Which one of the following is not a contact force

Options

A. Viscous force
B. Air resistance
C. Friction
D. Buoyant force

Answer

To determine which of the following options is not a contact force, we will analyze each option provided in the question:

1. **Viscous Force**:
- Viscous force arises when layers of liquid are in contact with each other. It opposes the relative motion between these layers. Since it acts between two layers of fluid that are in contact, it is classified as a contact force.

2. **Air Resistance**:
- Air resistance occurs when an object moves through air, and the air particles exert a force on the object. This force acts in the opposite direction of the object's motion. Since air particles are in contact with the object, air resistance is also a contact force.

3. **Friction**:
- Friction is the force that opposes the relative motion between two surfaces in contact. It acts when two solid bodies are in contact with each other. Therefore, friction is a contact force.

4. **Buoyant Force**:
- Buoyant force is the upward force exerted by a fluid on an object submerged in it. This force results from the pressure difference exerted by the fluid on the object. Although it is related to the presence of fluid, it does not arise from direct contact between the object and the fluid particles in the same way as the other forces mentioned. It is considered a non-contact force because it acts on the object due to the pressure exerted by the fluid surrounding it.

**Conclusion**: Among the options given, the **buoyant force** is not a contact force.

### Step-by-Step Solution:
1. **Identify the forces**: List the forces given in the options: viscous force, air resistance, friction, and buoyant force.

2. **Analyze each force**:
- **Viscous Force**: Acts between layers of liquid in contact; it is a contact force.
- **Air Resistance**: Acts on objects moving through air; it is a contact force.
- **Friction**: Acts between surfaces in contact; it is a contact force.
- **Buoyant Force**: Acts on submerged objects due to pressure differences in the fluid; it is not a contact force.

3. **Determine the answer**: The force that is not a contact force is the **buoyant force**.