Which one of the following statements about a satellite orbiting around the Earth is correct ?
- A.Satellite is kept in orbit by remote control from ground station.
- B.Satellite is kept in orbit by retro-rocket and solar energy keeps it moving around the Earth.
- C.Satellite requires energy from solar panels and solid fuels for orbiting.
- D.Satellite does not require any energy for orbiting.
Correct answer
Explanation
The correct answer is D, Satellite does not require any energy for orbiting. Once a satellite is placed in orbit with the right speed, the Earth's gravity keeps it going round without any engine.
The gravitational pull of the Earth supplies exactly the centripetal force the satellite needs to move in a circle. In a circular orbit this force is always at right angles to the motion, so it does no work, and above the atmosphere there is almost no air resistance to slow the satellite down. Its speed therefore stays the same with no energy input, much like the Moon going round the Earth. The orbital speed near the Earth's surface is about 7.9 km/s.
A is wrong: ground stations send commands for small corrections and operations, but they do not hold the satellite up. B is wrong: retro-rockets are fired to slow a craft down, for example to change orbit or come back to Earth, not to keep it orbiting. C is wrong: solar panels only power the instruments and radios on board, and fuel is used only for occasional orbit corrections.
Exam tip: Gravity provides the centripetal force; a satellite needs energy only for launch and orbit corrections, not for orbiting.
Practice Questions
View allA negative work is done when an applied force F and the corresponding displacement S are
- A.perpendicular to each other.
- B.parallel to each other.
- C.anti-parallel to each other.
- D.equal in magnitude.
Show answer
Work is negative when the force acts opposite to the displacement, because the angle between them is then 180°. Work done is W = F S cos θ, where θ is the angle between the force and the displacement.
Read the full explanation →Which one of the following is not a conservative force ?
- A.Frictional force
- B.Electric force
- C.Gravitational force
- D.Spring force
Show answer
Friction is a non-conservative force because the work it does depends on the path taken and the energy it removes is lost as heat.
Read the full explanation →A microphone converts
- A.electrical signals to sound waves
- B.sound waves to electrical signals
- C.microwaves to sound waves
- D.sound waves to microwaves
Show answer
A microphone is a transducer that changes the pressure variations of sound into a matching electrical signal. In a common dynamic microphone, sound waves make a thin diaphragm vibrate.
Read the full explanation →When a light ray passes through from air to water with a non-zero angle the ray will be
- A.bending towards the normal
- B.bending away from the normal
- C.propagating in straight line
- D.reflected towards the opposite direction
Show answer
Light slows down when it enters water from air, so a ray that strikes the surface at an angle bends towards the normal.
Read the full explanation →Light year is a measure of
- A.time
- B.distance
- C.total amount of light falling on the Earth in a year
- D.average intensity of light falling on the Earth in a year
Show answer
A light year is the distance that light travels in empty space in one year, so despite the word "year" it measures length, not time.
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