Equilibrium and forces

1. Anybody which is moving in a straight line at constant speed is in equilibrium; the stationary objects are merely special cases in which the constant speed is zero.

2. A book is probably lying on lying desk – it is motionless and in static equilibrium. What do we know about the forces which are no the book? Unless there is a high wind, there are no forces which tend to move it horizontally across the desk. We know, however, that there is a vertical force which acts on it and tries to move it downward toward the earth`s center. This gravitational attraction toward the earth is the book`s weight; you can fell this if you hold the book in your hand. We know, too, that in order to keep the book from falling, you must push upward with a force equal to its weight. If you withdraw you hand, the downward force of gravity acts without any opposition and the book falls down. We can then be sure that the book which is lying on the table is also being acted on by two forces: the downward pull of gravity and the equal and opposite upward push of the tabletop.

3. If we want to call an upward force «+» and downward force «–», a little more elegant way to say the same thing is to state that all the forces which act on a body in equilibrium add up to zero. This rule of course applies to other directions as well as to up and down. If you push the book north with a force of 5 lb., the book will move unless your push is opposed by some southward 5 lb. force.

4. In object that possesses a definite shape, we can single out an important point which is known as the center of gravity (CG). The force of gravity acts, of course, on all parts on any given object, but the center of gravity can be defined by saying that objects that are subjected to gravity behave as if there were only a single force applied at that point. If a body is supported at its center of gravity, it will be in balance and have no tendency to move or rotate in any direction.

Acceleration and gravity

1. One of the basic postulates of Einstein`s Special Theory of Relativity is that it is useless to speak about absolute motion through space and that only the relative motion of one system with respect to another system can be considered as a physical reality. An observer enclosed inside a windowless vehicle moving with a constant velocity cannot tell whether he is in a state of motion or at rest, no matter what kind of physical experiments (mechanical, electric, or magnetic) he performs inside his vehicle in order to answer this question.

2. But what about accelerated motion? When an airplane speeds up along the runway before the takeoff you are pressed to the back of your seat, and it is not necessary to look through the window to know that you are subjected to acceleration. If the flight is smooth, the conditions inside the airplane are exactly the same as if it were standing at the airport, but any accelerations caused by air currents in bad flying weather are certainly very noticeable to the passengers. Does this mean that, whereas one should not talk about absolute velocities, absolute accelerations have a definite physical meaning?

3. This problem was attacked by Einstein about 10 years after his original publication of the theory of relativity (now known as the Special Theory of Relativity), and its solution (published in 1916) resulted in further theoretical developments now known as the General Theory of Relativity. He showed that all the physical phenomena that are observed within an accelerated system are identical with those occurring in a resting system placed in a gravitational field . To understand this idea, let us consider events taking place in the passenger cabin of a rocket ship traveling through space, far away from any gravitating celestial bodies. If the rocket motors are shut off, the ship goes freely through space with constant velocity, in accordance with Newton`s first law. The travellers and all the objects in the ship are “weightless” and float about freely.

4. Suppose now that the motors are stared so that our rocket ship begins to gain speed. Since the velocity of the ship is now increasing, the things floating in the cabin will continue to move with the same velocity with which the ship was originally going through space, and they all will be collected at the back wall of the cabin and pressed to it by the force of the acceleration.