Chapter 21 - Gravitational Fields Flashcards
What is a gravitational field?
The region surrounding an object in which it exerts a gravitational force on any other object.
What is the strength of a gravitational field?
The force per unit mass on a small mass in the field.
g = F/m
Measured in N kg^-1
What is a field line?
The path which a smaller mass will follow.
What is a radial field?
A field where the field lines are like spokes, always directed to the centre.
What is a uniform field?
A field where the gravitational field strength is the same in magnitude and direction throughout the field. Field lines are parallel and equally spaced.
What is gravitational potential energy?
The energy of an object due to its position in a gravitational field.
What is the gravitational potential?
Gravitational potential, V, is the work done per unit mass to move a small object from infinity to that point.
V = W/m
Change in W = m X change in V
What is an equipotential surface?
A surface with constant potential - no work is done to move along an equipotential surface.
What is potential gradient?
Potential gradient at a point in a field is the change of potential (V) per metre at that point.
Gradient = change in V / change in r (where r is a small distance)
Gravitational field strength is equal to the negative of the gradient.
What is Kepler’s third law?
Kepler’s third law states that for any planet, r^3/T^2 = GM/4pi^2
where r is the mean radius of orbit, T is the time period, and M is the mass of the object being orbited.
What does Newton’s law of gravitation assume?
What equation does this give for gravitational force?
Gravitational force between any two point objects is always attractive, proportional to the mass of each object, and proportional to 1/r^2 where r is the distance apart.
Gravitational force, F = GMm / r^2
When using a spherical mass, what is r?
The distance between the centre of the spherical mass, and the other mass/object.
At distance r, g = GM / r^2
At distance r, g = GM / r^2
At or beyond the surface of a spherical planet, V = -GM / r
At or beyond the surface of a spherical planet, V = -GM / r
What is the escape velocity?
The minimum velocity required for an object to escape a planet when projected vertically from the surface.
v^2 (escape) = 2gR where R = the planet radius
A geostationary satellite orbits ______ km above Earth
A geostationary satellite orbits 36000 km above Earth.
The energy of an orbiting satellite, E = -GMm / r
The energy of an orbiting satellite, E = -GMm / r
What is the equation for gravitational potential energy?
E = -GMm / r
