NCERT Class-9 Science Solutions: Chapter 9-Force And Laws Of Motion Part 3

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Question 15:

A truck of mass M is moved under a force F. If the truck is then loaded with an object equal to the mass of the truck and the driving force is halved, then how does the acceleration change?


Calculate using

Mass of truck before loading

And driving force

Acceleration (before loading)

Mass of the truck after loading

And driving force after loading

Acceleration (after loading)

Thus, the acceleration reduced by a quarter

Acceleration becomes one-fourth of the original

Question 16:

Two friends on roller-skates are standing apart facing each other. One of them throws a ball of towards the other, who catches it, how will this activity affect the position of the two? Explain your answer.


Separation between them will increase. Initially the momentum of both of them are zero as they are at rest. In order to conserve the momentum the one who throws the ball would move backward. The second will experience a net force after catching the ball and therefore will move backwards that is in the direction of the force.

Question 17:

Water sprinkler used for grass lawns begins to rotate as soon as the water is supplied. Explain the principle on which it works.


The working of the rotation of sprinkler is based on third law of motion. As the water comes out of the nozzle of the sprinkler, an equal and opposite reaction force comes into play. So the sprinkler starts rotating.

Image The water sprinkle

Image the Water Sprinkle

Image The water sprinkle

Long Answer Questions

Question 18:

Using second law of motion, derive the relation between force and acceleration. A bullet of strikes a sand-bag at a speed of and gets embedded after travelling . Calculate

  1. The resistive force exerted by the sand on the bullet

  2. The time taken by the bullet to come to rest.


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