A common belief is that there is no gravity in space. In fact, at the height of the International Space Station, the Earth's gravitational pull is still about nine tenths of its value at the ground. Gravity is very much present.
The astronauts float because they are in continuous free fall. The station moves sideways so fast that as gravity pulls it down, the curved Earth falls away beneath it at the same rate, so it keeps missing the ground and circles instead. Everyone and everything inside falls at exactly the same rate, so the astronauts do not press on the floor and the floor does not push back. That absence of a support force is what we feel as weightlessness.
You feel a hint of this in a lift that suddenly drops: for a moment you feel lighter.
In SPM you should explain apparent weightlessness as free fall in orbit, not as the absence of gravity.
Common misconceptions
- There is no gravity in space -> Gravity is still strong at orbit height; it is what keeps the station circling.
- Astronauts float because they are far from Earth -> They float because they are in free fall together with their spacecraft.
- Weightlessness means an object has lost its mass -> Mass is unchanged; only the sensation of weight, from a support force, is absent.
The physics behind it
Weightlessness is not the same as having no weight. Weight is the gravitational force W = mg, and at the height where the International Space Station orbits, roughly 400 km up, the field strength g is still about 8.7 N kg⁻¹, nearly nine tenths of its value at the ground.
Gravity is very much present.
What an astronaut feels as weight is really the support force, the normal reaction from a floor or seat pushing up. In orbit the station and everyone in it are in continuous free fall, all accelerating toward Earth at the same rate g.
Applying Newton's second law to a 70 kg astronaut: the true weight is W = mg = 70 kg × 8.7 m s⁻² = 609 N, but because the astronaut accelerates with the station, the support force needed is R = W − ma = 609 N − (70 kg × 8.7 m s⁻²) = 0 N.
With no support force pressing back, the astronaut feels no weight. This absence of a reaction force, not the absence of gravity, is what we call weightlessness.
See it in daily life
You can feel a trace of this on Earth. Step into a lift and, at the instant it starts moving downward quickly, your stomach lurches and you feel lighter.
For that moment the floor is dropping away beneath you, so it presses up on you less and your apparent weight falls.
A fairground drop ride pushes this further: as the car plunges, riders and their loose belongings fall together and feel briefly weightless, which is why a phone left on the seat seems to hover. A diver experiences something similar in the short arc before hitting the water.
In every case the person is in free fall, accelerating downward at close to g, so the support force from any surface drops toward zero.
The orbiting astronaut simply lives in this state permanently, because the station keeps missing the Earth as it circles. Nothing has switched gravity off; the astronaut and the spacecraft are just falling around the planet side by side, so neither presses on the other.
How this comes up in SPM
In Paper 2 this appears under command words such as Explain and State, within the gravitation and free-fall content. A typical question describes an astronaut floating inside an orbiting spacecraft and asks you to explain why, expecting you to say that gravity still acts but the astronaut and craft are in free fall together, so there is no reaction force and hence apparent weightlessness.
You may be asked to compare mass and weight, stating that the astronaut's mass is unchanged while the sensation of weight is absent. Questions can connect this to gravitational field strength g = GM/r², showing that g decreases with height but does not vanish, and to satellite motion where gravity supplies the centripetal force.
Be ready to distinguish true weight, W = mg, from apparent weight, the support force, and to write field strength with units of N kg⁻¹. The topic sits beside Newton's law of universal gravitation, satellites and escape velocity in the gravitation chapter.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)