It feels natural to think a heavy stone falls faster than a light one, but in free fall they speed up at the same rate. A heavier object is pulled down with a larger gravitational force, yet it also has more mass and therefore more inertia, so the two effects cancel and the acceleration is the same for all objects, about 9.81 m/s squared near the Earth.
What we actually see is often air resistance at work. A feather drifts down slowly because air pushes back strongly compared with its tiny weight, while a coin falls quickly. Remove the air, as in a vacuum tube, and the feather and coin land together.
Dropping two different stones side by side is a simple demonstration: they hit the ground at the same time.
In SPM you should explain free fall as motion under gravity alone, and treat air resistance as a separate effect.
Common misconceptions
- Heavy objects always fall faster than light ones -> Only when air resistance differs; in free fall all objects accelerate equally.
- Gravity pulls heavier objects with the same force as lighter ones -> Weight is larger for heavier objects, but so is inertia, so acceleration stays the same.
- In a vacuum a hammer still beats a feather down -> With no air resistance they fall together and land at the same time.
The physics behind it
In free fall the only force acting on an object is its weight, W = mg, where m is the mass in kilograms and g is the gravitational field strength, about 9.81 N kg⁻¹ near Earth. Newton's second law states that acceleration a = F/m.
Putting the weight in as the force gives a = mg/m = g, so the mass cancels completely.
Take a 2 kg object: its weight is W = mg = 2 kg × 9.81 m s⁻² = 19.62 N, and its acceleration is a = W/m = 19.62 N ÷ 2 kg = 9.81 m s⁻². Now take a 10 kg object: its weight is ten times larger, but so is its mass, so a = 98.1 N ÷ 10 kg = 9.81 m s⁻² again.
The larger gravitational pull on a heavier object is matched exactly by its larger inertia. This is why, once air resistance is removed, every object gains speed at the same rate, and the acceleration of free fall is the same for all masses.
See it in daily life
Drop a small stone and a large stone from the same height at the same moment and listen: they strike the ground together. Their masses differ, yet they land at the same time because both accelerate at g.
What confuses people is air resistance. A flat sheet of paper flutters down slowly, but crush the same sheet into a tight ball and it drops much faster, even though its mass has not changed.
Only its air resistance changed. A feather and a coin dropped in ordinary air separate widely, yet inside a sealed tube with the air pumped out they fall side by side, a demonstration famously repeated on the Moon with a hammer and a feather.
A skydiver speeds up under gravity until air resistance grows equal to weight; then the acceleration becomes zero and the diver falls at a steady terminal velocity. Weight sets that final speed, but during the early free-fall stage every diver, heavy or light, quickens at the same 9.81 m s⁻².
How this comes up in SPM
In Paper 2 this idea is examined under command words such as Explain, State and Compare. You may be shown two objects of different mass released together and asked to explain why they reach the ground at the same time; the expected answer links the larger weight of the heavier object to its larger inertia, so the acceleration stays equal to g.
Questions often pair this with free-fall calculations using v = u + at and h = ut + ½at², taking a = g. A ticker-timer or light-gate experiment to determine g is a common practical, where you plot velocity against time and read the gradient.
The topic sits beside Newton's laws of motion, weight and mass, and gravitational field strength, so be ready to define free fall as motion under gravity alone and to treat air resistance as a separate force. Take care to write g with units, either 9.81 m s⁻² as an acceleration or 9.81 N kg⁻¹ as a field strength.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)