Motion, force and simple machines

If a body changes its position relative to other bodies it is moving; if not it is at rest. So both rest and motion are relative: a passenger sitting in a bus is at rest relative to the seat but moving relative to a tree by the road. The body we measure motion against is the reference body. The main types of motion are straight-line (a runner on a straight track), circular (a fan blade, a turning steering wheel) and oscillatory, i.e. repeated motion about one place (a swing, a clock pendulum). Speed shows the distance covered in unit time: speed = distance ÷ time (v = s ÷ t). Units are m/s or km/h; 1 m/s = 3.6 km/h. In an oscillation, the time for one full to-and-fro swing is the period.