1. Friction

2. Types of Friction
All moving objects are subject to friction. Friction is a force that opposes the motion of objects that touch as they move past each other. The types of friction include:
Static friction
Sliding friction
Rolling friction
Fluid friction

3. Static Friction
The friction force that acts on objects that are not moving. This type of friction always act in the opposite direction of the applied force. Every time you take a step you experience static friction. The static friction between the ground and your shoe keeps your shoe from sliding, as you push off with each step (see the image).

4. Sliding Friction
A force that opposes the direction of motion of an object as it slides over a surface. This friction is less than static friction therefore, less force is needed to keep the object in motion than to get it to begin to move. An example is sliding a match across the match box to light it.

5. Rolling Friction
The friction force that acts on rolling objects. When a round object is rolling both the object and the surface it is rolling on are bent slightly out of shape.

6. Fluid Friction
Friction acts on a submarine moving through water as well as an airplane flying through air. A mixture of gases such as air and water are known as fluids. The motion of an object is opposed by the force of fluid friction. An example of fluid friction is air resistance- resistance exerting on an object as it moves through air.