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Kinetic Friction

Newton's First Law is actually pretty non-intuitive, when you think about it. For example, suppose a box is sliding across the floor at 2m/s. The forces on the block are its weight and the normal force upward from the floor, which balance each other. There don't appear to be any horizontal forces, and so according to Newton's First Law the box should keep moving at a constant 2m/s forever, right?

Obviously we know that the box will come to a stop pretty quickly (unless the floor is very slippery), and if we believe Newton, that means that the forces on the box must be unbalanced: that is, something is pushing on the box from the left. The only thing touching the box is the floor, so it must be the floor's fault, and in fact the floor is exerting a new force called kinetic friction. (The word kinetic means “motion”.)

Kinetic and static friction are similar in that they both point along the surface of contact, but otherwise they are very different. Static friction is an adjustable force, taking whatever value it needs to, to maintain equilibrium. Kinetic friction, on the other hand, is a fixed force, with the approximate equation

where

• $N$ is the normal force between the two objects, and
• $\mu_K$ is called the coefficient of kinetic friction, and depends on the materials at the surface of contact.

Kinetic friction always points in the direction opposite of the object's motion, which means (as we’ll see later) that it tends to slow moving objects down. If you want to keep the box moving at a constant speed, you must balance the kinetic friction force by, for example, pushing on the box in the direction you want to go.

When static friction breaks, it changes immediately into kinetic friction: you can never have both from the same surface at the same time (though they could both appear, for example, if a box is sliding across the floor with another box resting on top of it).