# INTRODUCTION

The term weightlessness is a state in which the gravitational force acting on the object is compensated by another force, so the resulting impression is that the gravitational force does not act on the object. A typical situation in which a state of weightlessness arises near the surface of the Earth is a free fall or a projectile motion. These movements allow weightlessness for a very short time, a maximum of a few seconds. In the long run, a state of weightlessness occurs when an object moves in outer space, for example, when a satellite orbits the Earth. Staying in a state of weightlessness brings various health problems for a person. On the other hand, it will be possible to use weightlessness for a whole range of technological processes. Nowadays, this use has been mostly in the experimental phase.

# HOW TO ACHIEVE WEIGHTLESSNESS

## Free fall and projectile motion

As already mentioned, including the analysis of the force action, if an object falls in the gravitational field of the Earth in a free fall, it is in a state of weightlessness. The same situation occurs with compound movements, in which one component is a free fall, i.e. in all cases of projectile motion - vertical, horizontal and oblique. An example of a vertical motion can be jumps on a trampoline, in which the gymnast is in a state of weightlessness for the entire time he is without contact with the trampoline. The already mentioned free fall of the elevator cabin belongs to the same category. All these movements allow weightlessness for a very short time of tenths of a second or at most a few seconds. For example, the fall of a parachutist takes a long time. However, it is in a state of weightlessness only shortly after the beginning of the fall. After only a few seconds, the speed of the parachutist increases to such an extent that the resistive force of the surroundings equalizes with the gravity force, and the next fall already takes place with a uniform movement, in which the state of weightlessness does not occur. The movement along the trajectory of a horizontal or oblique projectile motion is used to simulate the state of weightlessness in aircraft. If the aircraft moves on a parabolic trajectory, along which the thrown object would move, a state of weightlessness occurs in the aircraft. In this way, it is possible to create weightlessness for about 25 seconds.

## Movement along the Earth´s orbit

State of weightlessness occurs for a long term when the object moves in outer space. The initial condition must still apply, i.e. the object must be located in a non-inertial reference frame that moves with an acceleration of the same magnitude and direction as the gravitational acceleration at that location. If another force applied to the object, typically the thrust of rocket engines, the state of weightlessness would not occur.

As the satellite orbits the Earth along a circular trajectory, a gravitational force acts on the center of the Earth. It is this force that is the centrifugal force in the reference system connected to the Earth, which causes the trajectory to be curved into a circle. If we observe the situation from a reference system connected to a satellite, it is again a non-inertial reference frame and the satellite (the objects inside the satellite) is subjected to an apparent inertial centripetal force as great as the gravitational force, but in the opposite direction. The result of gravitational and centripetal force is zero.

## Movement in outer space

An object in outer space does not have to orbit the Earth, but it can move through space in general. In such a case, a state of weightlessness occurs in the reference frame connected to the subject, if the object moves only under the influence of gravitational forces, i.e. with acceleration equal to the gravitational acceleration at the given location. In no case is it necessary for the gravitational force acting on the object to be zero. Most cosmic objects move in this way, i.e. by the action of gravitational forces. The only exceptions are sections of flight when the artificial space object is accelerated or decelerated by the thrust of the engines or changes the direction of flight. In these sections of the flight, no weightlessness will arise.

## Simulation of weightlessness

In connection with spaceflights, the study of weightlessness and training in being in it is very important. As mentioned, a state of weightlessness on the Earth's surface can only be achieved for a very short time. For some purposes, it is not necessary to use a state of weightlessness directly, but only to simulate a stay in it.

For these purposes, a stay in spacesuits in a pool with water is often used. The spacesuit is balanced so that the buoyancy force on it is as great as the gravity force. Then a person in a spacesuit floats in the water as in a state of weightlessness. This simulation has two shortcomings. The resistance of the aquatic environment is significantly greater than the resistance of the air, so that some movements are inhibited in the water and thus facilitated. The buoyancy force acts only on the surface of the spacesuit and the spacesuit lifts the person with force acting on the skin. A completely normal gravity force acts on the internal organs of a person inside, so it is not possible to achieve the effects of a balance disorder or nausea caused by real weightlessness.

The second way to simulate weightlessness is to stay in a wind tunnel. In it, a person is floated in a vertical tunnel vertically upwards by flowing air. The speed of the air can be used to regulate the acting force and the degree of effect. The disadvantages of this simulation are similar to those of a swimming pool, only the place of greater water resistance is affected by the flow of air along overalls.

## Problems with movement in state of weightlessness

Staying in a state of weightlessness has more side effects on humans (and other higher organisms). For a short time, for several hours or days, when you stay in a state of weightlessness, a seasickness associated with stomach upset and feelings of pressure in the head appears. This is caused by non-standard stimulation of the balance system in the inner ear, which normally determines the direction of the gravity of the body.

The longer-term effect is the redistribution of fluids in the body. On the surface of the Earth, the heart pushes the blood upwards against the action of gravity, in the state of weightlessness the blood flows into the upper half of the body much more easily. Cosmonauts show redness and sometimes swelling of the face. The different nature of movements in the state of weightlessness, when it is not necessary to overcome the action of gravity and overall physical exertion is less, gradually leads to muscle atrophy. Furthermore, hormonal changes, calcium leaching from the bones and also signs of liver damage were observed in experimental animals. Some consequences of weightlessness can be eliminated, for example, by regular exercise, while others must be subjected to further thorough research in the future.

Problems with staying in weightlessness include a complete change in motor habits when performing normal activities in orbit. Weightless personal hygiene, food, excretion must be subordinated, but of course also work activities such as assembly work inside and outside the space station. Much of the common events on the Earth's surface account for the ubiquitous gravity force that "provides support”.

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