NCERT Solutions for Class 9 Science Unit 8
Motion Class 9
Unit 8 Motion Exercise Solutions
Motion:- If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.
Rest:- When an object does not change its position with respect to a reference point with elapse of time, the object is said to be in rest.
Example When a vehicle changes its position with respect to an electric pole (a reference point) with elapse of time, then vehicle is called in motion. And if the same vehicle does not change its position with respect to that electric pole, the vehicle is called in the state of rest.
Thus to observe the motion of any object; two things are necessary a reference point and time.