Newton's Laws of Motion for JEE
Foundations of Motion
What Is Motion?
Look around you. Some things are still, like the chair you're sitting on. Other things are moving, like a fan spinning above you. In physics, we have simple terms for this: rest and motion.
An object is at rest if its position doesn't change over time relative to its surroundings. An object is in motion if its position does change. It seems simple, but there's a catch: rest and motion are always relative to an observer.
Imagine you're on a train, sitting perfectly still in your seat. To the person next to you, you are at rest. But to someone standing on the platform watching the train go by, you are in motion. Who is right? Both are. Motion is always described with respect to a frame of reference.
The Cause of Change
What causes an object at rest to start moving, or a moving object to stop? The answer is force. A force is simply a push or a pull. When you kick a football, you apply a force. When you lift your backpack, you apply a force.
Forces are the agents of change in the world of classical mechanics—the branch of physics that deals with the motion of everyday objects. They can change an object's speed, its direction, or both.
Without a force, an object's state of motion will not change.
Forces can be sorted into two main categories based on how they interact with objects.
| Force Type | Description | Examples |
|---|---|---|
| Contact Forces | Act when objects are physically touching. | Pushing a door, friction from the ground, the tension in a rope. |
| Non-Contact Forces | Act over a distance without physical touch. | Gravity pulling an apple down, a magnet attracting a paperclip. |
Now, consider a book resting on a table. Is gravity pulling it down? Yes. So why isn't it moving? Because the table is pushing up on it with an equal and opposite force. When all the forces acting on an object cancel each other out, we call them balanced forces. The object's motion doesn't change.
But if you push the book, you introduce a new, unbalanced force. The forces no longer cancel out, and the book starts to move. An unbalanced force is what causes a change in an object's state of motion.
Resisting Change
If you try to push a bicycle and then try to push a car with the same effort, you know the car will be much harder to move. This is because the car has more mass. Mass is a measure of how much matter, or "stuff," is in an object. It's a fundamental property that doesn't change no matter where you are, be it on Earth or the Moon.
Closely linked to mass is the concept of inertia. is the resistance of any physical object to any change in its state of motion. The more mass an object has, the more inertia it has, and the harder it is to start it moving or to stop it once it's in motion.
We can observe inertia in three different ways:
Inertia of Rest: An object's tendency to stay at rest. When you shake a branch, the leaves fall off because they tend to remain at rest while the branch moves.
Inertia of Motion: An object's tendency to stay in uniform motion. A person jumping from a moving bus falls forward because their feet stop upon hitting the ground, but the rest of their body continues to move forward.
Inertia of Direction: An object's tendency to resist a change in its direction of motion. When a car makes a sharp turn, passengers feel pushed to the side because their bodies want to continue moving in a straight line.
With these basic ideas of motion, force, and inertia, we now have the vocabulary to explore the rules that govern how everything moves.
According to physics, when is an object considered to be in motion?
You are sitting on a train that is moving at a constant speed. From your point of view, the passenger sitting opposite you is at rest. From the point of view of someone standing on the station platform, that same passenger is in motion. Who is correct?