Everything Science Grade 10

CHAPTER 21. MOTION IN ONE DIMENSION 21.2

You are standing on the platform watching the train move from left to right. To you it looks
as if the boy is moving from left to right, because relative to where you are standing (the
platform), he is moving. According to the boy, and hisframe of reference(the train), he is
not moving.

A frame of reference must have an origin (where you are standing on the platform) and
at least a positive direction. The train was moving from left to right, making to your right
positive and to your left negative. If someone else was looking at the same boy, his frame
of reference will be different. For example, if he was standing on the other side of the
platform, the boy will be moving from right to left.

b

boy is standing still train moving from left to right

From your frame of reference the boy is moving from left to right.

A boy inside a train which

is moving from left to right

Where you are standing

on the platform

(reference point or origin)

negative direction (towards your left) positive direction (towards your right)

For this chapter, we will only use frames of reference in thex-direction. By doing this we
restrict ourselves toone dimensional motion. We can use the sign of the position value
(positive or negative) to indicate the direction relative to the origin.

DEFINITION: One dimensional motion

An object is constrained to move back and forth along a line.

For example the blue dot in the figure below can only move along thex-axis.

origin x

Physics: Mechanics 389