Question

A stone is dropped from a height of 80 m. Find
(i) Time of fall
(ii) Velocity after 2 seconds of free fall
(iii) Velocity with which it hits the ground.​

Answer 1

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Answer 2

It has given that, a stone is dropped from a height of 80 m.

we have to find

1. Time of fall
2. velocity after 2 second of free fall
3. velocity with which it hits the ground

solution : The stone is dropped from a height of 80m.

so, initial velocity of stone, u = 0

using formula, s = ut + 1/2 at²

here s = -80 m , u = 0 and a = - g = -10 m/s²

⇒-80 = 0 + 1/2 × -10 × t²

⇒t² = 16

⇒t = 4 sec

therefore time of fall is 4 seconds.

using formula, v = u + at

here, u = 0, a = -10 m/s

velocity after 2 sec , v = 0 -10 × 2 = -20 m/s [ negative sign indicates that velocity of stone is downward ]

using formula, v² = u² + 2as

here u =0, a = - 10m/s² , s = -80 m

so, v² = 0 + 2(-10)(-80) = 1600

⇒v = 40 m/s

Therefore velocity with which it hits the ground is 40 m/s