This will be a continuation of Russell Stannard’s explanation of Einstein’s theories of sepcial and general relativity. General relativity involves the study of the effects of space and time on gravity. Having considered the theory of special relativity, let it be known that it is a special instance of the more general theory of relativity. It is event that, in a gravitational field, such as that on the surface of our planet, all objects, when released at an identical height above the ground, accelerate at the same time.
We do have to deal with air resistance, of course, which slows some objects more than others. An obvious example of this would be the contrast between a feather and a hammer. When air resistance is excluded, however, was the case on Apollo 15 when this experiment was conducted on the moon, the feather and hammer fell at the same height when dropped from the same height.
Galileo had previously established this universality of free fall. In this case, when an object is placed at such and such a point in space, and provided an initial velocity, its subsequent motion is independent of the object’s internal composition or structure as long as it is subject only to gravitational forces, rather than extrinsic forces such as wind resistance. Suppose we have gravity “g,” and the gravitational force “F.” We have the equation F = mGg where mG is a property of the object’s body, and is known as gravitational mass.
According to Newton, however, we have F = M1a, where a is the acceleration and mI is the object’s internal mass. This is a measure of the object’s inertia when responding to forces. When we get read of F, we get mGg = m1a. Indeed, the universality of free fall dictates that the acceleration of both objects, whatever they are, are identical. We then thus denote acceleration due to gravity by “g.” To say that a is identical to g is to say that mG = mI, and it is possible to speak of the mass of an object, which had previously been denoted as m.
When Einstein described special relativity, had realized that there was something amiss when trying to reconcile the principle of relativity with the fact that the speed of light was a constant. He was also confused by the fact that two very different forms of “mass” were found to have the same value. How did gravity make them both accelerate at the same rate? He concluded there must be a close association between gravity and acceleration.
Imagine dropping a hammer and a feather in a lift, the lift being the reference frame that can easily be accelerated vertically. Imagine dropping the feather and the hammer right when the lift is severed so that it falls. This will cause the lift to accelerate in precisely the same manner as the two objects. The lift, feather and hammer will all fall together, with their relative positions remaining static. To the unfortunate person in the lift, the hammer and feather will remain where they were, appearing “weightless.”
Most of us think of astronauts in space when we think of “weightlessness.” This is experienced before the craft has even left Earth’s orbit, however. These astronauts are still being pulled by Earth’s gravity. The reason they appear weightless is because the ship is in a state of free fall under the influence of the planet’s gravity in a manner similar to that of the observer in the lift.
Why does the craft not crash on the earth? This is because the planet’s gravitational attraction is being used up converting ordinary straight line motion into the orbital motion that can be observed. Therefore, none is left over and the astronaut is not pulled down to the earth. This is what makes the astronaut appear weightless.