Every student knows that in the presence of contact between two solid surfaces, the so-called friction force arises. We consider in this article what it is, concentrating our attention on the point of application of the friction force.
What types of friction force?
Before considering the point of application of the friction force, it is necessary to briefly recall what types of friction exist in nature and technology.
We begin to consider the friction of rest. This view characterizes the state of a solid at rest on some surface. The friction of rest prevents any displacement of the body from its state of rest. For example, due to the action of this same force, it is difficult for us to move the cabinet on the floor.
Sliding friction is another type of friction. He manifests himself in the case of contact between two surfaces sliding on each other. Sliding friction prevents movement (the direction of force friction is opposite to the speed of the body). A vivid example of its action is the sliding of a skier or skater on ice in the snow.
Finally, the third type of friction is rolling. It always exists when one body rolls on the surface of another. For example, rolling wheels or bearings are vivid examples when it is important to take into account the rolling friction force.
The first two of the described species arise due to roughness on rubbing surfaces. The third type arises due to the deformation hysteresis of the rolling body.
Points of application of friction forces of sliding and rest
It was said above that rest friction prevents an external acting force, which tends to move an object from its place along the contact surface. This means that the direction of the friction force is opposite to the direction of the external force parallel to the surface. The point of application of the considered friction force is in the contact area of two surfaces.
It is important to understand that the rest friction force is not constant. It has a maximum value, which is calculated by the following formula:
F t = μ t * N.
However, this maximum value appears only when the body begins its movement. In any other case, the static friction force modulo is exactly equal to the parallel surface of the external force.
As for the point of application of the sliding friction force, it does not differ from that for rest friction. Speaking about the difference between the friction of rest and sliding, it should be noted the absolute value of these forces. So, the sliding friction force for a given pair of materials is a constant. In addition, it is always less than the maximum rest friction force.
As you can see, the point of application of the friction forces does not coincide with the center of gravity of the body. This means that the forces in question create a moment that tends to overturn the sliding body forward. The latter can be observed when the cyclist brakes sharply with the front wheel.
Rolling friction and its application point
Since the physical reason for the appearance of rolling friction differs from that for the types of friction considered above, the point of application of the rolling friction force has a slightly different character.
Suppose the wheel of a car is on asphalt. Obviously, this wheel is deformed. The area of contact with asphalt is 2 * d * l, where l is the width of the wheel, 2 * d is the length of the side contact of the wheel and asphalt. The rolling friction force in its physical essence manifests itself in the form of a reaction moment of the support directed against the rotation of the wheel. This moment is calculated as follows:
M = N * d
If we divide it and multiply by the radius of the wheel R, then we get:
M = N * d / R * R = F t * R, where F t = N * d / R
Thus, the rolling friction force F t is actually a reaction of the support creating a moment of force that tends to slow down the rotation of the wheel.
The point of application of this force is directed vertically upward relative to the surface of the plane and is shifted to the right of the center of mass by a value of d (provided that the wheel moves from left to right).
Problem solving example
The action of friction of a force of any kind tends to slow down the mechanical motion of bodies, while translating their kinetic energy into thermal energy. We solve the following problem:
- the bar slides on an inclined surface. It is necessary to calculate the acceleration of its movement, if it is known that the coefficient for sliding is 0.35, and the angle of inclination of the surface is 35 o .
Consider what forces act on the bar. First, the component of gravity is directed down along the sliding surface. It is equal to:
F = m * g * sin (α)
Secondly, upward along the plane a constant friction force acts, which is directed against the acceleration vector of the body. It can be determined by the formula:
F t = μ t * N = μ t * m * g * cos (α)
Then Newton's law for a bar moving with acceleration a takes the form:
m * a = m * g * sin (α) - μ t * m * g * cos (α) =>
a = g * sin (α) - μ t * g * cos (α)
Substituting the data into the equality, we obtain that a = 2.81 m / s 2 . Note that the acceleration found does not depend on the mass of the bar.