A person places a hand on a closed book resting on a table and then presses downward while pushing outward. Either the book slides across the table or the hand slides across the book. What determines which event happens? Which type(s) of friction is (are) involved?

Imagine trying to push a heavy box across the floor. You apply some force, and yet, the box doesn't budge. This is where static friction comes into play. Static friction is the force that resists the initiation of sliding motion between two surfaces that are at rest with respect to each other.

It's a self-adjusting force that increases with the applied force up to a certain point—this point is known as the maximum static friction force. In the scenario of the book and the hand, static friction is what keeps the book stationary until the pushing force becomes strong enough to overcome this resistance. Similarly, it is static friction that decides if the hand will slide over the book or not, before any actual movement starts.

Now, if you've pushed that heavy box and it's started to slide, the type of friction that comes into play is kinetic friction. Kinetic friction, also known as sliding or dynamic friction, acts between two surfaces in relative motion.

Generally lower than static friction, kinetic friction exerts a constant force that opposes the direction of motion, acting as a resistance that slows down the moving object. When the hand slides over the book or the book slides over the table, kinetic friction is the force that needs to be overcome to keep the motion going. It is determined by the nature of the surfaces and is less dependent on the force pressing the two surfaces together compared to static friction.

The normal force is an essential component in understanding friction. It is the perpendicular force exerted by a surface on an object in contact with it. This force is a reaction to the object's weight or any other force pressing the object against the surface.

In the book and hand scenario, the weight of the book and the downward force applied by the hand contribute to the normal force between the book and the table, denoted as N. This normal force is then used to calculate the maximum static friction and kinetic friction when movement occurs. It's crucial to recognize that normal force directly affects how much static or kinetic friction can be exerted by a surface.

Understanding the Coefficient

Every material interaction has a unique coefficient of static friction, symbolized as \( \mu_s \)—a dimensionless number representing the ratio of the maximum static friction force to the normal force. This coefficient describes how resistant a surface is to sliding.

When calculating the maximum static friction force that can act on the book or the hand, multiplying the normal force by the respective coefficient gives us the threshold at which objects will begin to move. A higher coefficient means more effort is needed to start the movement—imagine the difference between pushing an object on a rough concrete floor versus a smooth, ice-covered surface.