With what velocity will an object strike the ground if it is let go from the roof of a building 40 m high?

Correct answer: b 28 ms⁻¹

v = √(2gh) = √(2 × 9.8 × 40) = √784 = 28 ms⁻¹.

Why the other options are wrong

• a 784 ms⁻¹: 784 is v², not the velocity: v² = 2gh = 2 × 9.8 × 40 = 784. Its square root is 28.
• c 19.79 ms⁻¹: 19.79 is √392, which drops the 2 and takes √(gh). The formula is v = √(2gh).
• d 14 ms⁻¹: 14 is half of 28, the average velocity. The question asks for the velocity of impact with the ground.