A ball is projected vertically upwards from the ground with speed 21 ms^–1. The ball moves freely under gravity once projected. What is the greatest height reached by the ball?

Set out information given in question, and taking the upward direction to be positive: s (displacement) = ?, u (initial speed) = 21ms^-1, v (final speed at maximum height) = 0ms^-1, a (acceleration when falling freely under gravity) = - 9.8ms^-2, t = ?. Using v² = u² + 2as: 0 = 21² + 2(-9.8)s, s = 441 / 19.6 = 22.5m. So maximum height reached = 22.5m