Difference between Static and Kinetic Friction

March 9, 2017 by Editorial Team

Friction is important in our daily lives. Regardless of the type, we won’t be able to drive our cars or even walk across a room without it. This article seeks to find out how the two types of friction, static and kinetic, are different and how they affect our lives.

Definitions

Static friction keeps this man from moving the rock

Static friction is the frictional force that keeps a stationary object from moving while resting on a surface of another object. When the surfaces of two solid objects are in contact, there is always a force keeping these objects from moving against each other. The main cause of the resistance is uneven surfaces. Microscopic “peaks” and “valleys” on opposing surfaces “lock” when the peaks of one surface touch the valleys of the other surface, thus creating a form of resistance. A block of wood with a string attached to it will not move an inch unless the right amount of force is applied to overcome the resistance and pull it forward. Under normal conditions, static friction gives car tires traction on the road surface, thus avoiding skidding and sliding. Although the tires are moving in relation to the ground, a portion of the tire is always in contact with the ground until the static force is overcome.

Continuing with our example, as the force is increased to pull the block forward, so does the force of the friction, and a state of equilibrium is reached. Once the pulling force is enough to move the block, friction is overcome, and the block finally moves. The point where the blocks starts to move is the threshold of the static force.

Kinetic friction
A penguin sliding down a snowy slope, experiencing kinetic friction

Kinetic friction is the frictional force produced as a result of two surfaces moving in relation to each other. Using the earlier scenario, the block moving across the table surface is producing kinetic friction. While the friction force will remain the same for the most part regardless of the average speed, the friction force will change over time within a fixed range. Kinetic friction is also referred to as sliding friction or dynamic friction.

Kinetic friction is usually less than static friction; that is, it takes lesser force to keep an object moving than to initially overcome static friction and get it to move in the first place. However, many studies are emerging that prove kinetic friction can be greater than static friction. In addition, it is now widely held that kinetic friction is not always caused by irregular surfaces, but by chemical bonding between these surfaces.

Kinetic friction exists in almost everything that involves two objects moving against each other. Rubbing your palms against each other creates kinetic friction. A sofa being pushed to the other side of the room starts from static friction until it starts to move. When it eventually moves, kinetic friction is produced.

Comparison

Static friction is the frictional force that keeps a stationary object from moving. In contrast, kinetic friction is the frictional force present between two surfaces moving against each other. Static friction increases as the applied force increases until it reaches its maximum value. On the other hand, the value of kinetic friction is constant no matter the amount of force applied.

While static friction has always been held as harder to overcome compared to kinetic friction, newer studies are coming out stating kinetic energy can be stronger than static friction.

Comparison Chart

Static FrictionKinetic Friction
Frictional force that keeps a stationary object from moving on the surface of another objectFrictional force present between two objects moving in relation to each other

Increases as the applied force is increased

Remains constant regardless of how fast the objects are moving
Will always try to keep an object stationaryWill always try to slow down the movement of an object

Video

For more about static and kinetic friction, check out this YouTube video.