# NCERT Class 9 Science Solutions: Chapter 9 – Force and Laws of Motion Part 6

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Question 18

Q. How much momentum will a dumbbell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration to be.

**Answer:**

Mass of the dumbbell,

Distance covered by the dumbbell

Acceleration in the downward direction,

Initial velocity of the dumbbell,

Final velocity of the dumbbell (when it was about to hit the floor)

According to the third equation of motion

Hence, the momentum with which the dumbbell hits the floor is

Question 1

The following is the Distance-time table of an object in motion;

Time in Seconds | Distance in metres |

0 | 0 |

1 | 1 |

2 | 8 |

3 | 27 |

4 | 64 |

5 | 125 |

6 | 216 |

7 | 343 |

Q. (a) what conclusion can you draw about the acceleration? Is it constant, increasing, decreasing, or zero?

**Answer:**

There is an unequal change of distance in an equal in an equal interval of time.

Thus, the given object is having a non-uniform motion. Since the velocity of the object increases with time, the acceleration is increasing.

Q (b) what do you infer about the forces acting on the object?

**Answer:**

According to Newton’s second law of motion, the force acting on an object is directly proportional to the acceleration produced in the object. In the given case, the increasing acceleration of the given object indicates that the force acting on the object it also increasing.

Question 2

Q. Two persons manage to push a motorcar of mass 1200 kg at a uniform velocity along a level road. The same motorcar can be pushed by three persons to produce an acceleration of 0.2. With what force does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort)

**Answer:**

Mass of the motor car

Only two persons manage to push the car. Hence, the acceleration acquired by the car is given by the third person alone.

Acceleration produces by the car, when it is pushed by the third person,

Let the force applied by the third person be F.

From Newton’s second law of motion:

Force

Thus, the third person applies a force of magnitude 240 N.

Hence, each person applies a force of 240 N to push the motor car.

Question 3

Q. A hammer of mass 500 g, moving at 50, strikes a nail. The nail stops the hammer in a very short time of 0.01 s. What is the force of the nail on the hammer?

**Answer:**

Mass of the hammer,

Initial velocity of the hammer,

Time taken by the nail to the stop the hammer,

Velocity of the hammer, (Since the hammer finally comes to rest from Newton’s second law of motion:

The hammer strikes the nail with a force of Hence, from Newton’s third law of motion, the force of the nail on the hammer is equal and opposite, i.e.