Solution of Motion (Page No - 22) - Physics by Lakhmir Singh, Class 9 | Extra Documents & Tests for Class 9 PDF Download

Question 55:
A body is moving along a circular path of radius R. What will be the distance travelled and displacement of the body when it completes half a revolution ?
Solution :
Distance travelled in half a rotation of a circular path is equal to the circumference of semi-circle, i.e.,

Distance travelled in half of a circular path is equal to the circumference of semi-circle, i.e = πR.

Displacement = diameter of circle = 2R

Question 56:
If on a round trip you travel 6 km and then arrive back home :
(a) What distance have you travelled ?
(b) What is your final displacement ?
Solution :
(i) Distance travelled = 6 km
(ii) Displacement = zero(since final position is same as initial position)

Question 57:
A body travels a distance of 3 km towards East, then 4 km towards North and finally 9 km towards East.
(i) What is the total distance travelled ?
(ii) What is the resultant displacement ?
Solution :
(i) Total distance travelled= 3 + 4 +9=16 km
(ii) The body travels a total distance of 12 km in east direction i.e. towards x-axis.
And it travels a distance of 4 km in North direction, i.e. towards y-axis. Hence, resultant displacement is

(i) Total distance travelled  = 3+4+9 = 16 km

(ii) The body travel a total distance of 12 km in east direction i.e toward  x-axis

And it travel a distance of 4 km in North direction, i.e toward  y- axis

Hence, resultant displacement is

Question 58:
A boy walks from his classroom to the bookshop along a straight corridor towards North. He covers a distance of 20 m in 25 seconds to reach the bookshop. After buying a book, he travels the same distance in the same time to reach back in the classroom. Find (a) average speed, and (b) average velocity, of the boy.
Solution :
(a) Total distance covered in going to the bookshop and coming back to the classroom = 20 + 20 = 40m
Total time taken= 25 + 25 = 50 sec 

(a) total distance covered in going to the bookshop and coming back to the classrooom = 20+20 = 40 km

Question 59:
A car travels 100 km at a speed of 60 km/h and returns with a speed of 40 km/h. Calculate the average speed for the whole journey.
Solution :
In the first case, car travels at a speed of 60 km/h for a distance of 100 km
In the second case, car travels at a speed of 40 km/h for a distance of 100 km
Total distance travelled = 200 km 

In the first case , car travels at a speed of 60 km/h for a distance of 100 km

In the second case, car travels at a speed of 40 km/h for a distance of 100km

Total distance travelled  = 200 km

Average speed=

Question 60:
A ball hits a wall horizontally at 6.0 m s^-1. It rebounds horizontally at 4.4 m s^-1. The ball is in contact with the wall for 0.040 s. What is the acceleration of the ball ?
Solution :
Initial velocity, u=6m/s
Final velocity ,v=-4.4m/s(the ball rebounds in opposite direction)
Time, t = 0.040 s