Motion and speed
To tell which body moves faster we compare how far each goes in the same time. Speed is the distance a body covers in a unit of time (for example 1 hour). If a car travels 120 km in 2 hours, its speed is 120 ÷ 2 = 60 km/h. Speed is measured in km/h or m/s. Among animals the cheetah runs fastest (around 100 km/h), the elephant is much slower and the snail very slow. Vehicles: a bicycle about 15–20 km/h, a car outside the city 60–90 km/h, a modern passenger plane 800–900 km/h. The first aeroplane (the Wright brothers) flew at about 48 km/h. A rocket going to space reaches several kilometres per second. Speed is measured by radar, which sends a signal and calculates from the signal returned. Road signs show the permitted speed.
Measuring speed in class: mark 10 metres. Children walk it in turns while the teacher counts seconds. Calculate speed: divide 10 m by the seconds. Who walked fastest? (Walk, do not run; be careful.)