**Solution:**

Given:
Initial speed of the stone, u = 4.9 m/s
Time taken to fall down, t = 2 s

To find:
Height of the bridge

**Assumptions:**
1. Neglecting air resistance
2. Taking acceleration due to gravity as 9.8 m/s^2

**Analysis:**

1. The stone is thrown vertically upward from the bridge. After reaching its maximum height, it starts falling down under the influence of gravity.
2. At the highest point, the stone comes to rest for a moment before falling back down.
3. The time taken to reach the maximum height is equal to the time taken to fall back down to the bridge.
4. The height of the bridge can be calculated using the equation of motion.

**Calculations:**

1. Let's consider the upward direction as positive and downward direction as negative.
2. Using the equation of motion, s = ut + (1/2)at^2, where s is the displacement, u is the initial velocity, a is the acceleration, and t is the time.
3. For upward motion, taking acceleration as -9.8 m/s^2, the equation becomes s = 4.9t - (1/2)(9.8)t^2.
4. For downward motion, the equation becomes s = 0 - (1/2)(9.8)t^2, as the initial velocity is 0 at the highest point.
5. Equating the two equations, we get 4.9t - (1/2)(9.8)t^2 = -(1/2)(9.8)t^2.
6. Simplifying the equation, we get 4.9t = 0, which gives t = 0.
7. This implies that the stone takes 0 seconds to reach the highest point and start falling back down.
8. However, this contradicts the given time of 2 seconds for the stone to fall down to the water.
9. Therefore, it is not possible for the stone to fall down to the water in 2 seconds if it is thrown vertically upward with an initial speed of 4.9 m/s.
10. Hence, we cannot determine the height of the bridge using the given information.

**Answer:**
The height of the bridge cannot be determined based on the given information.

Ans is 9.8 m we get the ans by using equation -s=ut -1/2gt²
I hope it will helpful.