Question

What is the difference between static and kinetic friction. Define both, write their SI unit and dimension. Write down the mathematical statement (formula) that allows us to calculate the force of friction. Explain each term with its unit. Let’s say you’re dragging a 50 kg rubber box on wet concrete. How do you calculate the force of friction on this box?

Answer 1

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.

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.

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

Kinetic friction remains constant regardless of the force applied

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.

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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:

where is again the size of the normal contact force between the two surfaces. 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. for steel on steel is about 0.57 while for aluminium on steel it is about 0.47.

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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 () between two surfaces is given by

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

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If mass of the box is m, then normal force (N) = mg

Therefore, Frictional force = N = mg