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# NCERT solutions Science Class 9 chapter 10 Gravitation

## Chapter 10 - Gravitation

#### Page 136

What do you mean by free fall?

Q 1 | Page 136

What do you mean by acceleration due to gravity?

Q 2 | Page 136

#### Page 138

What are the differences between the mass of an object and its weight?

Q 1 | Page 138

Why is the weight of an object on the moon 1/6th its weight on the earth?

Q 2 | Page 138

#### Page 141

Why is it difficult to hold a school bag having a strap made of a thin and strong string?

Q 1 | Page 141

What do you mean by buoyancy

Q 2 | Page 141

Why does an object float or sink when placed on the surface of water?

Q 3 | Page 141

#### Page 142

You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

Q 1 | Page 142

You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighingmachine. In reality, one is heavier than other. Can you say which one is heavier and why?

Q 2 | Page 142

#### Pages 143 - 144

Gravitational force acts on all objects in proportion to their masses. Why then, a heavy object does not fall faster than a light object?

Q 2 | Page 143

What is the importance of universal law of gravitation?

Q 7 | Page 144

What is the importance of universal law of gravitation?

Q 7 | Page 144

What is the acceleration of free fall?

Q 8 | Page 144

What do we call the gravitational force between the Earth and an object?

Q 9 | Page 144

Amit buys few grams of gold at the poles as per the instruction of one of his friends. He hands over the same when he meets him at the equator. Will the friend agree with the weight of gold bought? If not, why? [Hint: The value of g is greater at the poles than at the equator].

Q 10 | Page 144

Why will a sheet of paper fall slower than one that is crumpled into a ball?

Q 11 | Page 144

Gravitational force on the surface of the moon is only 1/6 as strong as gravitational force on the Earth. What is the weight in newtons of a 10 kg object on the moon and on the Earth?

Q 12 | Page 144

A ball is thrown vertically upwards with a velocity of 49 m/s. Calculate

(i) the maximum height to which it rises.

(ii)the total time it takes to return to the surface of the earth.

Q 13 | Page 144

A stone is released from the top of a tower of height 19.6 m. Calculate its final velocity just before touching the ground.

Q 14 | Page 144

A stone is thrown vertically upward with an initial velocity of 40 m/s. Taking g = 10 m/s2, find the maximum height reached by the stone. What is the net displacement and the total distance covered by the stone?

Q 15 | Page 144

A stone is allowed to fall from the top of a tower 100 m high and at the same time another stone is projected vertically upwards from the ground with a velocity of 25 m/s. Calculate when and where the two stones will meet.

Q 17 | Page 144

In what direction does the buoyant force on an object immersed in a liquid act?

Q 19 | Page 144

Why does a block of plastic released under water come up to the surface of water?

Q 20 | Page 144

The volume of 50 g of a substance is 20 cm3. If the density of water is 1 g cm−3, will the substance float or sink?

Q 21 | Page 144

The volume of a 500 g sealed packet is 350 cm3. Will the packet float or sink in water if the density of water is 1 g cm−3? What will be the mass of the water displaced by this packet?

Q 22 | Page 144

#### Extra questions

A ball thrown up vertically returns to the thrower after 6 s. Find

(a) the velocity with which it was thrown up,

(b) the maximum height it reaches, and

(c) its position after 4 s.

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