Which of the following is a contact force :

Options

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

Explanation

To determine which of the following is a contact force, let's analyze each option provided:

1. **Electrostatic Force**:
- The electrostatic force acts between charged objects. According to Coulomb's law, the force \( F \) between two charges \( q_1 \) and \( q_2 \) separated by a distance \( r \) is given by:
\[ F = k \frac{q_1 q_2}{r^2} \]
- Since this force acts at a distance without any physical contact between the charges, it is classified as a **non-contact force**.

2. **Gravitational Force**:
- The gravitational force acts between two masses. The force \( F \) between two masses \( m_1 \) and \( m_2 \) separated by a distance \( r \) is given by:
\[ F = G \frac{m_1 m_2}{r^2} \]
- Similar to the electrostatic force, the gravitational force also acts at a distance without physical contact, making it a **non-contact force**.

3. **Frictional Force**:
- The frictional force arises when two surfaces are in contact and there is relative motion (or an attempt of motion) between them. It is defined as:
\[ F_{\text{friction}} = \mu F_{\text{normal}} \]
- Since this force requires physical contact between the surfaces, it is classified as a **contact force**.

4. **Magnetic Force**:
- The magnetic force acts between magnets or moving charges. The force \( F \) between two magnetic poles \( m_1 \) and \( m_2 \) separated by a distance \( r \) is given by:
\[ F = \frac{\mu_0}{4\pi} \frac{m_1 m_2}{r^2} \]
- This force can also act at a distance without direct contact between the magnets, thus it is a **non-contact force**.

**Conclusion**: Among the options provided, the only contact force is the **frictional force**.

**Final Answer**: The contact force is **C. Frictional Force**.

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