NCERT Solutions for Class 9 Science Chapter 8 Motion

Ans : Yes. An object that has moved through a distance can have zero displacement. Displacement is the shortest measurable 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 in a square park of length 20 m (as shown in the following figure). He staffs walking from point A and after moving along all the corners of the park (point B, C, D), 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. 

Ans : The farmer takes 40 s to cover 4 x 10 =40 m. 
In 2 min and 20 s (140 s), he will cover a distance \( \frac { 40 } { 40 } \times 140\)=40m
Therefore, the farmer completes \( \frac { 140 } { 40 } = 3.5\) 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 comer 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 to the field. 
Hence, the displacement will be \( \sqrt { 10 ^ { 2 } + 10 ^ { 2 } }\)=14.1m
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. 

Ans : (a) Not true 
Displacement can become zero when the initial and final position of the object is the same. 
(b) Not true 
Displacement is the shortest measurable 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. 

Ans : \( \ = \frac { \text { Total distance covered } } { \text { Total time taken } }\)
\( \ = \frac { \text { Displacement } } { \text { Total time taken } }\) 

Ans : The odometer an automobile measures the distance covered by an automobile.