KINEMATICS-DISTANCE & DISPLACEMENT
REST AND MOTION ARE RELATIVE
A body can be both at rest and in motion at the same time. A body can be in motion relative to one set of objects while at rest relative to some other set of objects. Thus, rest and motion are relative. To understand this, consider the following examples.
Examples for relativity in rest and motion:
1. Suppose you are lying on your bed. You are at rest in relation to all other objects inside the bedroom. But the room (or home) is on earth and the earth itself is not at rest. The earth is revolving around the sun. It takes one year to complete one revolution around the sun. Thus, along with earth, you are also revolving around the sun. Hence in relation to the sun, you are in motion.
2. Next, consider a boy sitting on a bench in a park looking to a bus passing by on a road. For the boy, the bus is in motion but the trees in the park appear to be at rest. But to a boy sitting inside the bus, the trees and boy outside the bus will appear to move in the opposite direction and the roof of the bus or driver of the bus will appear to be at rest.
3. If two trains move at the same speed and in the same direction on two parallel railways then both trains will appear stationary to the passengers sitting in them.
Important Terms:
The mathematical quantities that are used to describe the motion of objects can be divided into two categories. The quantity is either a vector or a scalar. These two categories can be distinguished from one another by their distinct definitions:
Scalar:
Scalars are quantities that are fully described by a magnitude (or numerical value) alone.
Ex: Distance, the distance between Chennai to Hyderabad is 600 km.
Vector:
Vectors are quantities that are fully described by both a magnitude and a direction.
Ex: Displacement, a body is moving 4km towards south & after turned 5km towards north.