Question

7. A ball is gently dropped from a height of 20 m. If its velocity
increases uniformly at the rate of 10 m s2, with what velocity
will it strike the ground? After what time will it strike the
ground?​

Answer 1

Given :

• initial velocity of ball, u = 0

[ since, ball is gently dropped from a height ]

• distance covered by ball, s = 20 m
• uniform acceleration of ball, a = 10 m/s²

To find :

• final velocity of ball, v = ?
• time after which ball will strike the ground, t = ?

Formulae required :

• second equation of motion

     s = u t + 1/2 a t²

• third equation of motion

    2 a s = v² - u²

[ where s is distance covered, u is initial velocity, a is acceleration , t is time taken and v is final velocity ]

Solution :

Using third equation of motion

→ 2 a s = v² - u²

→ 2 ( 10 ) ( 20 ) = v² - ( 0 )²

→ 400 = v²

→ v = 20 m/s

therefore,

• velocity of ball with which it will strike the ground is 20 m/s .

Using second equation of motion

→ s = u t + 1/2 a t²

→ 20 = ( 0 ) t + 1/2 ( 10 ) t²

→ 20 = 5 t²

→ t² = 4

→ t = 2 sec

therefore,

• time taken by ball to reach the ground is 2 seconds.