What Is the Difference Between Static Friction & Sliding Friction?

Friction

• Surfaces like tabletops and tires may look smooth to the naked eye, but if you could see them through a microscope you'd discover there are countless small irregularities ̵2; a little like tiny hills and valleys in a miniature landscape. When you try to push an object across a surface, these irregularities catch on each other and resist motion. Other forces like interactions between molecules in the two surfaces also act to oppose any movement.

Static Friction

• Static friction is the force that resists you when you push a stationary object. Imagine, for example, that you have a 40 pound concrete block and you try to push it across an ice skating rink. Initially, nothing happens. That's because the force of static friction is exactly equal to the force you exert on the block, so the block doesn't experience any net force. The harder you push on the block, the harder static friction pushes back.

Sliding Friction

• If you keep on increasing the force you exert on the block, you'll eventually reach a point where static friction hits a maximum value or threshold. At that point, your force exceeds static friction and the block begins to move. If you stopped pushing on the block for a minute, however, it would gradually come to a stop. That's because the force of sliding friction opposes the block's movement. Unlike static friction, sliding friction is relatively constant and does not depend on the force you exert on the block or its speed.

Coefficients

• The maximum "push" from static friction is determined by the force holding the two surfaces together and the coefficient of static friction, a number that measures the strength of friction for two particular surfaces. Sliding friction is very similar, but the coefficient of static friction is generally less than the coefficient of static friction, which is why it's more difficult to start an object sliding than it is to keep it going. When your car tires are rolling and you hit the brakes, you're relying on static friction between your tires and the road to bring your car to a halt. A car in a skid, however, experiences sliding rather than static friction, and since the coefficient of sliding friction is less, the skidding car takes longer to come to a halt.