Gravitational Fields Flashcards
Gravitational field
Region surrounding an object in which it exerts a gravitational force on any other object.
Field line (line of force)
The direction of a filed line indicates the direction of the force. Point towards the centre of mass of the object.
Gravitational field strength, g
The attractive force per unit mass on an object placed in a gravitational field.
Radial field
A field in which all the field lines are straight, directed towards the centre and converge or diverge as if from a single point. The magnitude of g decreases as distance from the massive body increases.
Uniform field
Magnitude and direction of g is the same throughout the field. The field lines are parallel and equally spaced.
Gravitational potential energy
The energy of an object due to its position in a gravitational field. The gpe at any point is the work done to move a small object from infinity to that point. 0 gpe is at infinity. At the earth’s surface gpe is negative as you have to do work to escape from the field (get to 0).
Gravitational potential, V
V at a point in a gravitational field is the work done per unit mass to move a small object from infinity to that point.
Equipotential
A line or surface in a field along which the gravitational potential is constant. Represented as circles around the Earth. Get further apart for equal increases in potential when further from Earth’s surface.
Potential gradient
Potential gradient at a point in a field is the change of potential per unit change of distance along the field line at that point. Is the negative of g at that point.
Kepler’s third law
For any planet, the cube of its ran radius or orbit (r) is directly proportional to the square of its time period (T)
What is radius of orbit cubed over time period squared?
GM/4pi^2
Newton’s law of gravitation
The gravitational force between two point masses m1 and m2 at distance r apart is:
F=Gm1m2/r^2
Graph of gravitational field strength against distance from centre of planet radius r
Straight line from origin to point (r,g). Then curved line down following inverse square law. So has point (2r,1/4 g), (3r, 1/9 g).
Escape velocity
Minimum velocity an object must be given to escape from a planet’s gravitational attraction when projected vertically from the surface.
Why don’t rockets launched from earth’s surface need to achieve escape velocity to reach their orbit?
They are supplied with energy to keep the velocity constant. They don’t have to escape the earth’s gravitational field.
