Gravitational Fields Flashcards
Definition of a gravitational field
The force per unit mass on a small test mass object placed in the field
What are the two types of g fields?
Radial fields –> directed towards the centre
Uniform fields –> equally spaced and towards the ground
Define gravitational potential
The gravitational potential energy per unit mass of a test mass at a point in the gravitational field
The work done per unit mass to move a small object from infinity to that point, where the GF at infinity is 0
Define gravitational potential energy
The work done to move a small object from infinity to that point where GF at infinity is 0
Define equipotentials
Surfaces with equal potentials, so no work needs to be done to move the object across that surface
Define potential gradient
The change of potential per metre at that point
What are the assumptions of Newton’s Law of gravitation between any two objects
Force is always attractive
Proportional to the mass of an object
Inversely proportional to the square of the distance between the objects
What are the conditions for a satellite to have a stable orbit?
The gravitational field strength = centripetal force that keeps the satellite in orbit
The total energy of the satellite in orbit:
E = -GMm/2r
What happens to the speed of the satellite as it gets closer to Earth
Since v^2 = GM/r, as the satellite gets closer to the Earth, the speed increases.
Why do geostationary satellites orbit above the equator?
Because the rotation about the equator has a time period of exactly 24h
So the satellite will remain in a fixed position as its time period = time period of rotation of the Earth
Why is the GPE negative?
As GPE = 0 at infinity, work must be done to reach infinity as the force is attractive
