What is Motion?
Everything around us is in motion — from planets orbiting the sun to the blood flowing through your veins. In science, we describe motion very precisely using specific quantities and language.
A change in the position of an object over time, relative to a reference point.
The key phrase here is relative to a reference point. Motion is always described in comparison to something else. For example, when you sit on a moving bus, you are stationary relative to the seat, but moving relative to the road.
Think about watching a friend walk past you. From your point of view, they are moving. But from a bird flying at the same speed alongside your friend, your friend appears to be standing still! Motion depends on your point of view — your frame of reference.
To fully describe motion in physics, we need to measure and communicate:
- Distance or displacement (how far and in what direction)
- Speed or velocity (how fast and in what direction)
- Acceleration (how the velocity is changing)
- Time (the duration over which the motion occurs)
Relative velocity in one dimension
When two objects move along the same line, we can calculate how fast one appears to move relative to the other. If two cars travel in the same direction at 20 m/s and 15 m/s, the faster car moves at only m/s relative to the slower one. If they travel in opposite directions, the relative speed is m/s. This idea of a frame of reference becomes especially important when we study space travel — astronauts on the International Space Station are moving at about 7700 m/s relative to Earth's surface, but are stationary relative to each other inside the station!
This unit connects to the MYP Key Concept of Change and the Related Concept of Movement. As you study motion, keep asking: What is changing? How quickly is it changing? Relative to what? These questions are at the heart of physics.
Distance and Displacement
Distance and displacement both describe how far an object has moved, but they are fundamentally different.
The total length of the path travelled by an object, regardless of direction. It is a scalar quantity — it has magnitude (size) only.
The straight-line distance from the starting point to the finishing point, including the direction of that line. It is a vector quantity — it has both magnitude and direction.
A physical quantity that has magnitude (size) only, with no direction. Examples: distance, speed, mass, time, temperature.
A physical quantity that has both magnitude and direction. Examples: displacement, velocity, acceleration, force.
Example: The Marathon Runner
Imagine a runner completes a circular track of circumference 400 m.
- After one full lap, the distance travelled = 400 m
- After one full lap, the displacement = 0 m (they are back where they started!)
This shows that distance and displacement can be very different values.
Do not use distance and displacement interchangeably. In MYP and beyond, using the wrong term will cost you marks. If a question asks for displacement, you must include a direction in your answer (e.g., "50 m north" not just "50 m").
11 more sections in this topic
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