Rest & Motion

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Rest and Motion

Introductory Concepts


One of the fundamental properties of objects making up this physical world is that they exhibit movement. Movement or motion is all around us, both in the world that we can see, and in the microscopic world that we cannot. Understanding motion is essential to understanding the nature of the physical world that we inhabit.

An object is said to be at rest if its positionPosition refers to location of a body at an instant of time. does not change with time. If the object's position changes with time, it is said to be in motion. However, these are very simplistic definitions.


Consider a scenario. You are traveling in a car, seated with your friend in the back seat. Your friend is quietly minding his own business, and according to you, your friend is at rest since his position does not change with time. But for a pedestrian standing beside the road and seeing the car go by, your friend (as well as you, are in motion since both of you are changing position as far as the pedestrian is concerned.

So, at any point of time, an object can be at rest with respect to a second object, while being in motion with respect to a third.

Thus, an object's state of rest or motion is always relative to some other body with reference to which we are making the observation. Or in other words, motion is always relative, never absolute.



There are two ways in which we think about time.

One is the identification of a particular moment or instant in time, generally in response to a question like "What time is it?".

The other way is to identify an interval of time which has a beginning and end time moments, generally in response to a question like "How much time did this event take?".

In physics, the term time generally refers to an interval of time, and is represented by t. The SI unit for measuring time is the second.

A number of experiments in physics involve measurement of changes with time. When graphs are plotted describing the phenomena, time is always plotted along the horizontal x-axis.

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Cassidy, D, Holton, G & Rutherford, J, Understanding Physics, NY, USA: Springer-Verlag, 2002.
Hsu, T, Foundations of Physics, 1st edn, MA, USA: CPO Science, 2004.