how is the position of a particle moving along a straight line described by a number
The position of a particle moving along a straight line is described by a number called the "displacement" or "position coordinate". This number represents the distance and direction of the particle from a fixed reference point, usually called the "origin".
To determine the position of a particle, you need to know two things: the reference point and the displacement. The reference point is usually chosen as the starting point or origin of the motion, and the displacement is the distance and direction from that reference point.
The displacement can be positive or negative, depending on the direction of the particle's motion. A positive displacement indicates that the particle is moving in one direction away from the reference point, while a negative displacement indicates movement in the opposite direction towards the reference point.
To measure the position of a particle, you can use a measuring device such as a ruler or tape measure. Place one end of the measuring device at the reference point (origin) and measure the distance to the particle's current location along the straight line. The measured value will give you the position or displacement of the particle from the reference point, and it can be either positive or negative depending on the direction.
It is important to note that the position of a particle changes with time, so if you want to describe the particle's motion over a period of time, you will need to measure its position at different moments and track how it changes.