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Newton's law of gravitation
 Assumes that the gravitational force between any two point objects is: always an attractive force, proportional to the mass of each object and proportional to 1/r^{2} (r is distance apart)
 gravity is a universal force acting between all mater

Gravitational field strength
 force field: a region in which a body experiences a force
 Radial field: field lines are like spokes on a wheel, directed towards the centre
 Uniform field: field lines are parallel and equally spaced
 g=F/m
 In a radial field g=GM/r^{2}

Gravitational potential
 the gravitational potential at a point in a gravitational field is the gpe per unit mass of a small test mass. It is equal to the work done per unit mass to move a small object from infinity to that point as the gravitational potential at infinity is zero
 work done in moving mass: W=mV
 V in a radial field: V=GM/r
 gravitational field strength is the negative of the potential gradient: g=V/r

Orbits of planets and satellites
 v=square root (GM/r)
 T=square root(4pi^{2}r^{3}/GM)
 A satellite's total energy is constant, so kinetic and potential energy is also constant (KE + PE = TE)
 Geosynchronous orbit: always above the same point on Earth, same angular speed as the Earth

