NCERT Solutions Class 9 Science Chapter 7 - Motion

Motion – Important Questions with Answers

1. What is motion?
Answer: Motion is the change in position of an object with respect to time.

2. Define distance and displacement.
Answer:

• Distance is the total path covered by an object.

• Displacement is the shortest straight-line distance between the initial and final positions.

3. What is the SI unit of distance and displacement?
Answer: The SI unit of both distance and displacement is metre (m).

4. What is speed?
Answer: Speed is the distance travelled per unit time.
Speed = Distance / Time

5. Define uniform and non-uniform motion.
Answer:

• Uniform motion: Object covers equal distances in equal intervals of time.

• Non-uniform motion: Object covers unequal distances in equal intervals of time.

6. What is velocity?
Answer: Velocity is the displacement per unit time. It is speed in a given direction.
Velocity = Displacement / Time

7. What is acceleration?
Answer: Acceleration is the rate of change of velocity.
Acceleration = (Final velocity – Initial velocity) / Time

8. What is the SI unit of acceleration?
Answer: The SI unit of acceleration is metre per second squared (m/s²).

9. What is uniform acceleration? Give an example.
Answer: Uniform acceleration is when the velocity of an object changes by equal amounts in equal intervals of time.
Example: A freely falling object.

10. A car increases its speed from 20 m/s to 50 m/s in 10 seconds. Find its acceleration.
Answer:
Acceleration = (50 – 20) / 10 = 30 / 10 = 3 m/s²

11. Name the three equations of motion.
Answer:

1. v = u + at

2. s = ut + ½at²

3. v² = u² + 2as
   Where:
   v = final velocity,
   u = initial velocity,
   a = acceleration,
   s = displacement,
   t = time

12. What type of graph is obtained by uniform motion?
Answer: A straight line graph between distance and time.

13. What does the slope of a distance-time graph represent?
Answer: The slope of a distance-time graph represents speed.

14. What does the area under a velocity-time graph represent?
Answer: The area under a velocity-time graph represents displacement.

15. A bus starts from rest and moves with an acceleration of 2 m/s². Find the velocity after 5 seconds.
Answer:
Using v = u + at
v = 0 + 2 × 5 = 10 m/s

16. What is the displacement of the above bus in 5 seconds?
Answer:
Using s = ut + ½at²
s = 0 + ½ × 2 × 5² = 25 m

17. What is retardation?
Answer: Retardation is negative acceleration, i.e., decrease in velocity with time.

18. When is an object said to be at rest?
Answer: An object is at rest when it does not change its position with respect to time.

19. Can the displacement be zero even if the distance is not? Give an example.
Answer: Yes, for example, when a person walks around a circular track and comes back to the starting point, displacement is zero but distance is not.

20. An object moves with a uniform velocity of 10 m/s for 15 seconds. What is the distance travelled?
Answer:
Distance = Speed × Time = 10 × 15 = 150 m

21. Define average speed.
Answer: Average speed = Total distance / Total time taken

22. Define instantaneous speed.
Answer: Instantaneous speed is the speed of an object at a particular instant of time.

23. Is speed a scalar or vector quantity?
Answer: Speed is a scalar quantity.

24. Is velocity a scalar or vector quantity?
Answer: Velocity is a vector quantity because it has both magnitude and direction.

25. What kind of motion does a free-falling object exhibit?
Answer: Uniformly accelerated motion.