to -GM?

Question 41

How is total gravitational potential energy calculated?

Answer 41

It is a scalar quantity, so the total at any point is equal to the sum of the gravitational potentials from each mass at that point

Question 42

What are the formulae for GPE?

Answer 42

E = mgh E = - GMm / r E = mV_g

Question 43

What is escape velocity?

Answer 43

The velocity at which the loss of KE must equal the gain in GPE and so the object escapes a gravitational field. Equal for all objects on a planet

Question 44

What is the formula for escape velocity?

Answer 44

V = Sqrt( 2GM / r )