At its heart, physics is about finding the rules that govern how things move and interact. Classical mechanics is the rulebook for the world we see every day, from a thrown baseball to the orbit of the moon. It’s the original physics, and its core ideas were laid out by Isaac Newton centuries ago. These principles are so solid that we still use them to send rovers to Mars.

Newton's first law of motion is about inertia. It states that an object will keep doing whatever it’s doing. If it's sitting still, it will stay still. If it’s moving, it will keep moving in a straight line at a constant speed. This only changes if an outside force pushes or pulls on it.

Think about a hockey puck gliding across a perfectly smooth sheet of ice. Once you push it, it keeps going and going until it hits a wall or friction with the ice slows it down. The push is the force that changes its state of rest, and friction is the force that changes its state of motion.

This leads directly to the second law, which is the most famous of the three. It connects force, mass, and acceleration in a simple, powerful equation. The law says that the acceleration of an object is directly proportional to the force applied to it and inversely proportional to its mass.

In this formula, $F$ is force, $m$ is mass (how much “stuff” an object is made of), and $a$ is acceleration (the change in velocity). This makes intuitive sense. If you push a small grocery cart and a heavy car with the same amount of force, the cart will accelerate much more quickly because it has less mass.

The third law of motion is about action and reaction. It says that for every action, there is an equal and opposite reaction. Forces always come in pairs. When you jump, your legs push down on the Earth. That’s the action. At the same instant, the Earth pushes up on you with an equal force. That’s the reaction, and it's what launches you into the air.

Beyond Newton’s laws, two other ideas are critical in classical mechanics: the conservation of energy and the conservation of momentum. They are called conservation laws because they describe quantities that remain constant in a closed system, meaning no external forces are acting on it.

Energy is the capacity to do work. It comes in many forms, but in mechanics, we often focus on two: kinetic and potential energy.

• Kinetic energy is the energy of motion. A fast-moving object has more kinetic energy than a slow one.
• Potential energy is stored energy. An object held high off the ground has gravitational potential energy because gravity can make it fall.

The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another. Think of a roller coaster. At the top of the first big hill, it has a lot of potential energy and very little kinetic energy. As it rushes down the hill, potential energy is converted into kinetic energy. The total amount of energy, the sum of kinetic and potential, stays the same throughout the ride (ignoring friction).

Momentum is a measure of an object's motion, combining its mass and velocity. It's often described as “mass in motion.” You can calculate it with a simple formula:

Here, $p$ is momentum, $m$ is mass, and $v$ is velocity. A heavy truck moving slowly can have the same momentum as a light car moving quickly.

The law of conservation of momentum is especially useful for understanding collisions. It states that the total momentum of a closed system before a collision is equal to the total momentum after the collision. When a cue ball hits another billiard ball, the momentum from the first ball is transferred to the second. Momentum isn't lost; it just moves between objects.

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These concepts—Newton's laws and the conservation principles of energy and momentum—form the bedrock of classical mechanics. They give us a powerful framework for predicting the motion of almost everything in our everyday experience.