Friction is a force that is created whenever two surfaces move or try to move across each other.

• Friction always opposes the motion or attempted motion of one surface across another surface.
• Friction is dependant on the texture of both surfaces.
• Friction is also dependant on the amount of contact force pushing the two surfaces together (normal force).

In this simulation you see a block sitting on a level table. You can place an applied force on the object by pressing the "More Force" button. Each time you press the button the applied force will increase. As you use this simulation there are several things you should notice.

• On a level surface, the normal force (FN) is always equal and opposite to the weight (only on a level surface).
• The force of static friction (fs) cancels out the applied force right up to and including when static friction reaches its maximum (fsmax).
• For applied forces greater than the maximum force of static friction the block starts to slip and then the value for friction becomes kinetic friction (fk) and the box is then under a net force so it accelerates to the right.

The force of friction depends upon both surfaces in contact and the normal force. A mathematical relationship can be created here.

The coefficients for static and kinetic friction are listed in some reference tables. The coefficient of static friction is usually a little bit higher than coefficient of kinetic friction for the same two surfaces. When coefficients are listed they must be given for one surface on another surface (ie wood-on-asphalt). The higher the coefficient, the greater the force of friction. The table below lists the coefficients for a few common surfaces used in physics. They are arranged from "sticky" to slippery.

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