Force of braking is the friction force of sliding. If force applied to a body exceeds the maximum friction force on value, then the body begins to move. Sliding friction force always works in the direction opposite to speed.

## Instruction

1. To calculate friction force of sliding (Ftr), it is necessary to know time of braking and length of a brake way.

2. If you know braking time, but its brake way is not known, then you can execute calculation for a formula: s = υ0⋅t/2 where s is length of a brake way, t is braking time, υ0 – body speed at the time of the beginning of braking. For calculation of speed of a body at the time of the beginning of braking you will need to know the size of a brake way and time of braking. Calculate it by a formula: υ0 = 2s/t, where υ0 – body speed at the time of the beginning of braking, s – length of a brake way, t – braking time.

3. Pay attention that length of a brake way is proportional to a square of initial speed before braking and is inversely proportional to the sliding friction force size (braking forces). For this reason, for example, on the dry road (when calculating for cars) the brake way is shorter, than on slippery.

4. After all values became known to you, substitute them in the sliding friction force (braking force), m is the mass of a moving body, s is the size of a brake way, t is braking time.

5. Knowing braking force, but not knowing its time, you can make necessary calculations for a formula: t = m ⋅υ0/Ftr where t is time of braking, m is the mass of a moving body, υ0 – body speed at the time of the beginning of braking, Ftr – braking force.

6. Calculate sliding friction force by other formula: Ftr = μ ⋅ Fnorm, where Ftr – the sliding friction force (braking force), μ – coefficient of friction, Fnorm – force of the normal pressure pressing a body to a support (or mg).

7. Define friction coefficient experimentally. In school textbooks in physics it is usually already specified in statements of the problem if it is not required to calculate it for any concrete body during laboratory work. For this purpose place a body on the inclined plane. Define a tilt angle at which the body begins the movement then recognize by tables or calculate independently a tangent of the received corner size α (the relation of an opposite leg to adjacent). It will also be value of coefficient of friction (μ = tg α).