Difference between Static and Kinetic Friction

Main Difference – Static vs. Kinetic Friction

Static friction and kinetic friction are both types of friction: they act when two surfaces are moving or attempting to move against each other, and resist the motion. The main difference between static and kinetic friction is that static friction acts while the surfaces are at rest while kinetic friction acts when there is relative motion between the surfaces.

What is Static Friction

Static friction is the frictional force acting between two surfaces which are attempting to move, but are not moving. Think of having a block of wood on a table. If you attach a string to it and pull it with a very small force, it would not move. Since you are applying a force in the “forward” direction, according to Newton’s first law there must be another “backward” force to maintain the object at rest. This is the frictional force. It acts along the two surfaces, in the direction opposite to the direction of attempted motion.

Now imagine increasing the pulling force. For a while, the block would still not move. In order for the forces to remain balanced, the force of friction must be also increasing as you increase the force of the pull. Eventually, at some point, your pulling force will be able to overcome friction and the object will begin to move. When the object just begins to move, the force of static friction has reached its maximum value.

The maximum static friction ( $f_{s, max}$ ) between two surfaces is given by

$f_{s, max}=\mu_s n$

where $n$ is the size of the normal contact force between the two surfaces. $\mu_s$ is called the coefficient of static friction and has a constant value for two types of surfaces. e.g. $\mu_s$ for steel on steel is about 0.74 while for aluminium on steel it is about 0.61.

What is Kinetic Friction

Kinetic friction is the frictional force acting between two surfaces which are in motion against each other. Going back to our example of dragging a wooden block across a table, once the block is moving it will experience kinetic friction. Kinetic friction will remain largely the same for the two surfaces regardless of their relative speed. It will fluctuate over time, but it will do so about a fixed average value.

The size of kinetic friction is given by:

$f_{k}=\mu_k n$

where $n$ is again the size of the normal contact force between the two surfaces. $\mu_k$ is called the coefficient of kinetic friction and also has a constant value for two types of surfaces. The value for the coefficient of kinetic friction is always less than the coefficient for static friction between two surfaces, meaning that the force of kinetic friction experienced by an object is always less than the maximum force of static friction experienced between them. $\mu_k$ for steel on steel is about 0.57 while for aluminium on steel it is about 0.47.

Difference between Static and Kinetic Friction - Graph

Difference between Static and Kinetic Friction – The graph shows how the frictional force between two surfaces changes as the pulling force is increased. The static friction rises steadily while the object remains stationary, and it drops down to kinetic friction when the object is in motion.

No surface is perfectly smooth, a surface contains microscopic “bumps”. When two surfaces are moving against each other, these bumps interlock against each other and oppose the motion. Friction originates from this opposition.

Difference Between Static and Kinetic Friction

Definition of Static Friction and Kinetic Friction

Static friction is the frictional force acting between two surfaces which are attempting to move, but are not moving.

Kinetic friction is the frictional force acting between two surfaces which are in motion against each other.

When it acts

Static friction acts when the surfaces are not in relative motion against each other.

Kinetic friction acts when the surfaces are in relative motion against each other.

Range

Static friction increases linearly with the force applied until it reaches a maximum value.

Kinetic friction remains constant regardless of the force applied.

Comparative Size

Static friction could have a value less or greater than the value for kinetic friction.

Kinetic friction has a value less than the maximum value of static friction.

References

Young, H. D., & Freedman, R. A. (2012). Sears and Zemansky’s university physics: with modern physics. Addison-Wesley.

Image Courtesy

“As force is gradually increased on a mass, the frictional force increases with it until the object starts to move, at which point, it experiences kinetic friction.” by Loodog (Collected in a physics lab using a Vernier Dual-Range Force sensor) [CC BY-SA 3.0], via Wikipedia