The net force acting is not zero on

Options

A. A retarding train
B. A ball falling with terminal velocity
C. A kite held stationary
D. A truck moving with constant velocity

Answer

To solve the question, we need to analyze each of the given options to determine where the net force acting is not zero.

### Step-by-Step Solution:

1. **Understanding Net Force**: According to Newton's second law of motion, the net force acting on an object is given by the equation:
\[ F_{\text{net}} = m \cdot a \]
where \( F_{\text{net}} \) is the net force, \( m \) is the mass of the object, and \( a \) is its acceleration. If the acceleration \( a \) is not zero, then the net force \( F_{\text{net}} \) is also not zero.

2. **Analyzing Each Option**:
- **Option 1: Retarding Train**: A retarding train experiences a force opposite to its direction of motion (deceleration). This means there is a net force acting on it (the resistive force). Therefore, the net force is not zero.
- **Option 2: Ball Falling with Terminal Velocity**: When a ball falls with terminal velocity, the forces acting on it (gravitational force downward and drag force upward) are balanced. Thus, the net force is zero.
- **Option 3: Kite Held Stationary**: A stationary kite has no motion, and the forces acting on it are balanced (tension in the string and weight). Therefore, the net force is zero.
- **Option 4: Truck Moving with Constant Velocity**: A truck moving at constant velocity has no acceleration, which means the net force acting on it is zero.

3. **Conclusion**: The only scenario where the net force is not zero is in the case of the **retarding train**.

### Final Answer:
The net force acting is not zero on the **retarding train**.

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