### What is kinetic friction?

Kinetic friction refers to the opposing force experienced by a moving object. It is the minimum force necessary to sustain the motion of an object. Kinetic friction only occurs when objects are in relative motion, whereas static friction occurs when objects are at rest.

The coefficient of kinetic friction determines the magnitude of this force. The strength of kinetic friction is directly related to the normal force, similar to other frictional forces.

### Kinetic friction formula

fk = μk*N

The value of kinetic friction, denoted as fk, is determined by the coefficient of kinetic friction, represented by μk. N represents the net normal force exerted on the objects at their point of contact. The normal force refers to the total perpendicular forces acting on the bodies at the end of contact.

The unit of friction is Newton, while the coefficient of kinetic friction is dimensionless. Frictional forces always oppose the motion, so kinetic friction acts in a direction opposite to the applied force.

What are the laws of kinetic friction?

There are four laws of kinetic friction

#### 1. First Law of Kinetic Friction

The force of kinetic friction (Fk) is directly related to the normal reaction (N) between two contacting surfaces. The constant of proportionality, known as the kinetic coefficient of friction (k), determines this relationship.

#### 2. Second Law of Kinetic Friction

The force of dynamic friction remains unaffected by the shape and apparent area of the surfaces in contact.

#### 3. Third Law of Kinetic Friction

The contact surface’s nature and material determine the frictional force’s characteristics.

#### 4. Fourth Law of Kinetic Friction

The object’s speed in contact does not significantly affect the frictional force, as long as the relative speed between the object and the surface is within a certain range.

### 10 Examples of kinetic friction

1. A rock rolling down a slope
2. A person running in a 100 m race
3. A moving truck or car
4. A football rolling on the ground
5. The movement of a snake on the ground
6. Someone moving on roller skates
7. Skiing down a mountain
8. Sliding down a kid’s slide
9. Striking a matchstick against a surface
10. Mopping with a rag cloth

### Applications of Kinetic Friction

1. Friction plays a significant role in everyday situations, such as when two objects are rubbed together. The motion generated from this rubbing can generate heat, possibly leading to fires in certain cases.

2. Friction is also responsible for the gradual wearing down of surfaces, so it is necessary to use oil or lubricants to reduce friction and protect machine parts from excessive wear and tear.

3. When two objects are rubbed together, the frictional force generated during the interaction is transformed into thermal energy, which, in some cases, can result in the ignition of a fire.