NCERT Class9 Science Solutions: Chapter 9Force 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?
Answer:
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 onefourth of the original
Question 16:
Two friends on rollerskates 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.
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.
Answer:
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.
Long Answer Questions
Question 18:
Using second law of motion, derive the relation between force and acceleration. A bullet of strikes a sandbag at a speed of and gets embedded after travelling. Calculate

The resistive force exerted by the sand on the bullet

The time taken by the bullet to come to rest.
Answer: