Important Questions for Class 9 Chapter 8 - Motion

Important questions based on NCERT syllabus for Chapter 8 - Motion:

Question-1: Differentiate between the following:
(i) Speed and velocity
(ii) Distance and displacement
on basis of scalar and vector quantities.

Solution: Velocity and displacement are vector quantities as they have both a value as well as direction.
Whereas, speed and distance are scalar quantities as they only have magnitude.

Question-2: A body moving with a constant speed, say 10 m/s on a frictionless surface has a uniform motion, while a freely falling object has a non-uniform motion. Why does this happen?

Solution: Freely falling object accelerates due to the presence of force of gravity. While sliding on a frictionless surface, there is no net force on the body, so velocity doesn't change.

Question-3: The distance between two points A and B is 100 m. A person moves from A to B with a speed of 20 m/s and comes back from B to A with a speed of 25 m/s.
Calculate the average speed and the average velocity of the person.

Solution: Distance from A to B = 100 m
Distance from B to A = 100 m
Thus, total distance = 200 m
Time taken to move from A to B,
t1=100/20=5s
Time taken from B to A,
t2=100 /25=4s
Total time taken = t1+t2=(5+4)s=9s

Hence, average speed of the person =
Total Distance/Total Time=2009=22.2m/s

Since person comes back to initial position A, displacement will be zero, resulting in zero average velocity.