Chapter 5.4 - Gravitational Fields

Question 1

What causes gravitational fields

Answer 1

Objects having mass

Question 2

How a spherical mass can be modelled

Answer 2

As a point mass

Question 3

How do gravitational field lines work

Answer 3

They point in the direction that a mass would move and the closer together they are the stronger the field

Question 4

Gravitational field strength

Answer 4

The force in newtons per unit mass

Question 5

Equation for gravitational field strength in terms of forces (g=)

Answer 5

g=F/m

Question 6

How are gravitational fields related to electric fields

Answer 6

They are both a form of a field giving rise to a force

Question 7

What is the force between two masses proportional to

Answer 7

The product of the masses

The inverse square of the separation

Question 8

Equation for gravitational field strength in terms of distance

Answer 8

g = -GM/r^2

Question 9

Why do we not have to consider changing gravitational field strength when considering problems on earths surface

Answer 9

The change in distance is so small that the field can be considered uniform at the surface (this also applies to direction since we can consider the surface flat)

Question 10

Relation between acceleration of free fall and gravitational field strength

Answer 10

They have the same numerical value (9.81) but are different things with different units

Question 11

Keplers first law

Answer 11

Planets move around the Sun in an ellipse, with the Sun at one focus of the ellipse

Question 12

Keplers second law

Answer 12

A line joining the Sun to a planet will sweep out equal areas in equal times

Question 13

Keplers third law

Answer 13

T^2 ∝ r^3

where T is the period and r is the mean distance

Question 14

How centripetal force relates to gravity

Answer 14

The centripetal force on a planet is provided by the gravitational force between it and the Sun

Question 15

How to derive the constant for keplers third law

Answer 15

Equation centripetal force to the gravitational force

Question 16

Geostationary Orbit

Answer 16

An orbit directly over the equator with the same angular speed as the Earths rotation, causing it to always be in the same place in the sky relative to the Earth

Question 17

Period of a geostationary orbit

Answer 17

1 day

Question 18

Uses of geostationary orbits

Answer 18

Telecommunications. A satellite dish can be pointed at a geostationary satellite and can remain stationary because the satellite is stationary relative to the Earth

Question 19

How to calculate the orbital radius of a geostationary satellite

Answer 19

Keplers third law with the constant

Question 20

Gravitational potential at a point

Answer 20

The work done to bring a unit mass from infinity to that point

Question 21

Gravitational potential at infinity

Answer 21

0

Question 22

How to calculate the energy required to move an object over a gravitational potential difference

Answer 22

W=mΔV

where V is the gravitational potential

Question 23

What a force-distance graph for gravitational force from a sphere mass looks like

Answer 23

1/x

x > r where r is the radius of the sphere

Question 24

Area under a force-distance graph

Answer 24

Work done to move the object between the two points

Question 25

What is gravitational potential energy (over large distances) proportional to

Answer 25

Product of the masses

inverse of the distance

Question 26

Escape velocity

Answer 26

Minimum velocity required to escape a gravitational field i.e. the kinetic energy of the object is equal to the magnitude of its gravitational potential energy

Question 27

How to calculate escape velocity

Answer 27

KE=GPE

Question 28

How gravity relates to atmospheric thickness

Answer 28

The more massive a planet is the more kinetic energy will be needed to overcome its gravitational energy and therefore a higher escape velocity will be required, making it less likely that a particle will reach this velocity and escape. Therefore more massive planets have thicker atmospheres.