Question

If one object is falling from some height. It reaches to ground in 5 sec. Find out height fromwhich it fall down. [take g = 10 m/s2 ]​

Answer 1

125 m is the required answer.

GIVEN:

• Time taken to reach the ground, t = 5 sec

• Acceleration due to gravity, g = 10 m/s².

• Since the body falls from rest, its initial velocity is zero. u = 0

TO FIND:

Height from which it fall down , h

FORMULAE:

• According to first equation of motion, v = u + at.

• According to third equation of motion, v²- u² = 2as

SOLUTION:

STEP 1: To find final velocity

Here , a = g = 10 m/s²

u = 0

t = 5 sec

$v = u + at \\ \\ v = u + gt \\ \\ v = 0 + (10 \times 5) \\ \\ v = 50 \: \frac{m}{s}$

The velocity when it reaches the ground is 50 m/s.

STEP 2: To find height

Here, v = 50 m/s

a = g = 10 m/s²

u = 0

${v}^{2} - {u}^{2} = 2as \\ \\ {(50)}^{2} - {0}^{2} = 2 \times 10 \times s \\ \\ 2500 = 20 \times s \\ \\ s = \frac{2500}{20} \\ \\ s = \frac{250}{2} \\ \\ s = 125 \: m$

ANSWER:

Height from which it falls down is 125 m.

DIRECT FORMULA:

• Time, t = u/g

• Height, h = u²/2g