D.1 Gravitational fieldsIB Physics SL: Flashcards

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State Kepler's first law.

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State Kepler's first law.: Planets move in ellipses with the Sun at one focus.
State Kepler's second law.: The line joining the Sun and a planet sweeps out equal areas in equal times.
Where is an orbiting body fastest?: At perihelion, its closest point to the Sun.
State Kepler's third law.: T² ∝ r³ for bodies orbiting the same central mass.
Newton's law of gravitation?: F = Gm₁m₂/r², attractive, r between centres.
When can an extended body be treated as a point mass?: When it is spherically symmetric, or when its size is very small compared with the separation.
Define gravitational field strength.: The force per unit mass on a small test mass at that point: g = F/m.
g for a spherical mass M?: g = GM/r².
Units of g?: N kg⁻¹, equivalent to m s⁻².
What do gravitational field lines around a planet look like?: Radial, pointing inwards, getting further apart with distance.
What does field line density show?: Field strength: closer lines, stronger field.
Why do astronauts on the ISS float?: They and the station are in free fall together; g is still about 8.6 N kg⁻¹.
How does g change if r is doubled?: It falls to one quarter.