Measuring length and time, and describing how things move
Long ago people measured length with body parts such as a handspan or a foot, but these vary from person to person. To avoid confusion, scientists use standard units: the metre for length, the kilogram for mass and the second for time, all part of the SI system. A ruler or measuring tape measures length, and you should keep your eye directly above the mark to avoid errors. A curved line can be measured using a thread and then a scale.
Motion is a change in position over time. Rectilinear motion is along a straight line, circular motion follows a circle, and periodic motion repeats at regular intervals, like a swinging pendulum. Speed is the distance covered divided by the time taken, so a cyclist covering 30 km in 2 hours has an average speed of 15 km/h. A distance–time graph that is a straight sloping line shows uniform speed, and the steeper the line, the faster the motion.
Why it matters
Standard units let tailors, builders and shopkeepers everywhere agree on measurements, and accurate time-keeping is essential for trains and schools. Speedometers and odometers in vehicles measure speed and distance. Reading distance–time graphs is a skill you will use again when studying motion and velocity in Class 9.
Key terms
- SI unit
- A unit in the internationally agreed system, such as the metre, kilogram or second.
- Periodic motion
- Motion that repeats itself after a fixed interval of time.
- Speed
- The distance an object travels divided by the time it takes.
- Uniform motion
- Motion in which an object covers equal distances in equal intervals of time.