Static Friction: Definition, Formula & Examples

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  • 0:04 What Is Static Friction?
  • 0:49 What Causes Friction?
  • 1:20 Inequality for Static Friction
  • 3:41 Examples of Static Friction
  • 4:00 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

In this lesson you will learn what static friction is and how it is different from kinetic friction. You will also learn a formula that can be used to calculate the force of static friction and see some examples of static friction in real life.

What Is Static Friction?

Have you ever noticed that it's harder to get a shopping cart moving than it is to keep it moving. If you try to push your couch across the room, the first push is the hardest part. Maybe people assume that's nothing more than psychological, but there really is a physics reason for it. The reason is static friction. Friction, in general, is a force that makes it harder for two objects to slide alongside one another. Static friction is the friction that exists between a stationary object and the surface on which it's resting.

Once the objects have already started moving, kinetic friction takes over. This is the friction that exists between two objects moving relative to each other. Kinetic friction isn't as strong as static friction, and so it's easier to keep the shopping cart moving.

What Causes Friction?

If you were to see an extreme close up of an otherwise smooth surface, you'd see that it contains a whole landscape of mountains and valleys, pits and bumps. These imperfections cause two surfaces to grip each other and make it hard for things to slide.

But when an object is stationary, there's also something called adhesion between two surfaces. Adhesion is where two non-moving surfaces stick together slightly, due to some light chemical bonding between the materials. This is what makes static friction so strong.

Inequality for Static Friction

Force is a push or pull, measured in Newtons (N). Friction is one such force. We can calculate how many Newtons of frictional force there are between two surfaces using this inequality:

Inequality for Static Friction

The coefficient of friction is just a number that represents how much two surfaces grip each other. The normal force is the force a surface applies to an object to keep it sitting on a surface. Without a normal force, objects would fall through the ground, because there would be nothing to hold them up. On a flat surface, this normal force is equal to the force of gravity acting down on the object, and this can be calculated by taking the mass of the object (in kilograms) and multiplying it by the acceleration due to gravity:

Equation for the force of gravity
Equation for the Force of Gravity

So for example, if a block is resting on a slope with a coefficient of friction of 0.1, and the normal force acting on the block is 2 Newtons, we can use the equation for static friction to calculate the force of static friction between the slope and the surface:

0.1 * 2 = 0.2 Newtons

The equation for static friction is almost identical to the equation for other kinds of friction, such as kinetic friction. The only difference is that instead of the coefficient of static friction, you would use the coefficient of kinetic friction instead.

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