The purpose of mechanics is to describe how bodies change their position in space with "time." I

should load my conscience with grave sins against the sacred spirit of lucidity were I to formulate

the aims of mechanics in this way, without serious reflection and detailed explanations. Let us

proceed to disclose these sins.

It is not clear what is to be understood here by "position" and "space." I stand at the window of a

railway carriage which is travelling uniformly, and drop a stone on the embankment, without

throwing it. Then, disregarding the influence of the air resistance, I see the stone descend in a

straight line. A pedestrian who observes the misdeed from the footpath notices that the stone falls

to earth in a parabolic curve. I now ask: Do the "positions" traversed by the stone lie "in reality" on a

straight line or on a parabola? Moreover, what is meant here by motion "in space" ? From the

considerations of the previous section the answer is self−evident. In the first place we entirely shun

the vague word "space," of which, we must honestly acknowledge, we cannot form the slightest

conception, and we replace it by "motion relative to a practically rigid body of reference."

The positions relative to the body of reference (railway carriage or embankment) have already been

defined in detail in the preceding section. If instead of " body of reference " we insert " system of

co−ordinates," which is a useful idea for mathematical description, we are in a position to say : The

stone traverses a straight line relative to a system of co−ordinates rigidly attached to the carriage,

but relative to a system of co−ordinates rigidly attached to the ground (embankment) it describes a

parabola. With the aid of this example it is clearly seen that there is no such thing as an

independently existing trajectory (lit. "path−curve" 1) ), but only a trajectory relative to a particular

body of reference.

You can download the book from here,

Download Book

**Tags:**

Relativity: The Special and General Theory by Albert Einstein Book PDF free download

## No comments:

## Post a Comment