Understanding of physical terms and knowledge of the definitions of quantities plays an important role in the study of various laws and for solving problems in physics. One of the fundamental concepts is the concept of body weight. Let us consider in more detail the question: what is this - body weight?
History
Taking into account the modern view of physics, we can confidently say that body mass is a characteristic that manifests itself during movement, during interaction between real objects, as well as during atomic and nuclear transformations. However, this understanding of the masses finally took shape quite recently, literally in the first decades of the 20th century, thanks to the theory of relativity created by Einstein.
Returning further to history, we recall that some philosophers of ancient Greece believed that there was no movement, so there was no concept of body weight. Nevertheless, there was a concept of body weight. To do this, just recall the law of Archimedes. Weight is related to body weight. This, however, is not the same quantity.
In the era of the New Age, thanks to the work of Descartes, Galileo and especially Newton, concepts of two different masses were formed:
As it turned out later, both types of body mass are one and the same quantity, which is inherently characteristic of all objects surrounding us.
Inertial
Speaking of inertial mass, many physicists begin to give a formula for Newton’s second law, in which force, body mass and acceleration are connected into one equality. Nevertheless, there is a more fundamental expression from which Newton himself formulated his law. It's about the amount of movement.
In physics, the quantity of motion is understood as the value equal to the product of body mass m by the speed of its movement in space v, that is:
p = m * v
For any body, p and v are vector variable characteristics. The quantity m is a constant coefficient for the body under consideration that relates p and v. The larger this coefficient, the greater the p value at constant speed and the more difficult it is to stop the movement. That is, body mass is a characteristic of its inertial properties.
Using the written expression for p, Newton obtained his famous law, which mathematically describes the change in the momentum. It is customary to express it in the following form:
F = m * a
Here F is the force that acts on a body with mass m and tells it the acceleration a. As in the previous expression, the mass m is the coefficient of proportionality between two vector characteristics. The greater the mass of the body, the more difficult it is to change its speed (less than a) using a constant acting force F.
Gravity
Throughout history, mankind has followed the sky, stars and planets. As a result of numerous observations in the XVII century, Isaac Newton formulated his law of universal gravitation. According to this law, two massive objects are attracted to each other in proportion to the two constants M 1 and M 2 and inversely to the square of the distance R between them, that is:
F = G * M 1 * M 2 / R 2
Here G is the gravitational constant. The constants M 1 and M 2 are called the gravitational masses of interacting objects.
Thus, the gravitational mass of a body is a measure of the force of attraction between real objects, which has nothing to do with the mass of inertia.
Body weight and weight
If the expression above is applied to gravity on our planet, then we can write the following formula:
F = m * g, where g = G * M / R 2
Here M and R are the mass of our planet and its radius, respectively. The value of g is the acceleration of gravity familiar to every student. The letter m denotes the gravitational mass of the body. This formula allows you to calculate the force of attraction by the Earth of a body of mass m.
According to the third Newtonian law, the force F must be equal to the reaction of the support N, on which the body rests. This equality allows us to introduce a new physical quantity - weight. Weight refers to the force with which the body stretches the suspension or presses on a specific support.
Many people who are not familiar with physics do not distinguish between the concepts of weight and mass. At the same time, these are completely different quantities. They are measured in different units (mass in kilograms, weight in Newtons). In addition, weight is not a characteristic of the body, but mass is. Nevertheless, it is possible to calculate body mass m, knowing its weight P. This is carried out according to the following formula:
m = P / g
Mass is a single characteristic.
It was noted above that body mass is gravitational and inertial. In developing his theory of relativity, Albert Einstein proceeded from the assumption that the noted types of mass represent the same characteristic of matter.
To date, numerous measurements of both types of body masses have been carried out in various situations. All these measurements led to the conclusion: gravitational and inertial masses coincide with each other with the accuracy of the instruments that were used to determine them.
The rapid development of nuclear energy in the middle of the last century deepened the understanding of the concept of mass, which turned out to be related to energy through the speed constant of light. Energy and body weight are a manifestation of some single essence of matter.