Gravitational fields Flashcards
How do you represent the gravitational field around a mass?
Straight lines pointing towards the centre of the mass
Shape of the gravitational field of the Earth
Radial, but assumed to be uniform over short distances
What is gravitational field strength (g)?
The force per unit mass acting at a point, NKg^-1
What is Newton’s Law of gravitation?
Any two point masses attract each other with a force that is proportional to each of their masses and inversely proportional to the square of the distance between them
What are some of the functions of satellites?
Gps/navigation Communications Telescopes/astronomy Weather Military
How do satellites stay in orbit?
They have centripetal force provided by their weight
What are Kepler’s Laws?
1st law - Orbital paths are elliptical
2nd law - The speed of a planet changes i.e they move faster when close to the sun
3rd law - The time period squared is proportional to the mean distance squared
Don’t need to learn these, but they are useful, especially 3rd law which provides an easier route to answer some questions (T^2 / r^3 = T^2 / r^3 to compare 2 situations)
How do geostationary satellites work?
They have the same angular velocity as the Earth, so they will stay at the same point above the Earth’s surface. They have to be over the equator
Why are geostationary satellites particularly useful in telecommunications?
As they stay above the same point on the Earth, a receiver will point in one direction only and will not need to track the satellite, so there will be constant transmission
Why are geostationary satellites expensive?
They are expensive because of the cost required to get it to the very high orbit it needs, and to maintain the satellite itself
What is gravitational potential?
The amount of work done per unit mass to move an object from an infinite distance to that point in the field
Where does the minus sign in V = -GM/r come from?
As gravitational fields are always attractive, objects always gain energy when moving to a point in the field, so gravitational potential is always negative
What is gravitational potential energy?
The energy an object has due to its position in a gravitational field
What are the similarities between electric and gravitational fields?
Both are force fields - a quantity placed in the field experiences a force
Both are inverse square relationships with force (F∝1/r^2)
A particle moving at right angles to the field will follow a parabolic path
Both have an ‘infinite’ range
What are the differences between electric and gravitational fields?
Electric fields can be attractive and repulsive, gravitational fields can only be attractive
Gravitational forces are much weaker compared to electrical forces on a similar scale (gravity dominates the large scale, electrical dominate small scale)
A charged particle can be shielded from an electric field but a mass cannot be shielded from a gravitational field
