Mechanics is the branch of physics that explains how and why objects move. It’s the rulebook for everything from a thrown baseball to a planet orbiting the sun. We can break mechanics into two main parts: kinematics, which describes motion, and dynamics, which explains what causes it.

Before we can understand why things move, we need a clear way to talk about their movement. That's the job of kinematics. It gives us the language to describe motion using three key ideas: displacement, velocity, and acceleration.

Displacement isn't the same as distance. Imagine you walk 10 meters north and then 10 meters south, returning to your starting point. You've traveled a distance of 20 meters, but your displacement is zero because you ended up right where you began.

Just like displacement, velocity cares about direction. A car driving at 50 mph around a circular track has a constant speed, but its velocity is always changing because its direction is constantly turning.

Acceleration doesn't just mean "speeding up." When a car slows down, it's accelerating (just in the opposite direction of its motion). And because velocity includes direction, that car on the circular track is constantly accelerating, even if its speed never changes.

Now that we can describe motion, we can ask why it happens. The answer lies in the work of Isaac Newton, whose three laws of motion form the foundation of dynamics.

This law introduces the concept of inertia—an object's resistance to a change in its state of motion. In simple terms, things like to keep doing what they're already doing. If a book is sitting on a table, it will stay there forever unless something (a push, a pull, gravity) forces it to move.

So, what does it take to change an object's motion? A force. This brings us to the second law.

This is famously summarized in one of the most important equations in physics.

Here, $F$ is the net force, $m$ is mass, and $a$ is acceleration. This formula tells us a few things. To make something accelerate more, you need to apply more force. It also tells us that for the same amount of force, a heavier object (more mass) will accelerate less than a lighter one. It's much easier to push a shopping cart than a car.

This means that forces always come in pairs. When you push on a wall, the wall pushes back on you with the same amount of force. When a rocket expels gas downwards (the action), the gas pushes the rocket upwards (the reaction), launching it into space. The two forces are equal in strength but opposite in direction.

Forces can also transfer energy. In physics, work is done when a force causes an object to move a certain distance. If you push a box across the floor, you are doing work on the box. If you push on a wall and it doesn't move, you might feel tired, but you haven't done any work in the physics sense.

Work ($W$) equals the force ($F$) applied multiplied by the distance ($d$) moved. Work is a way of transferring energy, which is the ability to do work. A moving object has kinetic energy, or the energy of motion. An object held up high has potential energy, or stored energy due to its position.

One of the most fundamental principles in all of science is the Law of Conservation of Energy. It states that energy cannot be created or destroyed; it can only change from one form to another. When you drop a ball, its potential energy is converted into kinetic energy as it falls. The total amount of energy remains the same throughout the process.

Finally, power is the rate at which work is done. It's not just about how much work you do, but how fast you do it. Lifting a heavy weight quickly requires more power than lifting it slowly, even though the total work done is the same.