Why does a soccer ball roll across the grass when kicked? Why does a car slow to a stop when the driver steps on the brakes? And why do satellites continue circling the Earth without falling down? At first glance, these everyday observations might feel obvious. Objects move, and sometimes they stop. But behind these simple events lies one of the most important foundations of science: the study of motion and the forces that govern it.
Motion is everywhere. It defines the orbits of planets, the movement of ocean currents, and the beating of your heart. Physics, the science of matter and energy, gives us the tools to understand not just that motion happens, but how and why it happens. In this article, we’ll explore what motion really means, how speed, velocity, and acceleration describe it, and why forces—especially those described by Newton’s First Law—are the hidden causes behind everything that moves.
What Do We Mean by Motion?
At its simplest, motion is a change in position of an object over time. To say that something is “in motion,” we need a frame of reference—a point we use to judge whether it has moved. Imagine sitting in a train as it glides smoothly along the tracks. To you, the seat in front of you is still. But to a person standing at the station, that same seat is moving at 100 kilometers per hour. Motion, then, is not absolute—it depends on perspective.
This idea of relative motion is essential. It explains why astronauts floating inside the International Space Station can feel weightless even though they are moving nearly 28,000 kilometers per hour around Earth. From their perspective, they and their surroundings move together, so they feel at rest.
Speed, Velocity, and Acceleration: The Language of Motion
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To make sense of motion, physicists use precise terms. Although people often use “speed” and “velocity” interchangeably, in physics they are not the same.
Speed tells us how fast something moves. It is calculated by dividing the distance traveled by the time taken:
Speed = Distance / Time
If your car covers 60 kilometers in an hour, its speed is 60 km/h. Straightforward, right?
But speed alone doesn’t tell the full story. If we want to describe not only how fast, but also in what direction something moves, we use velocity. For example, saying “60 km/h east” is velocity, not just speed. Direction matters, because changing direction—even while keeping the same speed—counts as a change in motion.
That brings us to acceleration: the rate at which velocity changes. Most people think of acceleration as “speeding up,” but in physics, it also includes slowing down or changing direction. When a car turns a corner at a steady speed, it is accelerating because its direction changes. When a skydiver falls, their acceleration depends on the balance between gravity pulling them down and air resistance pushing back.
Acceleration is what makes roller coasters thrilling, rockets powerful, and sports exciting. Without it, everything would move in straight, steady lines, forever unchanged.
What Exactly Is a Force?
If motion describes what something is doing, then force explains why it is doing it. A force is simply a push or pull acting on an object. Forces can make objects start moving, stop moving, or change direction.
Physicists measure force in newtons (N), named after Sir Isaac Newton, the English scientist who laid down the foundations of classical mechanics in the 17th century. One newton is the amount of force needed to accelerate a one-kilogram object by one meter per second squared. In practical terms, the force of gravity acting on a small apple is about one newton—hence the famous story of Newton and the falling apple.
Not all forces are obvious. Some are contact forces, like pushing a door open or pulling a rope. Others act at a distance, like gravity or magnetism. When multiple forces act on an object, they can balance each other out or add up to change motion.
Balanced forces: When equal forces act in opposite directions, the object does not move or continues at a constant velocity.
Unbalanced forces: When one force is stronger, the object’s motion changes—it accelerates.
For example, a book resting on a table stays put because gravity pulling it down is balanced by the table pushing it up. But if you shove the book sideways, you create an unbalanced force, and the book slides until friction brings it to rest.
Newton’s First Law of Motion: The Law of Inertia
Few ideas in science are as elegant—and as counterintuitive—as Newton’s First Law of Motion, also known as the Law of Inertia. It states:
“An object at rest stays at rest, and an object in motion stays in motion at the same speed and in the same direction unless acted upon by an unbalanced force.”
This simple sentence reshaped our understanding of the universe. Before Newton, many people—including ancient philosophers like Aristotle—believed that objects naturally come to rest unless something keeps pushing them. Newton realized the opposite: motion is the natural state, and things only stop because forces like friction or air resistance act on them.
Consider a soccer ball. Left alone on the grass, it will not move. Kick it, and it rolls forward. On Earth, the ball eventually slows because friction from the grass and air resistance act as unbalanced forces. But if you could remove those forces—say, in the vacuum of space—the ball would keep moving forever in a straight line.
The same principle explains why astronauts continue drifting when they push away from the space station, or why Earth keeps orbiting the Sun. Inertia, the resistance of objects to changes in motion, is at the core of Newton’s First Law.
Why Does It Matter?
At first glance, studying motion and forces might seem abstract, but their importance is impossible to overstate. The concepts of speed, velocity, acceleration, force, and inertia explain almost everything we experience in daily life.
When you slam on the brakes in a car, your body lurches forward. That’s inertia—you wanted to stay in motion even as the car slowed. When engineers design bridges, roller coasters, or airplanes, they rely on these principles to predict how forces will act. Even in sports, from the curve of a soccer ball to the spin of a basketball, physics determines success or failure.
Beyond everyday life, understanding motion is the foundation for advanced fields of science and engineering. Space exploration, robotics, medical imaging, renewable energy, and countless technologies rest on the shoulders of Newton’s insights. Without the First Law, there would be no satellites, no modern transportation, no safe building design—and perhaps no physics as we know it.
Connecting Back to the Bigger Picture
So far, we’ve explored motion, forces, and Newton’s First Law as separate ideas. But together, they form the framework of classical mechanics—the branch of physics that studies the movement of objects. These ideas set the stage for everything that comes next: Newton’s Second and Third Laws, the concept of energy, and the forces that shape the universe.
By mastering these fundamentals, you not only learn why soccer balls roll, cars stop, and satellites orbit—you also gain the tools to explore deeper questions about how nature works. The journey through physics begins here, with motion and forces, and expands into understanding energy, light, electricity, magnetism, and beyond.
Conclusion: The First Step Into Physics
Physics begins with motion because motion is the most immediate evidence of the laws of nature at work. Whether it’s the graceful path of the Moon across the night sky or the sudden bounce of a ball on the floor, every movement tells a story of forces, inertia, and change. Newton’s First Law distilled that story into a single idea that continues to shape science centuries later.
As you continue exploring physics, remember that these fundamental principles are not just theoretical—they are lived experiences, woven into the fabric of your daily life. By asking questions, observing carefully, and thinking critically about the forces around you, you step into the same tradition of inquiry that propelled Newton, Galileo, and countless scientists to unravel the universe’s secrets.
If this introduction to motion and Newton’s First Law has sparked your curiosity, there is much more waiting. Dive deeper into the world of physics with Mentivia’s Foundations of Physics free course, where you’ll discover not only how the universe works but also how it connects to your world. Visit Mentivia.com today and begin your journey into science.