Which of these is a contact force ?

Options

A. friction
B. magnetic force
C. gravitational force
D. electrostatic force

Explanation

To determine which of the given forces is a contact force, we need to understand the definition of contact forces and analyze each option provided.

### Step-by-Step Solution:

1. **Understanding Contact Forces**:
- A contact force is a force that requires physical contact between two objects. It acts only when the two objects are touching each other.

2. **Analyzing the Options**:
- The options given are:
- Friction
- Magnetic force
- Gravitational force
- Electrostatic force

3. **Evaluating Friction**:
- Friction is the force that opposes the motion of an object when it is in contact with another surface. For example, when a box slides on the floor, friction acts between the box and the floor. Since friction requires contact, it is classified as a contact force.

4. **Evaluating Magnetic Force**:
- Magnetic force acts at a distance without the need for physical contact. For instance, a magnet can attract a paperclip without touching it. Therefore, magnetic force is not a contact force.

5. **Evaluating Gravitational Force**:
- Gravitational force also acts at a distance. An object falls towards the Earth due to gravity, even though it does not need to be in contact with the Earth to experience this force. Hence, gravitational force is not a contact force.

6. **Evaluating Electrostatic Force**:
- Electrostatic force is the attraction or repulsion between charged particles. This force can act over a distance without the charges needing to touch each other. Thus, electrostatic force is not a contact force.

7. **Conclusion**:
- After evaluating all the options, we find that the only force that requires contact between two objects is **friction**. Therefore, the answer to the question is that **friction is the contact force**.

### Final Answer:
**Friction is the contact force.**

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