# 5.4 Gravitational Flashcards

1
Q

What causes gravitational fields

A

Objects having mass

2
Q

How a spherical mass can be modelled

A

As a point mass

3
Q

How do gravitational field lines work

A

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

4
Q

Gravitational field strength

A

The force in newtons per unit mass

5
Q

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

A

g=F/m

6
Q

How are gravitational fields related to electric fields

A

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

7
Q

What is the force between two masses proportional to

A

The product of the masses

The inverse square of the separation

8
Q

Equation for gravitational field strength in terms of distance

A

g = -GM/r^2

9
Q

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

A

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)

10
Q

Relation between acceleration of free fall and gravitational field strength

A

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

11
Q

Keplers first law

A

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

12
Q

Keplers second law

A

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

13
Q

Keplers third law

A

T^2 ∝ r^3

where T is the period and r is the mean distance

14
Q

How centripetal force relates to gravity

A

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

15
Q

How to derive the constant for keplers third law

A

Equation centripetal force to the gravitational force

16
Q

Geostationary Orbit

A

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

17
Q

Period of a geostationary orbit

A

1 day

18
Q

Uses of geostationary orbits

A

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

19
Q

How to calculate the orbital radius of a geostationary satellite

A

Keplers third law with the constant

20
Q

Gravitational potential at a point

A

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

21
Q

Gravitational potential at infinity

A

0

22
Q

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

A

W=mΔV

where V is the gravitational potential

23
Q

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

A

1/x

x > r where r is the radius of the sphere

24
Q

Area under a force-distance graph

A

Work done to move the object between the two points

25
Q

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

A

Product of the masses

inverse of the distance

26
Q

Escape velocity

A

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

27
Q

How to calculate escape velocity

A

KE=GPE

28
Q

How gravity relates to atmospheric thickness

A

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.