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 upwards 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 following cases the net force is not zero, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Option 1: A kite skillfully held stationary in the sky.**
- The kite is held stationary, which means it is not moving.
- If an object is stationary, its acceleration is zero.
- According to Newton's first law, if the acceleration is zero, the net force acting on the object is also zero.
- **Conclusion:** The net force is zero in this case.

2. **Option 2: A ball freely falling from a height.**
- The ball is falling under the influence of gravity.
- The force acting on the ball is its weight, which is given by \( F = mg \) (where \( m \) is the mass of the ball and \( g \) is the acceleration due to gravity).
- Since the ball is in free fall, it is accelerating downwards due to gravity.
- Therefore, the net force acting on the ball is not zero.
- **Conclusion:** The net force is not zero in this case.

3. **Option 3: An aeroplane is rising upward at an angle of 45 degrees with the horizontal with a constant speed.**
- The aeroplane is moving at a constant speed, which means its acceleration is zero.
- If the acceleration is zero, the net force acting on the aeroplane must also be zero.
- **Conclusion:** The net force is zero in this case.

4. **Option 4: A cork floating on the surface of water.**
- The cork is floating, which means it is in equilibrium.
- Two forces act on the cork: the gravitational force (weight) acting downwards and the buoyant force acting upwards.
- For the cork to float, these two forces must balance each other (i.e., they are equal in magnitude but opposite in direction).
- Therefore, the net force acting on the cork is zero.
- **Conclusion:** The net force is zero in this case.

### Final Conclusion:
The only case where the net force is not zero is **Option 2: A ball freely falling from a height.**