In which of the following cases the net force is NOT zero?

Options

A. A kite skillfully held stationary in the sky
B. A ball freely falling from a height
C. An aeroplane rising upward at an angle of `45^(@)` with the horizontal with a constant speed
D. A cork floating on the surface of water

Answer

To determine in which of the given cases the net force is NOT zero, we will analyze each option one by one.

### Step-by-Step Solution:

1. **Option A: A kite skillfully held stationary in the sky.**
- A kite that is stationary in the sky is in a state of static equilibrium. In this condition, the forces acting on the kite (the gravitational force downward and the lift force from the wind upward) are balanced. Therefore, the net force acting on the kite is zero.
- **Conclusion:** Net force = 0. This option does not satisfy the condition.

2. **Option B: A ball free falling from a height.**
- When a ball is in free fall, the only force acting on it is the gravitational force (weight) which is directed downward. Since there are no other forces acting on the ball, the net force is equal to the gravitational force (mg), which is not zero.
- **Conclusion:** Net force ≠ 0. This option satisfies the condition.

3. **Option C: An aeroplane rising upward at an angle of 45 degrees with a horizontal with a constant speed.**
- If the aeroplane is moving at a constant speed, it means that the forces acting on it are balanced. The lift force generated by the wings of the aeroplane balances the weight (mg) of the aeroplane. Therefore, the net force acting on the aeroplane is zero.
- **Conclusion:** Net force = 0. This option does not satisfy the condition.

4. **Option D: A car floating on the surface of water.**
- A car floating on water is also in equilibrium. The buoyant force acting on the car balances its weight (mg). Since these two forces are equal and opposite, the net force acting on the car is zero.
- **Conclusion:** Net force = 0. This option does not satisfy the condition.

### Final Answer:
The only case where the net force is NOT zero is **Option B: A ball free falling from a height.**

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