Unit I: Introduction to Kinematics
The Science of Describing Motion.
This is the first chapter of the Physics curriculum. Welcome to the story of motion!
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The Problem: How Do We Describe Motion?
Physics begins with observing the world. Things move. A car drives down the street, a ball is thrown, the planets orbit the sun. But how can we talk about this motion with scientific precision? Saying "the car went far" isn't good enough.
To solve this, physicists developed two key concepts: distance and displacement. They sound similar, but they answer two very different questions.
- Distance answers: "How much ground did you cover?"
- Displacement answers: "How far are you from where you started?"
Use the interactive module below to discover the crucial difference. Click or tap to move the character around the screen.
Distance vs. Displacement Simulator
Click or tap on the canvas to move the character and create a path.
The "Aha!" Moment
Notice how distance is the total length of the winding path you took, always increasing. Displacement, however, is just the straight-line distance from your start point to your current position. If you walk in a circle and end up where you started, your distance is large, but your displacement is zero!
This is the key difference: distance is a scalar (just a number), while displacement is a vector (a number and a direction).
Speed vs. Velocity
Now that we can describe position, let's add time to the mix. This brings us to two more crucial concepts: speed and velocity.
- Speed is how fast you're going (distance ÷ time). It's a scalar.
- Velocity is how fast your displacement is changing (displacement ÷ time). It's a vector, meaning it has direction.
In physics, "negative" velocity doesn't mean slowing down; it simply means moving in the negative direction (e.g., left). Use the simulator below to see this. Set a velocity and watch the car move.
Use the slider to change the car's velocity. Try a negative value to see what happens!
The "Aha!" Moment
Notice that speed is always positive, representing the magnitude of motion. Velocity, however, can be positive (moving right) or negative (moving left). This distinction is critical for solving physics problems.
You Can Describe Motion. What's Next?
You've learned the fundamental language of kinematics. The next step is to explore how velocity itself can change—a concept known as acceleration.