JAMB 2015 Physics · Question 62

If a ball released from a height of 20m, How long would it take the ball to reach the ground? (assures g = 10m/s)

Options

A.2sec
B.3sec
C.35sec
D.2.4sec

Answer: A. 2sec

Explanation written with AI and checked against the answer key

1. Analyze the question

We are given the height ( $h$ ) from which a ball is dropped and the acceleration due to gravity ( $g$ ). We need to find the time ( $t$ ) it takes for the ball to hit the ground.

Height ( $h$ ) = 20 m
Acceleration due to gravity ( $g$ ) = 10 m/s²
Initial velocity ( $u$ ) = 0 m/s (since the ball is "released" or dropped)

2. Explain the concept

This is a problem involving equations of motion under gravity. When an object is dropped from rest, we use the second equation of motion:

h = ut + \frac{1}{2}gt^2

Since the initial velocity ( $u$ ) is 0, the equation simplifies to:

h = \frac{1}{2}gt^2

3. Show the solution

Step 1: Substitute the known values into the simplified formula.

20 = \frac{1}{2} \times 10 \times t^2

Step 2: Simplify the right side of the equation.

20 = 5 \times t^2

Step 3: Solve for $t^2$ by dividing both sides by 5.

t^2 = \frac{20}{5}

t^2 = 4

Step 4: Take the square root of both sides to find $t$ .

t = \sqrt{4}

t = 2 \text{ seconds}

4. Identify the answer

Comparing our result to the given options: A. 2sec B. 3sec C. 35sec D. 2.4sec

The correct option is A.

5. Tip

Whenever you see a question about an object being "dropped" or "released," always remember that the initial velocity ( $u$ ) is zero. This makes your calculation much faster because the $ut$ part of the equation disappears! Also, always ensure your units are consistent (meters and seconds) before you start calculating.

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