Measuring length and motion · लंबाई का मापन और गति
A free Physics lesson for Class 6 children.
A hand span or a footstep is not a reliable unit, because everyone's hands and feet are different.
So the world uses standard units. The SI unit of length is the metre, written m.
One metre has 100 centimetres, and one kilometre has 1000 metres.
A ruler measures short lengths. Keep your eye straight above the mark to read it correctly.
Things that change their position with time are in motion. A moving car and a swinging pendulum are both in motion.
Motion along a straight line is rectilinear, motion in a circle is circular, and to-and-fro motion is periodic.