Question

A cricket ball dropped from the top of a building takes 3s to reach the ground. Calculate the distance the ball moved in the third second of its motion. Take g=9.8m/s^2​

Answer 1

Given:-

• Initial velocity (u) = 0m/s

• Time taken (t) = 3s

• Acceleration due to gravity (a) = 9.8m/s²

To Find:-

• distance covered by ball in 3rd second (s).

Solution:-

By using 2nd equation of motion

→ s = ut +1/2at²

Substitute the value we get

→ s = 0×3 + 1/2×9.8×3²

→ s = 0 + 4.9 ×9

→ s = 44.1 m

∴ The total distance covered by the ball is 44.1 metre.

And , Now we have to calculate the distance moved by ball in 3rd second. So ,

Distance covered by ball in 2 second

Again using 2nd equation of motion

→ s = ut +1/2at²

Substitute the value we get

→ s = 0×2+ 1/2×9.8×2²

→ s = 0 + 4.9×4

→ s = 19.6m

∴ The distance covered by ball in 2 second is 19.6 m

Distance covered by othe ball in 3rd second = Total Distance covered by ball - Distance covered by ball in 2 second

→ Distance covered by ball in 3rd second = 44.1m - 19.6m = 24.5m

∴ The distance covered by the ball in 3rd second is 24.5 metre.

Answer 2

Given :-

◉ A cricket ball dropped from the top of a building of height h and took time t = 3s to reach the ground.

g = 9.8 m/s²

To Find :-

◉ Distance the ball moved in the third second of its motion.

Solution :-

We need to find the height h of the building. In other words, distance travelled by the ball.

We are given,

• u = 0 m/s
• t = 3s
• g = 9.8 m/s²

If you observe, we have taken g positive, It is because the ball is moving in the direction of g.

Using Second equation of motion, we get

⇒ s = ut + 1/2at²

⇒ s = 1/2 gt² [∴ u = 0 m/s)

⇒ s = 1/2 × 9.8 × 3²

⇒ s = 4.9 × 3²

⇒ s = 44.1 m

Now, we have got the height of the building, Let us calculate the distance that the ball travelled from 0 to 2 second.

⇒ s = ut + 1/2 × at²

⇒ s = 1/2 × gt² [∵ u = 0 m/s]

⇒ s = 1/2 × 9.8 × 4

⇒ s = 9.8 × 2

⇒ s = 19.6 m

Further,

⇒ Distance Travelled in Third second = Height - Distance travelled from 0 to 2 second.

⇒ d = 44.1 - 19.6

⇒ d = 24.5 m

Hence, The ball travelled 24.5 m in the third second.