A stone was thrown vertically upwards from the ground with a velocity of 50 m/sec. After 5 seconds another stone was thrown vertically upwards from the same place. If both the stones strike the ground at the same time, then the velocity with which the second stone was thrown should be (Assume g = 10 m/sec2) A. 15 m/sec B. 25 m/sec C. 40 m/sec D. 50 m/sec

The correct answer is $\boxed{\text{B}}$.

The first stone was thrown upwards with a velocity of $50$ m/s. After $5$ seconds, it will have a velocity of $50 – 5 \times 10 = -25$ m/s, since the acceleration due to gravity is $-10$ m/s$^2$.

The second stone was thrown upwards from the same place, so it will also have a velocity of $-25$ m/s when it reaches the top of its trajectory. In order for both stones to strike the ground at the same time, the second stone must have been thrown upwards with a velocity of $50 – 25 = 25$ m/s.

Here is a more detailed explanation of each option:

• Option A: $15$ m/s. This is the velocity with which the second stone would have to be thrown upwards in order to reach a height of $50$ m. However, the question states that both stones strike the ground at the same time, so this option is incorrect.
• Option B: $25$ m/s. This is the velocity with which the second stone would have to be thrown upwards in order to reach a height of $25$ m. Since the question states that both stones strike the ground at the same time, this option is correct.
• Option C: $40$ m/s. This is the velocity with which the second stone would have to be thrown upwards in order to reach a height of $40$ m. However, the question states that both stones strike the ground at the same time, so this option is incorrect.
• Option D: $50$ m/s. This is the velocity with which the first stone was thrown upwards. However, the question states that both stones strike the ground at the same time, so this option is incorrect.