Out of friction force and viscous force which depends upon velocity.

Explanation

To determine which force, between friction force and viscous force, depends on velocity, we can analyze both forces step by step.

### Step 1: Understanding Friction Force
1. **Definition**: Friction force is the force that opposes the relative motion of two surfaces in contact.
2. **Types**: There are two types of friction: static friction (when the object is at rest) and kinetic friction (when the object is in motion).
3. **Formula**: The friction force can be expressed as:
- For static friction: \( F_f = \mu_s \cdot N \)
- For kinetic friction: \( F_f = \mu_k \cdot N \)
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