Instruction

1

In order to calculate the

**force**of sliding friction (Fтр), you need to know the time**of braking**and stopping distance.2

If you know the time

**of braking**, but not known for its braking distance, you can calculate by the formula:s = υ0⋅t/2, where s is the stopping distance, t – time**braking**, υ0 is the velocity of the body at the beginning of**braking**.To calculate the speed of the body at the beginning**of the braking**you'll need to know the braking distance and time**of braking**. Calculate it by the formula:υ0 = 2s/t, where υ0 is the velocity of the body at the beginning of**braking**, s – stopping distance, t – time**braking**.3

Note that the stopping distance is proportional to the square of the initial speed before

**braking**and inversely proportional to the magnitude of the force of sliding friction (power**braking**). That is why, for example, on dry roads (when calculating for cars) shorter braking distance than slippery.4

Once you become aware of all the values, substitute them in the formula: Fтр = 2m⋅s/t2, where FTR silyl sliding friction (power

**braking**), m is the mass of a moving body, s is the braking distance, t – time**braking**.5

Knowing

**the power of****braking**, but not knowing his time, you can make the necessary calculations according to the formula:t = m⋅υ0/ Fтр, where t is the time**of braking**, m is the mass of the moving body, υ0 is the velocity of the body at the beginning of**braking**, FTR silyl**braking**.6

Calculate the

**force**of sliding friction according to another equation:Fтр = μ⋅ Fнорм where FTR silyl sliding friction (power**braking**), μ – coefficient of friction, Fнорм – force of normal pressure of the pressing body to the support (or mg).7

Determine the coefficient of friction experimentally. In school textbooks in physics it is usually already indicated in the conditions of the problem, if you do not want to calculate it for some particular body during laboratory work. To do this, place the body on an inclined plane. Determine the angle at which the body starts moving, and then find out the tables or independently calculate the tangent values of the angle α (the ratio of opposite over adjacent). This will be the value of the coefficient of friction (μ = tg α).

# Advice 2: How to find frictional force

Friction – the process of interaction of two bodies, causing deceleration at offset relative to each other. Find

**force****friction**means to determine the magnitude of the impact is directed in the direction opposite to the movement, because of which the body loses energy and eventually stops.Instruction

1

The force

**of friction**is a vector quantity, which depends on many factors: the pressure of bodies upon each other, the materials from which they were made, speed. Surface area in this case is irrelevant, because the bigger it is, the more reciprocal pressure (reaction force bearing N), which is already involved in finding the force**of friction**.2

These quantities are proportional to each other and connected with a coefficient

**of friction**μ, which can be considered constant, if greater accuracy of calculations is not required. So to find the**force****of friction**, you need to calculate the product:Fтр = μ•N.3

Given the physical formula refers to the friction caused by sliding. It can be dry and wet, if between the bodies is present in the liquid layer. The force

**of friction**must always be taken into account in determining the totality of the forces acting on the body in solving problems.4

Rolling friction occurs during rotation of a body on the surface of another. It is present on the boundary of contact bodies that is constantly changing. However, the force

**of friction**constantly opposes the motion. On this basis, it is equal to the ratio of the product of the coefficient**of friction**of the rolling and pressing forces to the radius of a rotating body:Fтркач = f•N/r.5

We must distinguish between the coefficient

**of friction**of sliding and rolling. In the first case is the value with no dimension, in the second it represents the distance between the straight lines that characterize the direction of clamping forces and the reaction forces of the supports. Therefore, it is measured in mm.6

The coefficient

**of friction**of rolling is usually a known value for common materials. For example, for iron for iron it is equal to 0.51 mm for iron on wood – 5,6, wood on wood – 0,8-1,5, etc. it can be Found by the formula the ratio of the moment**of friction**by the pressing force.7

The force

**of friction**of rest you receive at the minimum displacement or deformation. This force is always present in the dry slide. Its maximum value equals μ•N. There is also internal friction within the body between its layers or parts.Note

Uniform motion of a body is characterized by the balance between the external force and the frictional force.