NCERT Class 9 Science Solutions Chapter 1 Motion

NCERT Class 9 Science Solutions Chapter 1 Motion 

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Solution 1

Yes. An object that has moved through a distance can have zero displacement. Displacement is the shortest distance between the initial and the final position of an object. An object which has covered a distance can have zero displacement if it comes back to its starting point i.e., the initial position.

Consider the following situation. A man is walking along the boundary of a square park of side 20 m (as shown in the following figure). He starts walking from point A and after moving along all the sides of the park (AB, BC, CD, DA), he again comes back to the same point i.e., A.

    

In this case, the total distance covered by the man is 20 m + 20 m + 20 m + 20 m = 80 m. However, his displacement is zero because the shortest distance between his initial and final position is zero.

Concept Insight - Distance and displacement may or may not be the same. Displacement is always less than or equal to the distance covered.

Solution 2

The farmer takes 40 s to cover 4 × 10 = 40 m. 

In 2 min and 20 s (140 s), he will cover a distance =  = 140 m
Therefore, the farmer completes =3.5 rounds (3 complete rounds and a half round) of the field in 2 min and 20 s. 

That means, after 2 min 20 s, the farmer will be at the opposite end of the starting point. 

Now, there can be two extreme cases. 

Case I: Starting point is a corner point of the field. 

In this case, the farmer will be at the diagonally opposite corner of the field after 2 min 20 s. 

Therefore, the displacement will be equal to the diagonal of the field. 

Hence, the displacement will be  = 14.14 m

Case II: Starting point is the middle point of any side of the field. 

In this case the farmer will be at the middle point of the opposite side of the field after 2 min 20 s. 

Therefore, the displacement will be equal to the side of the field, i.e., 10 m. 

For any other starting point, the displacement will be between 14.1 m and 10 m. 
 
Concept Insight: - Be careful about considering the cases, as the displacement in both the cases is different. 

Solution 3

(a) Not true 
Displacement can become zero when the initial and final positions of the object are the same.   (b) Not true   Displacement is the shortest distance between the initial and final positions of an object. It cannot be greater than the magnitude of the distance travelled by an object. However, sometimes, it may be equal to the distance travelled by the object. 

Concept Insight - Displacement is always less than or equal to the distance covered. 

Page 102

Solution 1

Speed	Velocity
Speed is the distance travelled by an object per unit time. It does not have any direction.	Velocity is the displacement of an object per unit time. It has a unique direction.
Speed is a scalar quantity.	Velocity is a vector quantity.
The speed of an object can never be negative. At the most, it can become zero. This is because distance travelled can never be negative.	The velocity of an object can be negative, positive, or equal to zero. This is because displacement can take any of these three values.