What is work, everyone knows. Even children work, in kindergarten - toddlers. However, the generally accepted, everyday representation is far from the same as the concept of mechanical work in physics. For example, a person is standing and holding a bag. In the usual sense, he does the work, holding the load. However, from the point of view of physics, he does nothing of the kind. What is the matter here?
Once such questions arise, it's time to recall the definition. When a force acts on an object, and under its action the body moves, then mechanical work is performed. This value is proportional to the path traveled by the body and the applied force. There is an additional dependence on the direction of application of force and the direction of movement of the body.
Thus, we introduced the concept of mechanical work. Physics defines it as the product of the magnitude of the force and displacement, multiplied by the cosine of the angle, available in the most general case between them. As an example, we can consider several cases that will allow you to better know what is meant by this.
When does mechanical work fail? There is a truck, we push it, but it does not move. The force is applied, but there is no movement. The work done is zero. And here is another example - a mother carries a child in a stroller, in this case the work is done, force is applied, the stroller moves. The difference in the two cases described is the presence of displacement. And accordingly, the work is performed (example with a stroller) or not performed (example with a truck).
Another case - a boy on a bicycle accelerates and quietly rolls along the path, he does not pedal. The work is being done? No, although there is movement, but there is no applied force, the movement is by inertia.
Another example is a horse carrying a cart; a chariot sits on it. Does he do the work? There is movement, there is applied force (the weight of the driver acts on the cart), but the work is not performed. The angle between the direction of movement and the direction of the force is 90 degrees, and the cosine of the angle 90 ° is zero.
The above examples make it possible to understand that mechanical work is not just a product of two quantities. It should also consider how these quantities are directed. If the direction of movement and the direction of the force are the same, then the result will be positive, if the direction of movement is against the direction of application of force, then the result will be negative (for example, work performed by the force of friction when moving the load).
In addition, it must be taken into account that the force acting on the body can be the result of several forces. If this is so, then the work of all the forces applied to the body is equal to the work performed by the resulting force. Work is measured in joules. One joule is equal to the work that a force of one Newton does when moving the body one meter.
From the considered examples, an extremely interesting conclusion can be drawn. When we examined the cart driver, we determined that he was not doing work. The work is done in the horizontal plane, because it is there that the movement is made. But the situation will change a little when we consider the pedestrian.
When walking, the center of gravity of a person does not remain motionless, he moves in a vertical plane and, therefore, does the job. And since the movement is directed against gravity, the work will occur against the direction of gravity. Although the movement is small, but with long walking, the body will have to do additional work. So a proper gait reduces this unnecessary work and reduces fatigue.
Having analyzed several simple life situations selected as examples, and taking advantage of the knowledge of what mechanical work is, we examined the main situations of its manifestation, as well as when and what work is performed. We determined that such a concept as work in everyday life and in physics is of a different nature. And it was established through the application of physical laws that a wrong gait causes additional fatigue.