When two surfaces aren’t moving but are pressing together, they have the chance to interlock on the microscopic level. When this happens, it creates static friction. In physics, the coefficient of static friction is
You experience static friction when you push something that starts at rest. This is the friction that you have to overcome to get something to slide.
For example, say that the static coefficient of friction between the ingot in the figure and the ground is 0.30, and the ingot has a mass of 1,000 kilograms (quite a fortune in gold). What’s the horizontal force that a thief has to exert to get the ingot moving? You know that the magnitude of the force of friction is related to the magnitude of the normal force by
And because the surface is flat, the normal force — the force that presses the two surfaces together — is in the opposite direction of the ingot’s weight and has the same magnitude. That means that
where m is the mass of the ingot and g is the acceleration due to gravity near the surface of the Earth. Plugging in the numbers gives you
The thief needs about 2,900 newtons of force just to get the ingot started. There are 4.448 newtons to a pound, so that translates to about 650 pounds of force. Pretty respectable force for any thief.
