1. Statement of Newton’s First Law
Newton’s first law of motion states that:
“An object remains at rest, or continues to move with uniform motion in a straight line, unless acted upon by an unbalanced external force.”
This law describes the natural behaviour of objects — they do not change their state on their own.
1.1. What the Law Means
If something is not moving, it will not start moving unless a force pushes or pulls it. If something is already moving, it will keep moving in the same way unless some force changes it.
1.2. Another Name: The Law of Inertia
Newton’s first law is also called the law of inertia because it is based on the idea that objects resist changes in their state of motion.
2. Rest and Motion According to the First Law
Rest and motion are both natural states for objects. They do not change without a reason.
2.1. Objects at Rest
An object at rest stays at rest until an unbalanced force acts on it. Example: A football on the ground stays still unless you kick it.
2.2. Objects in Motion
An object in motion keeps moving in a straight line at the same speed unless a force stops it or changes its direction. Example: A moving ball slows down only because friction and air resistance act on it.
3. Understanding Unbalanced Forces
The first law works only when no unbalanced external force acts on the object. If forces are balanced, the object's motion does not change.
3.1. Examples of Unbalanced Forces
- A bat hitting a moving ball — the bat applies an unbalanced force and changes the ball's motion.
- Brakes in a car — friction applies an unbalanced force and stops the vehicle.
- Wind pushing a light object — it starts moving because of an unbalanced force.
4. Inertia and the First Law
Newton's first law is built around the idea of inertia. The more mass an object has, the more it resists changes to its motion.
4.1. Mass and Inertia
Heavier objects have greater inertia, so they need greater force to start or stop them. Example: It is harder to push a car than a bicycle.
4.2. Real-Life Examples of Inertia
- Passengers jerk backward when a bus starts suddenly (inertia of rest).
- Passengers jerk forward when a bus stops suddenly (inertia of motion).
- Clothes shaken vigorously throw off water drops (inertia of direction).
5. Why We Don’t See the First Law Perfectly on Earth
On Earth, friction and air resistance almost always act on objects. These forces make moving objects slow down even if no one pushes them. Because of this, uniform motion is harder to observe in everyday life.
5.1. If Friction Were Absent
On a smooth surface with very little friction (like ice), objects move much longer without stopping. In outer space, where there is almost no friction, an object will keep moving indefinitely.