Rest and Motion in Physics

Concept of rest and motion

The concept of rest and motion is one of the basic concepts of physics and is introduced while studying kinematics. In simple terms, if an object is changing its position then it is in motion and if it does not change its position then it is at rest. 
In this article, learn about rest and motion in physics. By the end of reading this article, you would be able to,

• Recognize the difference between rest and motion in physics.
• Define terms like rest and motion.
• Understand why motion is relative.

Watch this video to get the concept of rest and motion in physics

Tutorial on rest and motion in physics

Rest and Motion examples

Now if you look around then you might find some things moving and some things not moving at all.
Moving things around us include cars, buses, trains, birds, animals, human beings, etc. All these things move from one place to another. Apart from these examples things are in motion inside of the body too. For example, blood moves through blood vessels, our muscles move when we work or play, our heart pumping blood and many more search examples can be found in nature.
We do not see these motions but we know about them from their effect because our life depends on the heart pumping blood and we need muscle movement to perform various tasks.
So we can say that

Motion occurs due to movement of a body

Now let’s talk about some objects like a tree, house, School, electric pole, etc. These objects are stationary objects. They remain fixed in their position. From these examples, we can see that few things around us are moving and few of them are stationary.

 Define motion and rest

Let’s now define the terms, rest and motion

A body is said to be at rest if its position does not change with the passage of time if the position of an object changes with the passage of time it is said to be in motion.

One common feature of all moving bodies is that they change their position with time. The position of a body is known by its surroundings.
So for a book kept on a table inside a room, the table, walls, and ceiling of a room make the surroundings.

To be more precise, the state of motion or, state of the rest of the body should be defined with respect to its surroundings.

As an example, look at the book lying on the table in a room. When nobody is near it then the position of the book with respect to the tabletop, walls, or ceiling of the room is not changing.

So the book as seen from the room is at rest. We can say that the book is at rest with respect to the room.
Now you entered the room, pick up the book, and begin to move out of the room with the book. Now in this case, the distance of the book from the walls changes as time passes. So the book is in motion with respect to the room.

Why rest and motion are relative terms

Now let us look at the fact that both rest and motion are relative.
This statement means that any object which is in motion with respect to one person or object might be at rest with respect to another person or object.
For example, consider the case of the book kept on the tabletop. Now you picked up the book and started moving.
Now this book in your hand when you are moving is at rest with respect to you but the same book is in motion or moving with respect to the walls of the room.
To get more clarity on this statement let us take a case of a person observing another person inside a hot air balloon.

Now for a person A who is observing other person B in a hot air balloon from the ground, person B is in motion with respect to him or the ground. This same person B is at rest with respect to the hot air balloon or another person say person C sitting beside him in a hot air balloon. This is because both of them are sitting inside the balloon and both are not moving with respect to each other as time passes.

So, both rest and motion are relative and depend on respect to which object or body we are observing it.
I hope you now understand the concept of rest and motion. You are now aware of the fact that both rest and motion are relative.

Key Ideas

Frequently Asked Questions

Further Reference

• http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html#motcon– Visit for concept map on motion and other variables involved in kinematics.