A satellite is not held up by anything; gravity is pulling it down all the time. What keeps it in orbit is speed in the sideways direction. Gravity provides the centripetal force that bends its straight-line motion into a circle.
Imagine throwing a ball horizontally. The faster you throw it, the further it travels before landing. If you could throw it fast enough, the ground would curve away beneath it as fast as it falls, and it would never land, it would orbit. A satellite is simply moving at that speed, high above the air so nothing slows it.
At a lower orbit the pull is stronger, so the satellite must move faster; higher up it moves slower. A geostationary satellite orbits at just the right height to circle once a day, staying above the same spot, useful for communications over Malaysia.
In SPM you should explain orbits using gravity as the centripetal force.
Common misconceptions
- Satellites stay up because there is no gravity so high -> Gravity is exactly what provides the centripetal force keeping them in orbit.
- A satellite needs its engine running constantly to stay up -> Once in orbit above the air, it coasts; gravity alone bends its path.
- All satellites orbit at the same speed -> Lower orbits require higher speed; higher orbits are slower.
The physics behind it
A satellite in a circular orbit is not held up by anything; gravity pulls it toward Earth the whole time. That gravitational pull is exactly the centripetal force that bends its path into a circle.
Setting the gravitational force equal to the centripetal force gives GMm/r² = mv²/r, and the satellite's mass m cancels, leaving the orbital speed v = √(GM/r), where G is the gravitational constant, M is the Earth's mass and r is the distance from the Earth's centre.
For a low orbit, taking GM = 3.99 × 10¹⁴ m³ s⁻² and r = 6.77 × 10⁶ m, the speed is v = √(3.99 × 10¹⁴ ÷ 6.77 × 10⁶) = √(5.89 × 10⁷) ≈ 7.68 × 10³ m s⁻¹, about 7.7 km s⁻¹.
Because m cancels, a heavy satellite and a light one at the same height orbit at the same speed. A lower orbit has a smaller r, so the required speed is higher; higher orbits need less speed.
See it in daily life
Newton pictured this with a cannon on a very tall mountain. Fire the cannonball gently and it falls to the ground nearby.
Fire it faster and it lands further away, its path curving as it falls. Fire it fast enough and the ground curves away beneath it exactly as quickly as the ball falls, so it never lands, it circles the Earth.
A satellite is doing precisely that, moving so fast sideways that it keeps falling around the planet, high above the air so nothing slows it down.
This is the same principle you feel when you swing a ball on a string in a circle: the inward pull of the string is the centripetal force, and gravity plays that role for a satellite.
Communication and weather satellites over Malaysia rely on this. A geostationary satellite is placed at the one height where the orbital period is exactly 24 hours, so it circles once a day and stays fixed above the same point on the equator, letting a dish antenna point at one spot in the sky.
How this comes up in SPM
In Paper 2 this is examined with command words such as Explain, State and sometimes Calculate. You may be asked to explain what provides the centripetal force for a satellite, with the expected answer being the gravitational force of the Earth.
A common calculation gives you the orbital radius and asks for the speed or period using gravity as the centripetal force, GMm/r² = mv²/r, so keep track of units: r in metres, v in m s⁻¹.
Questions frequently contrast low-orbit satellites with geostationary ones, asking you to state why a geostationary satellite must sit at a particular height and above the equator. The topic sits within the gravitation chapter beside Newton's law of universal gravitation, gravitational field strength and Kepler's third law, T² proportional to r³.
Be careful to explain, not just assert: state that the satellite is in free fall and that gravity continuously changes its direction, providing the centripetal acceleration that keeps it moving in a circle.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)