Class 9th Physics // Chapter - Motion // Short Answer type Questions

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Question 1. An object has moved through a distance. Can it have zero displacement ? If yes, support your answer with an example. 

Answer:- Yes, The displacement of the object can be zero. Let a boy completes one round of a circular track in 5 minutes. The distance travelled by the boy = circumference of the circular track. However, displacement of the boy is zero because his initial and final positions are same.

Question 2. A farmer moves along the boundary of a square field of side 10 m in 40 s.
What will be the magnitude of displacement of the farmer at the end of 2 minutes 20 seconds ? 

Answer:-
ABCD is a square field of side 10 m.
The farmer moves along the boundary of the field from the corner A via the corners B, C and D.
After every 40 s, the farmer is again at the corner A, so his displacement after every 40 s is zero.
At the end of 2 minutes 20 seconds = (2 x 60 + 20) = 140 s, the farmer will be at the corner C.

Question 3. Distinguish between speed and velocity. 
Answer:

Speed	Velocity
1. Distance travelled by an object per unit time is known as its speed.	The distance travelled by an object in a particular direction (i.e. displacement) per unit time is known as its velocity.
2. Average speed of a moving object cannot be zero.	Average velocity of a moving object can be zero.
3. tells how fast an object moves.	Velocity tells how fast an object moves and in which direction it moves.
4. Speed is a scalar quantity.	Velocity is a vector quantity.
5. Speed of an object is always positive.	Velocity of an object can be positive or negative.

Question 4. Under what conditions) is the magnitude of the average velocity of an object equal to its average speed ? 
Answer:- When an object moves in one direction along a straight line.

Question 5. What does the odometer of an automobile measure ? 
Answer:- Odometer of an automobile measures distance travelled by the automobile.

Question 6. What does the path of an object look like when it is in uniform motion ? 
Answer:- Straight path.

Question 7. During an experiment, a signal from a spaceship reached the ground station in five minutes. What was the distance of the spaceship from the ground station ? The signal travels at the speed of light, that is, 3 x 108 m s-1
Answer:-

Question 8. When will you say a body is in

1. uniform acceleration ?
2. non-uniform acceleration (CBSE 2012, 2013)

Answer:

1. A body has uniform acceleration if its velocity changes by an equal amount in equal intervals of time. .
2. A body has non-uniform acceleration if its velocity changes by unequal amount in equal intervals of time.

Question 9. A bus decreases its speed from 80 kmh-1 to 60 kmh-1 in 5 s. Find the acceleration of the bus.
Answer:-
Here,

Question 10. A train starting from a railway station and moving with a uniform acceleration attains a speed of 40 km h-1 n 10 minutes. Find its acceleration.
Answer:

Question 11. What is the nature of the distance-time graphs for uniform and non-uniform motion of an object ?
Answer:

1. For uniform motion, distance-time graph is a straight line having constant gradient or slope as shown in figure.
2. For non-uniform motion, distance-time graph is a curve having increasing gradient or slope or decreasing gradient as shown in figures.
   

Question 12. What can you say about the motion of an object whose distance-time graph is a straight line parallel to the time axis ? 
Answer:- When distance-time graph is a straight line parallel to the time axis, it means, the distance of the object is not changing with time. Thus, the object is stationary.

Question 13. What can you say about the motion of an object if its speed-time graph is a straight line parallel to the time-axis ?
Answer:- In this case, speed of the object is constant. That is, the object travels equal distances in equal intervals of time along a straight line. Hence, motion of the object is uniform motion.

Question 14. What is the quantity which is measured by the area occupied below the velocity-time graph ?
Answer:- Magnitude of the displacement of a body is measured by the area under the velocify-time graph.

Question 15. A bus starting from rest moves with a uniform acceleration of 0.1 m s-2 for 2 minutes. Find
(a) the speed acquired,
(b) the distance travelled. (CBSE 2012)
Answer:

Question 16. A train is travelling at a speed of 90 km h-1. Brakes are applied so as to produce a uniform acceleration of – 0.5 m s-2 . Find how far the train will go before it is brought to rest ?
Answer:

Question 17. A trolley, while going down an inclined plane has an acceleration of 2 cm s-2. What will be its velocity 3 s after the start ?
Answer:
Here, u = 0,v = ?, a = 2 cm s-2, t = 3 s
Using, v = u + at, we get
v = 0 + 2 x 3 = 6 cm s-2.

Question 18. A racing car has a uniform acceleration of 4 m s-2. What distance will it cover in 10 s after start ?
Answer:

Question 19.
A stone is thrown in a vertically upward direction with a velocity of 5 m s-1 If the acceleration of the stone during its motion is 10 m s-2 in the downward direction, what will be the height attained by the stone and how much time will it take to reach there ?
Answer:


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