Which of the following are contact forces ?
A. Friction B. Gravitational force C. Magnetic force D. Muscular force

Options

A. A and B
B. B and C
C. A and D
D. B and D

Answer

To determine which of the given forces are contact forces, we can analyze each option based on the definition of contact forces. Contact forces are those that occur when two objects are physically touching each other.

### Step-by-Step Solution:

1. **Understanding Contact Forces**:
- Contact forces are forces that occur when two objects are in direct contact with each other. Examples include friction, tension, and muscular force.

2. **Analyzing Option A: Friction**:
- Friction is the force that opposes the relative motion of two surfaces in contact. Since friction occurs when two surfaces touch, it is classified as a contact force.
- **Conclusion**: Friction is a contact force.

3. **Analyzing Option B: Gravitational Force**:
- Gravitational force is the attraction between two masses. For example, when an apple falls to the ground, it is pulled by the Earth's gravity. However, this force does not require the two objects (apple and Earth) to be in contact.
- **Conclusion**: Gravitational force is a non-contact force.

4. **Analyzing Option C: Magnetic Force**:
- Magnetic force is the attraction or repulsion between magnetic poles. For instance, a magnet can attract iron filings from a distance without any physical contact.
- **Conclusion**: Magnetic force is a non-contact force.

5. **Analyzing Option D: Muscular Force**:
- Muscular force is the force exerted by muscles when they contract. This force requires physical contact with the object being moved or pushed. For example, when a person pushes a box, their muscles exert a force through direct contact with the box.
- **Conclusion**: Muscular force is a contact force.

6. **Final Conclusion**:
- Based on the analysis, the contact forces among the options provided are:
- A. Friction
- D. Muscular force
- Therefore, the correct answer is that options A and D are contact forces.