____ is a force that opposes motion.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Concept of Force:**
- A force is defined as any interaction that, when unopposed, will change the motion of an object. Forces can be categorized into different types, including contact forces.

2. **Identifying Contact Forces:**
- Contact forces are those that occur when two objects are in physical contact with each other. There are two main types of contact forces: normal force and friction force.

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