8.3 Orbits and satellites
9 exam questions worth 21 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What provides the force that keeps a satellite in orbit around the Earth?
Choose one answer.
Correct Not quite: the answer is The gravitational force
Mark scheme
B: The gravitational force
Model answer
The gravitational force (B).
A satellite moves in a circular orbit at a constant speed. Explain why its velocity is changing.
Mark scheme
velocity includes direction (1); the direction of motion is changing all the time (1)
Model answer
Velocity is a vector, and the satellite's direction is constantly changing as it goes around the orbit.
State the direction of the gravitational force on a satellite in a circular orbit, and the direction of its velocity.
Mark scheme
force: towards the centre of the Earth (1); velocity: along the tangent / at right angles to the force (1)
Model answer
The force is towards the centre of the Earth, and the velocity is along the tangent, at right angles to the force.
Explain why the force on an orbiting satellite changes its direction but not its speed.
Mark scheme
the force is always at right angles to the velocity (1); so it does no work along the direction of motion / it only changes direction (1)
Model answer
The force acts at right angles to the direction of motion, so it can change the direction but it does not speed the satellite up or slow it down.
Two satellites orbit the Earth. Satellite A is in a lower orbit than satellite B. State which one is moving faster and explain why.
Mark scheme
A (1); gravity is stronger closer to the Earth (1); so a higher speed is needed to stay in a circular orbit at the smaller radius (1)
Model answer
Satellite A is moving faster. Gravity is stronger at the smaller radius, so it needs a higher speed to stay in a stable circular orbit.
The Moon orbits the Earth at a radius of 3.8 × 10⁸ m, with a period of 2.4 × 10⁶ s. Calculate the orbital speed of the Moon. (use π = 3.14)
Mark scheme
v = 2 π r ÷ T (1); 2 × 3.14 × 3.8 × 10⁸ ÷ 2.4 × 10⁶ (1); 990 m/s (about 1000 m/s) (1)
Model answer
v = 2 π r / T = 2 × 3.14 × 3.8 × 10⁸ / 2.4 × 10⁶ ≈ 990 m/s.
Describe the orbit of a geostationary satellite and give one use.
Mark scheme
above the equator (1); period of 24 hours, so stays above the same point on the Earth (1); use: communications / TV broadcasting (1)
Model answer
It orbits above the equator with a period of 24 hours, so it stays above the same point. It is used for communications and television.
Explain why weather satellites are placed in low polar orbits rather than geostationary orbits.
Mark scheme
a low polar orbit passes over the whole of the Earth as it rotates underneath (1); a geostationary satellite can only see one part of the Earth / is very far away (1)
Model answer
A low polar orbit lets the satellite pass over every part of the Earth as it rotates beneath, whereas a geostationary satellite only sees one region.
Astronauts on the International Space Station appear to float. A student says that this is because there is no gravity in space. Explain why the student is wrong.
Mark scheme
gravity is still about 90% as strong at that height (1); the astronauts and the station are in free fall (1); they are both falling at the same rate, so there is no contact force between them and they feel weightless (1)
Model answer
Gravity is still strong at the height of the ISS, about 90% of its value on the ground. The astronauts and the station are both in free fall together, so no force pushes them against the station, and they feel weightless.
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