A balloon made of rubber and filled with air is floating in water in such a way that its top surface just coincides with the level of the water. If the balloon is gently pushed a short distance downward in water and then released, it will

Options

A. remain wherever it was left
B. move a short distance downward and then shot upward
C. move with a constant speed downward
D. sink and settle at the bottom

Answer

To solve the problem, let's analyze the situation step by step.

### Step 1: Understand the initial condition
The balloon is floating in water such that its top surface is at the water level. This indicates that the buoyant force acting on the balloon is equal to the weight of the balloon.

**Hint:** Remember that an object floats when the buoyant force equals its weight.

### Step 2: Identify the forces acting on the balloon
When the balloon is floating, two main forces act on it:
- The buoyant force (upward) which is equal to the weight of the water displaced by the balloon.
- The weight of the balloon (downward) which is equal to the mass of the balloon multiplied by the acceleration due to gravity (mg).

**Hint:** The buoyant force can be calculated using Archimedes' principle.

### Step 3: Analyze what happens when the balloon is pushed down
When the balloon is gently pushed down and then released, it will experience a change in the forces acting on it. While it is pushed down, the buoyant force remains the same, but the weight of the balloon does not change.

**Hint:** Consider how the buoyant force and weight interact when the balloon is displaced.

### Step 4: Determine the net force when released
Upon release, the balloon will experience a net upward force because the buoyant force is greater than the weight of the balloon. This is due to the fact that the buoyant force acts on the volume of water displaced, which remains constant as the balloon is submerged.

**Hint:** The net force can be calculated as: Net Force = Buoyant Force - Weight of the Balloon.

### Step 5: Conclude the motion of the balloon
Since the net force is upward, the balloon will accelerate upwards and return to its original position at the water surface. It will not sink but will instead float back to the level where it was initially resting.

**Hint:** Think about the principle of equilibrium and how forces balance out when the balloon is at rest.

### Final Answer
When the balloon is gently pushed downward in water and then released, it will rise back to the surface and float at the water level again.