The pull of gravity
Every object near the Earth is pulled towards its centre by gravitational force. This force gives a falling object a steady gravitational acceleration, given the symbol g. Near the Earth's surface g ≈ 9.81 m s⁻² (often rounded to 10 m s⁻² in calculations). This means a falling object speeds up by about 9.81 m s⁻¹ every second.
Key idea
Weight is the gravitational force on an object: W = mg, where m is the mass in kg and g is the gravitational acceleration. Weight is measured in newtons (N).
Free fall
Free fall is the motion of an object falling under the pull of gravity only, with no air resistance acting on it. In free fall the acceleration is constant and equal to g. A key result is that in a vacuum all objects fall at the same rate, no matter how heavy or light they are.
Example
If a coin and a feather are dropped together inside a vacuum tube, they land at exactly the same time. In ordinary air the feather falls more slowly, but this is because of air resistance, not because of its smaller mass.
Simple calculations
For an object released from rest and taking g = 10 m s⁻²:
- Velocity after time t: v = gt. After 2 s, v = 10 × 2 = 20 m s⁻¹.
- Distance fallen: s = ½ g t². After 3 s, s = ½ × 10 × 3² = 45 m.
- Weight of a 2 kg object: W = mg = 2 × 10 = 20 N.
Remember
Mass does not affect how fast an object falls in a vacuum. What changes with location is the weight, because g is slightly different on the Moon or on other planets.