Motion in the UniverseEdexcel IGCSE Physics: Flashcards

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What is a galaxy?

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What is a galaxy?: A huge collection of billions of stars held together by gravity.
Which galaxy is our Solar System in?: The Milky Way.
Define gravitational field strength.: The gravitational force exerted per unit mass at a point in a gravitational field, measured in N/kg.
Why is gravitational field strength different on the Moon than on Earth?: Because it depends on the mass of the body and distance from its centre; the Moon has much less mass than Earth, so its g is smaller.
What force causes planets to orbit the Sun?: Gravitational force.
What force keeps artificial satellites orbiting the Earth?: Gravitational force, which provides the centripetal force for the curved orbital path.
Describe the shape of a typical planet's orbit.: Nearly circular.
Describe the shape of a typical comet's orbit.: Highly elliptical (elongated), bringing it very close to the Sun at one point and very far away at another.
State the equation for orbital speed.: v = 2πr/T, where r is the orbital radius and T is the time period.
What does T represent in the orbital speed equation?: The time period — the time taken to complete one full orbit.
Why does a comet's speed vary a lot around its orbit but a planet's does not?: Because a comet's elliptical orbit takes it through very different distances from the Sun, while a planet's near-circular orbit keeps it at a roughly constant distance.
What provides the force that keeps a moon orbiting a planet?: The gravitational force between the moon and the planet.