Distinguish scalar and vector descriptions and communicate reproducible measurements.
Method and worked examples
A measurement combines a value and unit. Convert SI prefixes before substitution: milli means 10−3, micro means 10−6 and kilo means 103. Convert kilometres per hour to metres per second by dividing by 3.6. Retain extra digits during intermediate calculations, then round the final result as requested.
Distance is total path length; displacement is the directed change from initial to final position. Average speed is distance divided by elapsed time; average velocity is displacement divided by elapsed time. Choose a positive direction for one-dimensional vectors. A return trip can have non-zero distance but zero displacement. Distinguish measurement repeatability from calibration accuracy when reading a reported result.
Example 1
A runner travels 60m east, then 20m west in 16s. Take east as positive. Find average speed and velocity.
Distance =80m, displacement =+40m. Average speed =1680=5.0m/s; average velocity =1640=+2.5m/s, east.
Example 2
Convert a measured interval of 240ms to seconds and a speed of 72km/h to m/s.
240ms=240×0.001=0.240s. 72km/h=72×36001000=20m/s. The conversions change the numerical representation, not the motion.
Watch out:A return journey has zero displacement and average velocity, while its distance and average speed can be non-zero.
1. Measuring and describing motion | Year 11 Physics | Ascent Arcade