A bird resting on the floor of an airtight box which is being carried by a boy star flying. The boy will fell that the box is now :

Options

A. heavier
B. lighter
C. same in weight
D. Lighter in the beginnig and heavier later

Explanation

To solve the problem, we need to analyze the situation step by step:

### Step 1: Understand the System
We have a bird resting on the floor of an airtight box. The box is being carried by a boy. The key point here is that the box is airtight, meaning that the air inside the box is not able to escape.

**Hint:** Consider the implications of the box being airtight on the air pressure and the forces acting on the bird.

### Step 2: Identify Forces Acting on the Bird
When the bird is resting on the floor, it exerts a downward force equal to its weight (let's denote it as \( W_b = mg \), where \( m \) is the mass of the bird and \( g \) is the acceleration due to gravity). This force is transmitted through the air inside the box.

**Hint:** Remember that the weight of the bird is the force that needs to be overcome for it to take off.

### Step 3: Bird Takes Off
When the bird starts to fly, it must exert a force downwards to create lift. This is done by flapping its wings, which pushes air downwards. According to Newton's third law, for every action, there is an equal and opposite reaction. Therefore, as the bird pushes air down, it experiences an upward lift.

**Hint:** Think about how the bird's action of flying affects the air pressure inside the box.

### Step 4: Effect on the Boy
The boy holding the box feels the weight of the box plus the weight of the bird. When the bird flies, it still exerts a force on the air inside the box, which in turn exerts a force on the bottom of the box. The total weight that the boy feels is the weight of the box plus the effective weight of the bird (which is still acting on the air inside the box).

**Hint:** Consider how the forces balance out when the bird is flying versus when it is resting.

### Step 5: Conclusion
Since the box is airtight, the air pressure inside the box adjusts to the bird's actions. Therefore, the boy will feel the same weight as before, as the bird's weight is still accounted for by the air pressure inside the box. Thus, the boy will feel that the box is still heavy as if the bird were resting on the floor.

**Final Answer:** The boy will feel that the box is still heavy, experiencing the same weight as when the bird was resting.