Mechanical work is not what you think

What is work, everyone knows. Even children work in the kindergarten - kids. However, the generally accepted, everyday notion is far from being the same as the concept of mechanical work in physics. For example, a man stands and holds a bag in his hand. In the ordinary sense, he does the job, holding the load. However, from the point of view of physics, he does not do anything like this. What's the matter?

Once such questions appear, it's time to remember the definition. When a force acts on the object, and the body moves under it, mechanical work is performed. This value is proportional to the path traveled by the body and the applied force. There is also an additional dependence on the direction of application of force and the direction of motion of the body.

Thus, we introduced such a concept as mechanical work. Physics defines it as the product of the magnitude of the force and displacement multiplied by the cosine of the angle that exists in the most general case between them. As an example, you can consider several cases that will allow you to better know what is meant by this.

When does the 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 - Mom is carrying a child in a wheelchair, in this case the work is done, the force is applied, the carriage is moved. The difference in the two described cases is the presence of displacement. And accordingly, the work is carried out (example with a stroller) or not performed (example with a truck).

Another case - a boy on a bike accelerated and quietly rolls along the path, pedal does not twist. The work is being done? No, although there is movement, but there is no applied force, the movement is carried out by inertia.

Another example - a horse carries a cart, on it sits a driver. Does he do the work? Moving is, the applied force is (the driver's weight affects the cart), but the work is not performed. The angle between the direction of movement and the direction of the action of the force is 90 degrees, and the cosine of the angle of 90 degrees is zero.

These examples allow us to understand that mechanical work is not just the product of two quantities. It must also take into account how these quantities are directed. If the direction of travel and the direction of the force action coincide, the result will be positive, if the direction of movement is opposite to the direction of application of the force, the result will be negative (for example, work done by the frictional force when moving the load).

In addition, it must be taken into account that the force acting on the body can be the resultant of several forces. If this is so, then the work of all the forces applied to the body is equal to the work done by the resulting force. The work is measured in joules. One joule is equal to the work done by force in one newton when the body moves one meter.

From the examples considered, an extremely curious conclusion can be drawn. When we looked at the driver on the cart, it was determined that he was not committing a job. The work is done in a horizontal plane, because it is there that the movement is made. But the situation will change slightly when we consider a pedestrian.

When walking, the center of gravity of a person does not remain immobile, it moves in a vertical plane and, therefore, does work. And since the motion is directed against the force of attraction, the work will occur against the direction of the action of gravity. Let the movement and a small, but with a long walk the body will have to do additional work. So the right gait reduces this extra work and reduces fatigue.

Having analyzed several simple life situations chosen as examples, and using knowledge of what mechanical work is, we have examined the basic situations of its manifestation, as well as when and what work is being done. They determined that such a concept as work in everyday life and in physics is of a different nature. And it was established by means of application of physical laws that incorrect gait causes additional fatigue.