Rest and Motion in Physics

Concept of Rest and Motion

The concept of rest and motion is one of the basic concepts of physics. It is introduced while studying how motion is classified in physics. In simple terms, an object is in motion if it changes its position with time. An object is at rest if its position does not change with time.

Quick summary: An object is at rest if its position does not change with time; it is in motion if its position does change. Both rest and motion are relative — they depend on the observer’s frame of reference. The same object can be at rest with respect to one observer and in motion with respect to another, at the same instant.

By the end of this article, you will be able to:

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

Tutorial on Rest and Motion in Physics

Watch this short video to understand the concept of rest and motion in physics.

Rest and Motion Examples

Look around you. Some things are moving, and some things are not.

• Moving things around us include cars, buses, trains, birds, animals, and human beings. All of these move from one place to another.
• Apart from these visible cases, many examples of motion occur inside the body too. Blood moves through blood vessels, muscles move when we work or play, and the heart pumps blood continuously.
• We do not directly see these internal motions, but we know about them from their effect — our life depends on the heart pumping blood, and we need muscle movement to perform tasks.

So, we can say that:

An object is in motion if its position changes with respect to its surroundings over time.

Now consider objects like a tree, a house, a school, or an electric pole. These are stationary objects — they remain fixed in position. So, some things around us are moving, and some are stationary.

Define Motion and Rest

Let us now define rest and motion formally.

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

One common feature of all moving bodies is that they change their position with time. The position of a body is known with respect to its surroundings.

• For a book kept on a table inside a room, the table, walls, and ceiling form the surroundings.

To be precise, the state of motion or the state of rest of a body must be defined with respect to a chosen frame of reference — a coordinate system, fixed to an observer, used to specify the position of an object over time.

Consider a book lying on a table in a room. When nobody disturbs it, its position with respect to the tabletop, walls, or ceiling of the room does not change.

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, someone enters the room, picks up the book, and starts moving out with it. 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.

Rest vs Motion — Quick Comparison

Why Rest and Motion Are Relative Terms

Both rest and motion are relative — this means that any object in motion with respect to one observer might be at rest with respect to another observer.

• Consider the book kept on the tabletop. Suppose you pick it up and start moving.
• The book in your hand is at rest with respect to you, but the same book is in motion with respect to the walls of the room.

For more clarity, consider a person observing another person inside a hot air balloon.

• Person A observes person B in a hot air balloon from the ground. With respect to A (or the ground), person B is in motion.
• The same person B is at rest with respect to the hot air balloon, or with respect to person C sitting beside them in the balloon — since neither is moving with respect to the other as time passes.

So, both rest and motion are relative — they depend on which object or body we choose as the frame of reference.

Key Ideas

• Rest: position does not change with time, with respect to a chosen frame of reference.
• Motion: position changes with time, with respect to a chosen frame of reference.
• Relativity of motion: the same object can be at rest in one frame of reference and in motion in another, at the same instant.
• Frame of reference: a coordinate system, fixed to an observer, used to specify position and describe motion.

Frequently Asked Questions

Why are rest and motion called relative terms?

Rest and motion are relative because they are defined with respect to a chosen frame of reference. An object at rest in one frame of reference can appear to be moving in another. So, whether an object is at rest or in motion depends on the observer’s frame of reference.

What are some examples of an object at rest?

Common examples of rest include:

1. A sleeping person
2. A standing person
3. A stopped clock
4. A book kept on a table
5. A car parked in a parking lot

What causes an object’s state of rest or motion to change?

An unbalanced force acting on an object changes its state of rest or motion.

What are some examples of motion?

Common examples of motion include:

1. A cricketer hitting a ball
2. Walking
3. Kicking a football
4. Jumping
5. Dancing

Is all motion in physics relative?

Yes, all motion is relative, since it is always defined with respect to a chosen frame of reference. No object is in absolute motion or absolute rest independent of a frame of reference.

Can an object be at rest and in motion at the same time?

Yes. An object can be at rest with respect to one frame of reference and in motion with respect to another, at the same instant. For example, a passenger seated in a moving car is at rest with respect to the car but in motion with respect to the road.

Further Reference

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