Gravitational fields - A level Physics Edexcel

Define gravitational field

A region in which a mass experiences a force

What does Newton's third law mean for the force?

Newton's third law means that force is the same for both masses

What are gravitational forces always?

Attractive

What do the field lines indicate?

The direction of the gravitational force

What do the arrows on gravitational field lines point towards?

The centre of the mass that causes the field

What kind of field is around a mass?

A radial field

What are the field lines like in a uniform field?

Parallel and equidistant

What are the two possible units for g?

Nkg^-1 or ms^-2

Define gravitational field strength

Force per unit mass

What is a satellite?

A satellite is a body that circles around another body in space

Examples of natural satellites

Moons, stars, asteroids, planets and comets

What can stars be satellites around?

Around the galactic centre or in a binary system

What are a star and planet actually orbiting when a planet appears to orbit a star?

Their common centre of mass

Examples of artificial satellites

Hubble and ISS

What's Kepler's law?

Consider two masses in orbit under each others gravity. If one of the masses is a lot bigger than the other, it stays virtually at rest whilst the other orbits around it.

What is centripetal force the only?

Gravitational force on a body in orbit

Define T

The time it takes for the satellite to make one complete circuit

What is Kepler's 3rd law?

T^2=(4pi^2/GM)r^3

What is Kepler's law of proportionality?

T^2 is proportional to r^3

Where do geostationary satellites orbit above?

The equator

What do geostationary satellites have a common axis of?

Rotation centred on the Earth's centre

How high are geostationary satellites above the Earth's surface?

36000km

How far can one geostationary satellite transmit?

To nearly half the globe

What are the advantages of geostationary satellites?

Satellite dishes do not need to be moved to point at it and you can pick up a signal 24 hours a day

What are geostationary satellites used for?

Communications, weather observation and satellite TV

Describe polar orbits

Lower and faster than geostationary

What satellites use LEOs?

ISS and spy satellites

Whats the equation for gravitational potential using energy?

V=E/M

What does V depend on?

The mass that causes the field and distance from the centre of mass

Where is the grand level of gravity?

Infinity

Define V in a gravitational field

The work done against gravity to transfer a 1kg mass from its position in a G field to infinity

Why must V be negative?

Gravity pulls masses together. You only do work when the mass is moved further away from another mass and closer to infinity.

What is an equipotential in a gravitational field?

A line which joins regions of equal gravitational potential

Why is no work done if you travel along an equipotential?

There is no change in GPE

Why aren't equipotential lines equally spaced?

They get further apart as the field strength drops off