A stone is thrown horizontally with initial velocity 15m/s from a tower 20 m high. How long does it take to reach the ground? [g= 10m/s$^2$

Analyze the question

A stone is thrown horizontally from a height of 20 m. We need the time to hit the ground. The horizontal speed (15 m/s) doesn't affect the fall time, because horizontal and vertical motions are independent.

Concept

Vertically, the stone starts with zero velocity (it was thrown horizontally) and accelerates downward under gravity:

h = ut + ½gt², where u = 0 (vertical)

So h = ½gt², which gives t = √(2h/g)

Solution

• h = 20 m
• g = 10 m/s²

t = √(2 × 20 / 10) t = √4 t = 2 s

Check: the vertical distance fallen in 2 s is ½ × 10 × 2² = 20 m ✓

Answer: The stone takes 2 seconds, so the correct option is A.

Tip: In projectile questions where an object is launched horizontally, ignore the horizontal speed when finding the time of flight. Use only the vertical formula t = √(2h/g). The 15 m/s would only be needed if the question asked for the horizontal range (R = 15 × 2 = 30 m). Many WASSCE questions include extra data like this to test whether you know which information to use.