Question

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Here is an question from the topic Free fall -:
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A body is falling freely from rest Find-:
(a) Distance it falls in 3 second
(b) It's speed after falling 20 m
(c)Time required to reach a speed of 25 m/s
[TAKE g=10 m/s²

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Answer 1

1.initial velocity (u) = 0

time(t) = 3 seconds.

∴ S = ut + 1/2at²

⇒ S = 0 + 1/2 × 10 × 9

∴ S = 45 m.

2.v² - u² = 2aS

v² - u² = 2aSv² = 2 × 10 × 20

v² - u² = 2aSv² = 2 × 10 × 20∴ v = √400

3.∴ v = m/s

v - u = at

v - u = at∴ 25 - 0 = 10 × t

v - u = at∴ 25 - 0 = 10 × t∴ t = 2.5 seconds.

Answer 2

Explanation:

Given: A body is falling freely from rest.

To find:

› Distance it fall in 3 seconds.

› It's speed after falling 20 metre.

› Time required to reach a speed of 25 m/s.

Solution:

The body is falling freely from rest.

∴ Acceleration = Acc. due to gravity

• 10 m/s²

Since the body is starting from rest.

⇒ Initial velocity (u) = 0 m/s

⇒ Time taken (t) = 3 sec

Finding the distance it falls in 3 seconds:

• S = ut + ½ at²

⇒ S = 0(3) + ½ (10)(3)²

⇒ S = 0 + ½ (10)(9)

⇒ S = 0 + 5(9)

∴ Distance travelled (S) = 45 m.

Finding it's speed after falling 20m:

• v² - u² = 2as

⇒ v² - 0² = 2(10)(20)

⇒ v² = 20(20)

⇒ v² = 400

⇒ v = √400

∴ It's speed after falling 20 m (v) is 20 m/s.

Finding time required to reach a speed of 25 m/s:

• v = u + at

⇒ 20 = 0 + 10(t)

⇒ 20 = 10t

⇒ t = 20/10

∴ The time taken required to reach a speed of 25 m/s is 2 sec.