The movement in actual practice cannot continue indefinitely. The reason for that – friction force. It arises at contact of a body with other bodies and is always sent to an opposite direction of the movement. It means that friction force always performs negative work that needs to be considered when calculating.
It is required to you
- - roulette or range finder;
- - table of la of determination of coefficient of friction;
- - concept about kinetic energy;
- - scales;
- - calculator.
Instruction
1. If the body moves evenly and rectilinearly, find force which sets it in motion. It compensates friction force therefore it is in number equal to it, but is directed towards the movement. Measure by a roulette or a range finder distance of S on which force of F moved a body. Then work of friction force will be equal to the work of force on distance with the sign "minus" of A=-F∙S.
2. Example. The car moves on the road evenly and rectilinearly. What work is performed by friction force at a distance of 200 m if force of draft of the engine is equal to 800 N? At the uniform rectilinear motion force of draft of the engine is equal on the module to friction force. Then its work will be equal to A=-F∙S =-800∙200=-160000 J or-160 kJ.
3. Property of surfaces to keep the friend for the friend shows friction coefficient μ. For each couple of contacting surfaces it is different. He can be calculated or recognized in the special table. There is a coefficient of static friction and coefficient of a sliding friction. When calculating work of friction force take coefficient for sliding as without movement work is not performed. For example, the tree sliding friction coefficient on metal is equal to 0.4.
4. Define work of friction force operating on the body located on a horizontal surface. For this purpose determine its weight by m in kilograms by means of scales. Increase weight by sliding friction coefficient for these surfaces μ, acceleration of gravity (m/s g≈10²) and distance on which the body, S moved. Before a formula deliver the sign "minus" as the body moves aside, opposite to the direction of friction force (A=-μ ∙ to m∙g∙S).
5. Work of friction force when only she acts, is equal to change of kinetic energy of a body. For its definition find initial v0 and final v of speed of a body on the explored site of a way. Increase m body weight by the difference of squares of initial and final speed of a body, and divide result into number 2 (And =m ∙ (v²-v0²)/2). For example, if the car weighing 900 kg moving with a speed of 20 m/s stops, then work of friction force will be equal to A=900 ∙ (0²-20²)/2=-180000 J or-180 kJ.