Question

A girl standing on a stationary lift (open from above) throws a ball upwards with initial speed $50m/s$. The time taken by the ball to return to her hands is (Take g = $10m/{{s}^{2}}$ )

(A). 5 s
(B). 10 s
(C). 15 s
(D). 20 s

Answer 1

Hint: Wherever a ball is thrown upwards the velocity with which it is thrown is equals to the initial velocity of the ball, as the acceleration due to gravity is working as a negative force here thus we are considering it as a negative force. After that write the equation of displacement with respect to time, acceleration and velocity then calculate the time t.

Complete step-by-step answer:
We know that the ball is thrown upwards,
Which means,
Initial velocity u=50 m/s(Given)
As the ball returns to the same place,
Displacement (s)=0,
Acceleration (a)=-g=-$10m/{{s}^{2}}$.
We know that ,
S=ut + $\dfrac{1}{2}$ at$^{2}$ .
0=50t + $\dfrac{1}{2}$(-$10m/{{s}^{2}}$) t$^{2}$,(placing the value of u, s and t)
On solving this we get,
t=10 s.
Therefore option B is the correct option.

Additional Information:
The rate of change of its position of an object with respect to a frame of reference, and is a function of time is known as velocity.
Acceleration due to gravity is the force with which the earth pulls us towards itself.
Initial velocity is the velocity that a body has from the start of its movement.
Final velocity is the velocity gained by the body at the end of its movement.

Note: As the body is returning to the same hands as before therefore the displacement is zero. After throwing the ball the initial velocity is the velocity with which the ball is thrown but the final velocity is zero as after reaching the topmost point it will stop at a point of time then it will again regain its velocity.