At any movement between the surfaces of bodies or in the environment in which it moves there are always resistance forces. They are called still friction forces. They can depend on types of the rubbing surfaces, reactions of a support of a body and its speed if the body moves in the viscous environment, for example, water or air.
It is required to you
- - dynamometer;
- - table of coefficients of friction;
- - calculator;
- - scales.
Instruction
1. Find force of resistance to the movement which affects evenly rectilinearly moving body. For this purpose by means of a dynamometer or in a different way measure force which needs to be put to a body that it moved evenly and rectilinearly. Under the third law of Newton it will be in number equal to force of resistance of the movement of a body.
2. Determine force of resistance to the movement of a body which moves on a horizontal surface. In this case friction force is directly proportional to force of reaction of a support which, is in turn equal to the gravity operating on a body. Therefore force of resistance to the movement in this case or friction force of Ftr is equal to the work of body weight of m which is measured by scales in kilograms, on the acceleration of gravity of g≈9.8 of m/s² and proportionality coefficient μ, Ftr =μ ∙ m∙g. The number μ is called coefficient of friction and depends on the surfaces contacting at the movement. For example, for friction of steel on a tree this coefficient is equal to 0.5.
3. Calculate force of resistance to the movement of the body moving on the inclined plane. Except friction coefficient μ, body weights of m and acceleration of gravity of g, it depends on a plane tilt angle to the horizon α. To find force of resistance to the movement in this case, it is necessary to find works of coefficient of friction, body weight, acceleration of gravity and a cosine of the angle under which the plane is inclined to Ftr horizon =μ ∙ by m∙g∙сos(α).
4. At the movement of a body in air at low speeds force of resistance to the movement Fc is directly proportional to the speed of the movement of a body of v, Fc=α ∙ to v. The coefficient α depends on properties of a body and viscosity of the environment and pays off separately. At the movement at high speeds, for example, when falling a body from considerable height or the movement of the car, force of resistance is directly proportional to Fc=β speed square ∙ to v². The coefficient β in addition pays off for high speeds.