An air bubble rises from the bottom of a lake of large depth. The rising speed of air bubble will

Options

A. go on increasing till it reaches surface
B. go on decreasing till it reaches surface
C. increases in two beginning, then will become constant
D. be constant all throughout.

Explanation

To solve the problem of how the rising speed of an air bubble changes as it rises from the bottom of a lake, we can analyze the forces acting on the bubble and how they affect its motion.

### Step-by-Step Solution:

1. **Understanding the Forces**:
- When the air bubble is at the bottom of the lake, it is subjected to the pressure of the water above it. The pressure increases with depth.
- The bubble experiences two main forces: the buoyant force acting upwards and the weight of the bubble acting downwards.

2. **Buoyant Force**:
- The buoyant force is given by Archimedes' principle, which states that the buoyant force is equal to the weight of the fluid displaced by the bubble. As the bubble rises, it displaces a volume of water, and the buoyant force acts to push it upwards.

3. **Weight of the Bubble**:
- The weight of the bubble remains constant as it rises since it depends on the mass of the air inside the bubble and gravity.

4. **Net Force and Acceleration**:
- Initially, when the bubble starts rising, the buoyant force is greater than the weight of the bubble, resulting in a net upward force. This causes the bubble to accelerate upwards.
- As the bubble rises, the pressure decreases, and consequently, the buoyant force decreases.

5. **Terminal Velocity**:
- Eventually, the bubble will reach a point where the buoyant force equals the weight of the bubble. At this point, the net force acting on the bubble becomes zero, and the bubble stops accelerating. It then moves at a constant speed known as the terminal velocity.

6. **Conclusion**:
- Therefore, the speed of the air bubble will initially increase as it rises due to the buoyant force being greater than the weight. After some time, it will reach a constant speed (terminal velocity) as the forces balance out.

### Final Answer:
The rising speed of the air bubble will **increase initially and then become constant** as it reaches the surface.

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