Static Friction and Kinetic Friction

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What Is Friction?

In science, friction is defined as a resisting force that acts opposite the direction of the object's motion.

Two conditions are needed for the force of friction to be present. First, the two surfaces must be in contact with each other. Second, there should be an attempt to move the object against the surface of another object, or the objects are already in motion sliding past each other.

Imagine a box sitting on the floor:

• Is there a friction force?

The first condition is satisfied because the surface of the box and the floor are in contact.

However, if there is no attempt to move the box by pushing it, then the second condition is not satisfied. Therefore, the force of friction is not present.

Now, let's say the box is being pushed to the right:

In this example, there is a man applying a force to the box in one direction. If the applied force is to the right, the friction force acts in the opposite direction to the left.

• Does the box need to move before we can say that friction is present?

The answer is no. Keep reading to find out why.

Static Friction and Kinetic Friction: What's the Difference?

Static friction is a "grippy" type of force that does not allow the two surfaces to slide past each other even if a force is applied.

This happens when you try to push a heavy box, and it does not move. The box will never move if the amount of static friction force is the same amount of the opposing applied force.

If you are able to increase your applied force, you may reach the maximum static friction force and begin moving the box. This is what made the car in the scenerio above difficult to start moving.

Kinetic friction, also called sliding friction, is the force that exists between the surfaces of the objects that are in contact and sliding past each other.

As soon as you apply a greater force on the box and it starts moving, kinetic friction is present. This kinetic friction force occurs opposite the direction of the object's motion.

The amount of kinetic friction force will depend on the type of materials that are in contact.

Take a look at this graph comparing static friction force and kinetic friction force:

When you increase your applied force (push) on the box, you also increase your static friction force. However, there will come a point when you will reach your maximum static friction force, and the box will start moving.

It is important to note how the graph above shows that the strength of static friction force is greater than the kinetic friction force. This is why it is harder to start an object moving than to keep it moving.

It would be difficult to push the heavy box when it is at rest, but it would require less force as it starts sliding. This is why the car above was easier to push once it got going.

For further explanation, watch Does Static Friction exist? | Physics | Don't Memorise from Don't Memorise:

Coefficient of Friction

Later in your mathematical discussion of static and kinetic friction, you will encounter the term coefficient of friction.

• What does that mean?

The coefficient of friction is a number usually between 0 to 1 but can be greater than 1. This value represents the amount of friction between surfaces in contact and may vary depending on the type of material.

It is dimensionless, and it does not have any unit.

The coefficient of friction is assigned the Greek letter mu (μ). We use the subscript s for coefficient of static friction and a subscript k for coefficient of kinetic friction.

If you are curious about some of the values for different types of materials, check out the Table of Common Static and Kinetic Friction Coefficients found under the Downloadable Resources in the right-hand sidebar.

• Are you ready to review what you have learned about the difference between static friction and kinetic friction forces?

Then head over to the Got It? section!