Kinetic friction, also known as sliding friction or dynamic friction, is the force that opposes the motion of two objects sliding or moving past each other. It comes into play when an object is already in motion on a surface. Unlike static friction, which acts to prevent the initiation of motion, kinetic friction acts to slow down or resist the existing motion.
Kinetic friction is also called Dynamic friction.
The formula for kinetic friction force (Fk) is given by:
Fk=μkN
Where:
- Fk is the kinetic friction force,
- μk is the coefficient of kinetic friction (a dimensionless constant),
- N is the normal force between the two surfaces in contact.
In this formula, the coefficient of kinetic friction (μk) represents the ratio of the force of kinetic friction to the normal force. It is a measure of the “slipperiness” or “stickiness” of the surfaces in contact.
The kinetic friction force acts in the direction opposite to the motion of the object. The formula helps in calculating the force needed to maintain a constant velocity when an object is already in motion on a surface.
Daily Life Examples of Kinetic Friction
- Skidding of car tires on a road.
- Sliding down a playground slide.
- Braking a bicycle.
- Ice skating on an ice rink.
- Rubbing hands together to generate warmth.
- Sliding a book across a table.
- Sliding a pencil on a paper.
- Using sandpaper on wood.
- Sliding a box across the floor.
- Braking a car to come to a stop.
Difference Between Static and Kinetic Friction
| # | Difference Aspect | Static Friction | Kinetic Friction |
|---|---|---|---|
| 1 | Initiation of Motion | Prevents initiation of motion. | Acts to resist motion in progress. |
| 2 | Nature of Surfaces | Depends on coefficient of static friction (μs) and normal force. | Depends on coefficient of kinetic friction (μk) and normal force. |
| 3 | Direction of Force | Opposite to applied force attempting motion. | Opposite to the direction of motion. |
| 4 | Magnitude of Force | Varies up to a maximum, below or equal to applied force for initiation. | Generally remains constant once motion is initiated. |
| 5 | Energy Dissipation | Converts potential energy into internal energy without relative motion. | Converts kinetic energy into heat during sliding motion. |
| 6 | Coefficient of Friction | μs is used in calculations. | μk is used in calculations. |
| 7 | Application | Applied to situations where objects are at rest, requiring an external force. | Applied to situations where objects are already in motion, resisting their movement. |
| 8 | Formula | Fs≤μsN | Fk=μkN |
