Motion and measuring distance

Motion is a change in the position of a body relative to other bodies. To tell whether a body is moving, we compare it with something else: a tram driver is still relative to the tram but moving relative to a tree. The path of motion can be a straight line (a coin dropped down), a circle (the tip of a clock hand), a spiral, a broken line, an arc (a pendulum) or a parabola (a ball thrown into a hoop). To set a body moving, stop it or change its direction, a force must act on it: a push or a pull. Wind blows a dandelion seed away, and the Earth pulls every body toward itself, so things fall down.