Consider the contact force exerted by one object on another, while the two surfaces are not moving in respect to each other. The vector component of the contact force which is not the normal force is the static friction force. The maximum static friction force is proportional to the normal force. The constant of proportionality is the coefficient of static friction between those particular two objects.

Static friction is stronger than low-speed kinetic friction.

In somewhat simpler terms, static friction is why your keyboard doesn't move across the desk while you type on it, and why everything you touch doesn't go flying across the room. It's also why cars have low gears and why you can keep your balance standing on the ground.

Static friction is like a threshold force that has to be overcome in order for an object to move. Your keyboard is not moving while you type on it (well, it is, but infinitesimally). But if you firmly push the keyboard from one of its sides, it will slide a short ways. The minimum amount you have to push in order to make it move is essentially equal to the force of static friction (with adjustments for gravity, etc).

How much static friction is there for an object? It depends mostly on the object's weight. The weight of an object is determined not only by its mass, but also by the force of gravity, as well as all other forces acting in the vertical axis. The table that the keyboard is resting on exerts an upward force on the keyboard. It has to, if you think about it -- otherwise, the keyboard would simply sink into the table. This is an example of Newton's third law: "for every action, there is an equal and opposite reaction." The force of the table pushing upward on the keyboard is called the "normal force" (in physics and mathematics, the word "normal" is used to mean "perpendicular"). The normal force is equal to the object's weight, and acts in the opposite direction of the weight. This is why you can easily slide the keyboard across the table, but you cannot easily slide a parked car that has its brakes on. The weight of the car makes the force required very large.

Briefly consider the normal force exerted on the keyboard by the table or desk. It would be equal to the keyboard's weight, which is equal to the acceleration of gravity (9.8 m/s^2) times the mass of the keyboard (1.3 kg, for a typical keyboard). Remember that force is measured in newtons, and is equal to the mass of an object times its acceleration. So we have:

W = (9.8 m/s2)(1.3 kg) = 12.74 N

The normal force is therefore:

N = -W = -12.74 N

There is one more factor to consider before we can calculate the static friction, which is both obvious and a pain. Different objects have different degrees of slipperiness. It takes much less force to get a 1.3 kg block of ice moving across the table than a 1.3 kg block of plastic (like a keyboard). Similarly, it takes more force if the keyboard has sandpaper on the bottom instead of a piece of plastic. This difference is reflected in a quantity called the "coefficient of static friction". It does not vary with an object's weight, but with the material it is made out of. The only way to determine the coefficient of static friction for a particular object is to consult a table or conduct an experiment. Furthermore, we would also need to figure out the coefficient of static friction for the table as well, since the bottom of the keyboard and the top of the table are in contact. Coefficients of friction get smaller as objects get slipperier. Teflon on teflon is very close to 0, while sandpaper on sandpaper is very large. We will disregard the coefficient of static friction for the table in this small example, assuming it to be close to 0*. The coefficient of static friction for the bottom of the keyboard might be around .4**.

The force of static friction is equal to the coefficient of static friction times the normal force:

f = μN = (.4)(-12.74 N) = -5.09 N

Note that the coefficient of friction was unitless, because it's only a coefficient by which to increase or decrease the action of the normal force. According to this calculation***, you would need to exert around 5.09 newtons of force sideways on the keyboard to make it move.

An experiment to figure out the coefficient of static friction for two surfaces could be as follows:

  1. Gather up a block (made of material A), a pulley, a bunch of weights, a rope or string, and a scale, plus a horizontal surface made of material B.
  2. Determine the mass of the block by weighing it on the scale, and multiply by gravity (9.8 m/s2 to calculate the weight.
  3. Tie one end of the rope to the block.
  4. Put the block on the surface (which needs to be elevated above the ground -- on a table is perfect. Of course, you can measure the coefficient for the table itself).
  5. Run the rope through the pulley and dangle it over the edge of the table.
  6. Add weights to the dangly end until the block just begins to move.
  7. Record the amount of weight you had to add, then add weights on top of the block, and repeat.
  8. Divide the added-weight numbers by the weight of the block. These are approximate coefficients of static friction (if you performed the experiment correctly) for surface A against surface B. Average them to get a better approximation.
This is only one method for determining the coefficient of static friction. More accurate methods are available.
* This is inaccurate.
** This is inaccurate -- I guesstimated.
*** The results of the calculation are grossly inaccurate for the real world, but correct in principle. The force REALLY required could be anything, as many factors have been glossed over or assumed out of existence.

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