Motion describes where an object is, how fast it moves—speed, the distance travelled per second—and how its direction changes over time. It covers everyday motions, from a ball rolling on the floor to a car accelerating along a road, and it uses precise measurements to describe these changes. It does not ask why the motion happens, only how it looks in space and time.
Why it matters
Two concrete cases show why clear motion descriptions matter. A car travelling at 20 m/s brakes with a constant deceleration of 5 m/s^2; the stopping distance is about 40 m, and if braking starts 0.5 s late, roughly 10 m extra is required, raising crash risk. A football kicked at 25 m/s at 45 degrees travels about 63 m before hitting the ground, illustrating how speed and angle determine where a projectile lands.
How to approach this
Begin with the first two ideas, then practise simple straight-line cases. Expect around 6–8 hours of focused work to work through the nine sub-pages with short problems. The hardest part is tying velocity, displacement, and acceleration together across different situations.
The learning path
1. Position and displacement
Position is where an object is located in space, and displacement is the straight-line change in position between two moments, with a direction, as described in Position and displacement. These concepts provide the reference frame you use to describe motion. They feed into velocity and acceleration and are the base for all later topics. How far does a car move along a straight road in three seconds?
2. Speed and velocity
Speed, the distance travelled per second, and velocity, speed with a direction, are defined here as the ways to describe motion; speed is the rate of distance change, velocity adds direction, as described in Speed and velocity. Knowing velocity lets you describe motion with direction and sets the stage for acceleration and motion graphs. In practice, you can deepen intuition in The Feynman Lectures on Physics, Volume I The Feynman Lectures on Physics, Volume I. What is the velocity after 4 seconds if motion starts from rest and acceleration is 1 m/s^2?
3. Acceleration
Acceleration is the rate at which velocity changes over time; it has magnitude and direction, defined in Acceleration. It is the bridge from describing motion to predicting future states; it links one-dimensional motion and projectile motion. How does velocity change if a car starts from rest and accelerates at 2 m/s^2 for 5 seconds?
4. Motion graphs
Motion graphs plot how position, velocity, or acceleration change over time, as described in Motion graphs. They turn rates into slopes and areas, producing an interpretable picture of motion. They prepare you for constant-acceleration problems and for two-dimensional motion. How would a curved position-time graph reflect changing velocity?
5. One-dimensional motion
One-dimensional motion studies movement along a straight line and uses velocity, displacement, and acceleration to predict future position, as described in One-dimensional motion. It provides the core toolkit for predicting straight-line motion and introduces the equations that relate these quantities. If a car starts from rest and accelerates at 2 m/s^2 for 5 seconds, where does it end up?
6. Projectile motion
Projectile motion analyzes motion in two dimensions under gravity by splitting motion into horizontal and vertical components, as described in Projectile motion. It explains everyday phenomena like thrown balls and sports trajectories. What is the horizontal range of a ball launched with speed 20 m/s at 30 degrees?
7. Relative motion
Relative motion describes how motion looks from different frames of reference; velocity depends on the observer, as described in Relative motion. Understanding frames of reference clarifies why measurements of motion vary and prepares you for more advanced dynamics. What is your speed relative to the platform if you walk inside a train moving at 20 m/s and you walk forward at 1 m/s inside the train?
8. Galileo Galilei
Galileo Galilei was a 17th-century scientist who performed early experiments on motion, such as rolling balls and falling bodies, as described in Galileo Galilei. His quantitative, experimental approach established motion as testable and measurable, shaping later work on dynamics. How did his experiments anchor the idea that motion can be described with numbers?
9. Two New Sciences
Two New Sciences — Galileo's Discourses and Mathematical Demonstrations Relating to Two New Sciences documents experiments and mathematical arguments about motion and materials, as described in Two New Sciences. It anchors the historical basis for kinematics and shows how experiments shaped the concepts taught here. What experiments and mathematical arguments did Galileo present that influenced modern kinematics?
Where this leads
With this path completed, you can describe motion in straight lines and two dimensions, and begin predicting future positions from initial data. This groundwork supports Newton's laws, energy, and oscillations, and it opens the door to more advanced dynamics and engineering analyses.