Understanding physical terms and knowing 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 mass. Let's take a closer look at the question: what is body weight?
History
Taking into account the modern view of physics, it is safe to say that the mass of a body is a characteristic that manifests itself during movement, during the interaction between real objects, as well as during atomic and nuclear transformations. However, this understanding of mass took shape quite recently, literally in the first decades of the 20th century, thanks to the theory of relativity created by Einstein.
Returning further into history, we recall that some philosophers of ancient Greece believed that movement does not exist, so there was no concept of body mass. Nevertheless, there was a concept of body weight. To do this, it is enough to recall the law of Archimedes. Weight is related to body weight. However, they are not the same value.
BIn the modern era, thanks to the works of Descartes, Galileo and especially Newton, the concepts of two different masses were formed:
- inertial;
- gravitational.
As it turned out later, both types of body mass are the same value, which by its nature is characteristic of all objects around us.
Inertial
Speaking of inertial mass, many physicists are beginning to give a formula for Newton's second law, in which force, body mass and acceleration are connected in one equality. However, there is a more fundamental expression from which Newton himself formulated his law. It's about the amount of movement.
In physics, momentum is understood as a value equal to the product of body mass m and the speed of its movement in space v, that is:
p=mv
For any body, the values p and v are vector variables of the characteristic. The value m is some coefficient constant for the considered body, which connects p and v. The greater this coefficient, the greater will be the value of p at a constant speed and the more difficult it is to stop the movement. That is, the mass of a body is a characteristic of its inertial properties.
Using the written expression for p, Newton obtained his famous law, which describes mathematically the change in momentum. It is usually expressed in the following form:
F=ma
Here F is the force that acts on a body with mass m and tells it an acceleration a. As inin the previous expression, the mass m is the proportionality factor between the two vector characteristics. The greater the mass of the body, the more difficult it is to change its speed (less than a) with the help of a constant acting force F.
Gravity
Throughout history, mankind has followed the sky, the stars and the planets. As a result of numerous observations in the 17th century, Isaac Newton formulated his law of universal gravitation. According to this law, two massive objects are attracted to each other in proportion to two constants M1 and M2 and inversely proportional to the square of the distance R between them, that is:
F=GM1 M2 / R2
Here G is the gravitational constant. The constants M1 and M2 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 inertial mass.
Body weight and mass
If the expression above is applied to the force of gravity on our planet, then the following formula can be written:
F=mg, where g=GM / R2
Here M and R are the mass of our planet and its radius, respectively. The value of g is the acceleration of free fall familiar to every schoolchild. 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 with a mass of m.
According to Newton's third law, the force F must beis 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 is the force with which the body stretches the suspension or presses on a certain support.
Many people who are not familiar with physics do not distinguish between the concepts of weight and mass. At the same time, they are completely different values. 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, you can calculate the mass of a body m, knowing its weight P. This is done using the following formula:
m=P / g
Mass is a single characteristic
It was noted above that the mass of a body can be gravitational and inertial. In developing his theory of relativity, Albert Einstein proceeded from the assumption that the marked types of mass represent the same characteristic of matter.
Until now, numerous measurements of both types of body masses have been carried out in various situations. All these measurements led to the conclusion that the 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. The energy and mass of a body is a manifestation of some single essence of matter.
