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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/r2 (r is distance apart)
- gravity is a universal force acting between all mater
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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/r2
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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
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Orbits of planets and satellites
- v=square root (GM/r)
- T=square root(4pi2r3/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
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