Two balls of identical size, one of lead (mass = 12 kg) and another of aluminium (mass = 3 kg), both initially at rest, are dropped from a height of 50 m at the same time. When they are 10 m above the ground, which of the following quantities remains the same for both? Neglect air resistance.

Correct answer: (a) Speed and acceleration

Explanation

Without air resistance, every body falls with the same acceleration g, whatever its mass. Both balls start from rest at the same height at the same time, so after falling 40 m (50 m − 10 m) both have:

• the same acceleration (g), and
• the same speed: v² = 2gh = 2 × 10 × 40 = 800, so v ≈ 28 m/s.

Kinetic energy (½mv²), momentum (mv) and potential energy (mgh) all depend on mass, so they are four times larger for the 12 kg lead ball than for the 3 kg aluminium ball.