5.4  Gravitational Fields

5.4.1  Point and spherical masses

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) gravitational fields are due to objects having mass

(b) modelling the mass of a spherical object as a point mass at its centre

This just means that you consider all of the mass actually to be at the centre rather than worrying about where exactly the mass is.  You do this because it make things really hard if you don't.

(c) gravitational field lines to map gravitational fields

(d) gravitational field strength; g = F/m .

(e) the concept of gravitational fields as being one of a number of forms of field giving rise to a force.

5.4.2  Newton's law of gravitation

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) Newton’s law of gravitation; for the force between two point masses

(b) gravitational field strength for a point mass

(c) gravitational field strength is uniform close to the surface of the Earth and numerically equal to the acceleration of free fall.

5.4.3  Planetary motion

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) Kepler’s three laws of planetary motion

(b) the centripetal force on a planet is provided by the gravitational force between it and the Sun

Remember the circular motion stuff.  You will need it all again here.  There is not much else to say about this statement really.

(c) the equation Learners will also be expected to derive this equation from first principles.

This looks tricky but in fact if you equate the centripetal force equation with that for gravitational force and  force, cancel some stuff and substitute in for the velocity using the period and it will all be OK.

(d) the relationship for Kepler’s third law applied to systems other than our solar system

(e) geostationary orbit; uses of geostationary satellites.

5.4.4  Gravitational potential and energy

Learners should be able to demonstrate and apply their knowledge and understanding of:

(a) gravitational potential at a point as the work done in bringing unit mass from infinity to the point; gravitational potential is zero at infinity

(b) gravitational potential at a distance r from a point mass M; changes in gravitational potential

(c) force–distance graph for a point or spherical mass; work done is area under graph

(d) gravitational potential energy at a distance r from a point mass M   