NCERT Solutions Science Chapter 9 Gravitation for Class 9th
In Chapter 9 of the NCERT Science textbook for Class 9, titled “Gravitation,” the concept of gravity is introduced as a fundamental force that attracts objects toward one another. This chapter explains the universal law of gravitation, formulated by Isaac Newton, which states that every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. The chapter discusses the importance of gravitational force in daily life, including its role in keeping planets in orbit around the sun and the moon’s influence on tides. It also explores concepts like weight, mass, and the acceleration due to gravity, highlighting that weight varies depending on the location due to differences in gravitational pull. The chapter includes numerical problems and illustrations to help students understand the calculations involved, fostering a deeper comprehension of how gravitation affects objects in motion and their interactions. Overall, this chapter lays the groundwork for understanding gravitational phenomena in both scientific and practical contexts.
NCERT Solutions Science Chapter 9 Gravitation
Page: 134
1. State the universal law of gravitation.
Solution:
The universal law of gravitation states that every object in the universe attracts every other object with a force called the gravitational force. The force acting between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
2. Write the formula to find the magnitude of the gravitational force between the earth and an object on the surface of the earth.
Solution:
Consider F is the force of attraction between an object on the surface of earth and earth.
Also consider ‘m’ is the mass of the object on the surface of earth and ‘Me’ is the mass of earth.
Therefore the formula for magnitude of the gravitational force between the earth and an object on the surface is given as
Page: 136
1. What do you mean by free fall?
Solution:
Earth’s gravity attracts each object to its center. When an object is dropped from a certain height, under the influence of gravitational force it begins to fall to the surface of Earth. Such an object movement is called free fall.
2. What do you mean by acceleration Q due to gravity?
Solution:
When an object falls freely from a certain height towards the earth’s surface, its velocity changes. This velocity change produces acceleration in the object known as acceleration due to letter g denoted gravity.
The gravity-based acceleration value is,
Page: 138
1. What are the differences between the mass of an object and its weight?
Solution:
The differences between the mass of an object and its weight are tabulated below.
| Mass | Weight |
| Mass is the quantity of matter contained in the body. | Weight is the force of gravity acting on the body. |
| It is the measure of inertia of the body. | It is the measure of gravity. |
| It only has magnitude. | It has magnitude as well as direction. |
| Mass is a constant quantity. | Weight is not a constant quantity. It is different at different places. |
| Its SI unit is kilogram (kg). | Its SI unit is the same as the SI unit of force, i.e., Newton (N). |
2. Why is the weight of an object on the moon 1/6th its weight on the earth?
Solution:
The moon’s mass is 1/100 times and 1/4 times the earth’s radius. As a result, when compared to earth, the gravitational attraction on the moon is about one sixth. Thus, an object’s weight on the moon is 1/6th its earth weight.
Page: 141
1. Why is it difficult to hold a school bag having a strap made of a thin and strong string?
Solution:
It is tough to carry a college bag having a skinny strap as a result of the pressure on the shoulders. This can be as the result of the pressure is reciprocally proportional to the expanse on that the force acts. The smaller the surface area, the larger is going to be the pressure on the surface. Within the case of a skinny strap, the contact expanse is extremely tiny. Hence, the pressure exerted on the shoulder is extremely huge.
2. What do you mean by buoyancy?
Solution:
The upward force possessed by a liquid on an object that’s immersed in it is referred to as buoyancy.
3. Why does an object float or sink when placed on the surface of water?
Solution:
An object float or sink when placed on the surface of water because of two reasons.
(i) If its density is greater than that of water, an object sinks in water.
(ii) If its density is less than that of water, an object floats in water.
Page: 142
1. You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?
Solution:
When weighing our body, it is acting by an upward force. The buoyant force is this upward force. As a result, the body is pushed up slightly, resulting in the weighing machine showing less reading than the actual value.
2. You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?
Solution:
The bag of cotton is heavier than the bar of iron. The cotton bag has a larger air thrust than the iron bar. The weighing machine therefore indicates a smaller cotton bag weight than its actual weight.
Page: 143
1. How does the force of gravitation between two objects change when the distance between them is reduced to half?
Solution:
Consider the Universal law of gravitation,
According to that law, the force of attraction between two bodies is
Where,
m1 and m2 are the masses of the two bodies.
G is the gravitational constant.
r is the distance between the two bodies.
Given that the distance is reduced to half then,
r = 1/2 r
Therefore,
F = 4F
Therefore once the space between the objects is reduced to half, then the force of gravitation will increase by fourfold the first force.
2. Gravitational force acts on all objects in proportion to their masses. Why then does a heavy object not fall faster than a light object?
Solution:
All objects fall on the bottom with constant acceleration called acceleration thanks to gravity (g). It’s constant and therefore the price of ‘g’ doesn’t depend on the mass of associate object. So serious objects don’t fall quicker than light-weight objects provided there’s no air resistance.
3. What is the magnitude of the gravitational force between the earth and a 1 kg object on its surface? (Mass of the earth is 6 × 1024 kg and radius of the earth is 6.4 × 106m.)
Solution:
From Newton’s law of gravitation, we know that the force of attraction between the bodies is given by
4. If the moon attracts the earth, why does the earth not move towards the moon?
Solution:
According to universal law of gravitation and Newton third law, we all know that the force of attraction between 2 objects is the same, however in wrong way. So the planet attracts the moon with the identical force because the moon exerts on earth however in opposite directions. Since earth is far larger in size than moon, that the acceleration cannot be detected on earth surface.
5.What is the importance of universal law of gravitation?
Solution:
The universal law of gravitation explains many phenomena that were believed to be unconnected:
(i) The motion of the moon round the earth
(ii) The force that binds North American nation to the world
(iii) The tides because of the moon and therefore the Sun
(iv) The motion of planets round the Sun
6. What is the acceleration of free fall?
Solution:
When anybody is in free fall, the sole force functioning on the article is that the earth’s field of force. By Newton’s second law of motion all the forces manufacture acceleration, therefore all the objects accelerate toward the world’s surface thanks to attraction of the earth.
This acceleration is thought as acceleration thanks to gravity close to earth’s surface. It’s denoted by ‘g’ and its worth is 9.8m/s2 and it’s constant for all objects close to earth’s surface (irrespective of their masses).
7. What do we call the gravitational force between the earth and an object?
Solution:
Gravitational force is known as the object’s weight between the earth and an object.
8. 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.]
Solution:
The weight of a body on the earth’s surface;
W = mg wherever (m = mass of the body and g= acceleration thanks to gravity)
The value of g is a lot of at poles as compared to equator. So gold can weigh less at the equator as compared to poles.
Therefore, Amit’s friend won’t believe the load of the gold bought.
9. Why will a sheet of paper fall slower than one that is crumpled into a ball?
Solution:
A sheet of paper has a lot of area as compared to a crumpled paper ball. A sheet of paper must face a lot of air resistance. As result a sheet of paper falls slower than the crumpled ball.
10. 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 newton’s of a 10 kg object on the moon and on the earth?
Solution:
Given data:
Acceleration due to earth’s gravity = ge or g = 9.8 m/s2
Object weight m = 10 kg
Acceleration due to moon gravity = gm
Weight on the earth= We
Weight on the moon = Wm
Weight = mass x gravity
gm = (1/6) ge (given)
So Wm = m gm = m x (1/6) ge
Wm = 10 x (1/6) x 9.8 = 16.34 N
We = m x ge = 10 x 9.8
We = 98N
11. In what direction does the buoyant force on an object immersed in a liquid act?
Solution:
The buoyant force on an object that is immersed in a liquid will be in a vertically upward direction.
12. Why a block of plastic does released under water come up to the surface of water?
Solution:
The density of plastic is a smaller amount than that of water, therefore the force of buoyancy on plastic block are going to be bigger than the load of plastic block displaced. Hence, the acceleration of plastic block are going to be in upward direction, and comes up to the surface of water.
13. 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?
Solution:
To find the Density of the substance the formula is
Density = (Mass/Volume)
Density = (50/20) = 2.5g/cm3
Density of water = 1g/cm3
Density of the substance is greater than density of water. So the substance will sink.
14. 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?
Solution:
Density of sealed packet = 500/350 = 1.42 g/cm3
Density of sealed packet is greater than density of water
Therefore the packet will sink.
Considering Archimedes Principle,
Displaced water volume = Force exerted on the sealed packet.
Volume of water displaced = 350cm3
Therefore displaced water mass = ρ x V
= 1 × 350
Mass of displaced water = 350g.