Question

A stone reaches the ground in 5 Sec when dropped from certain height. If it becomes steady
after 3 Sec while falling then falls again, time taken by it to reach the ground is
A. 6 Sec
C. 6.5 Sec
B. 7 Sec
D. 7.5 Sec​

Answer 1

Hope this will help you

Answer 2

Answer:

B. 7 sec

Explanation:

$\\\\ S=u t+\frac{1}{2} {g}{ t}^{2} \\\\ S_{1}=0 \times 3+\frac{1}{2} \times 10 \times 5^{2}=125 \mathrm{m} \\\\ S_{2}=0 \times 3+\frac{1}{2} \times 10 \times 3^{2}=45 m$

We need to find time for $S_{1}-S_{2}=80m$

$t=\sqrt{\frac{80}{5}}=4 \sec$

total time = 4+3=7sec

The stone is freefalling from a certain height therefore it is under the influence of gravity. In the first 3sec, it was moving at a steady speed and we calculate the distance travelled in that time. Also the distance in 5 sec is required to determine the time taken to travel after it is dropped again.  

The total time travelled would be time taken in the first 3 seconds and lastly in 4 seconds.  

Therefore, the time taken to reach the ground is 7 seconds