What is the SI unit of force of friction ?

Options

A. Newton
B. joule
C. `n//m^(2)`
D. `N//m`

Answer

To find the SI unit of force of friction, we can follow these steps:

### Step 1: Understand the Concept of Friction Force
Friction force is the force that opposes the relative motion of two surfaces in contact. It is given by the formula:
\[ F_f = \mu \cdot N \]
where:
- \( F_f \) is the friction force,
- \( \mu \) is the coefficient of friction (a dimensionless quantity),
- \( N \) is the normal force.

### Step 2: Identify the Normal Force
The normal force \( N \) is the force exerted by a surface to support the weight of an object resting on it. For an object resting on a horizontal surface, the normal force is equal to the weight of the object, which can be expressed as:
\[ N = m \cdot g \]
where:
- \( m \) is the mass of the object,
- \( g \) is the acceleration due to gravity (approximately \( 9.81 \, \text{m/s}^2 \)).

### Step 3: Determine the Unit of Normal Force
The unit of force in the SI system is the Newton (N). Therefore, the unit of the normal force \( N \) is:
\[ [N] = \text{kg} \cdot \text{m/s}^2 \]

### Step 4: Calculate the Unit of Friction Force
Since the coefficient of friction \( \mu \) is dimensionless, the unit of the friction force \( F_f \) is the same as the unit of the normal force \( N \):
\[ [F_f] = \mu \cdot [N] = \text{dimensionless} \cdot \text{N} = \text{N} \]

### Conclusion
Thus, the SI unit of the force of friction is:
\[ \text{Newton (N)} \]