You must have studied in class 8 about Newton’s dilemma: why does the apple always fall down from the tree? In this chapter of class 9, you will understand more about gravitational force. It is a very important chapter related to Physics. This chapter will give you a vivid idea about how gravitational force works. There are quite a few numerical in this chapter. So, you will require regular practice to score full marks in them.

**NCERT Solution** Class 9 Science of Chapter 10 includes all Intext exercises 1, 2, 3, 4, 5, and an **NCERT** exercise for you. The NCERT exercise includes solutions to all 22 questions given in your NCERT Textbook. You can practice these questions to test and enhance your numerical abilities, application skills, and memory.

## NCERT Solution for Class 9 Science Chapter 10 : Gravitation Ex 10.1

**Exercise 10.1.1**

**Question 1.**

**State the universal law of gravitation.**

**Solution**:

According to universal law of gravitation, every particle in the universe attracts every other particle with a force which is directly proportional to the distance between them. The direction of the force is along the line joining the two particles.

**Question 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**:

The formula for the magnitude of gravitational force between the earth and an object on its surface is

F = GMm / R2

Where

F is the gravitational force. G is the gravitational constant. M is the mass of the earth, m is the mass of the object on the surface of the earth, R is the radius of the earth.

**Exercise 10.2.2**

**Question 1.**

**What do you mean by free fall?**

**Solution**:

All objects falling towards earth under the action of gravitational force of earth alone are said to be in free fall.

**Question 2.**

**What do you mean by acceleration due to gravity?**

**Solution**:

The acceleration with which an object fall freely towards the earth is known as acceleration due to gravity. It is denoted by g ad its value is 9.8 m/s.

.

**Exercise 10.4.1**

**Question 1.**

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

**Solution**: Refer pdf.

**Question 2.**

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

**Solution**:

The weight of an object depends on the value of acceleration due to gravity g. The value of g onearth is 6 times more than that of moon because, the mass and the radius of the earth is more than the mass and radius of the moon.

We have:

g = GM/ R2 and W = mg

Weight of a body of mass m on earth is

We = mg = mGM / R2

Weight of a body of mass on moon is

Wm = mgm = mGMm / R2

m

As mass of moon is 1/100 times the mass of earth and radius of moon is ¼ times the radius of

earth.

2 16 1 ( )

100 6

m m e

e e m

W M R

W M R

(approx)

**Exercise 10.5.2**

**Question 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 is sort of giant. This can be as a result of the pressure is reciprocally proportional to the expanse on that the force acts. The smaller is that the surface area; the larger are 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 giant.

**Question 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.

**Question 3.**

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

**Solution**:

An object floats or sinks 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.

**Exercise 10.6**

**Question 1.**

**You find your weight to be 42 kg on a weighing machine. ls your mass more or ****than 42 kg?**

**Solution**:

In fact, a weighing machine is a sort of spring balance which measure the weight of a body. When we stand on the weighing machine, our weight (which is due to gravitational attraction of the earth) acts vertically downwards. But the buoyancy due to air on our body acts vertically upwards. As a result of this, our apparent weight (true weight buoyant force) is less than the true weight. Since the

weighing machine measures the apparent weight, our true weight is more, i.e., more than 42 kg.

0R

On weighing machine we are actually measuring weight and considering this as a measure of mass. As the weighing is being done in air there is force of buoyancy due to air also. Hence, weight of 42 kg True weight Force of buoyancy.

So it is clear that true weight must be more than 42 kg. Hence, mass of said boy is more than 42 Kg.

**Question 2.**

**You have a bag of cotton and an iron bar, each indicating a m 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**:

We know that true weight = apparent weight + upthrust. The cotton bag is heavier than the iron bar .This is due to the reason that the bag of cotton which has more volume (as it is less dense) the iron bar (which has more density) experiences more upthrust due to air.

**Exercise Chapter 10**

**Question 1.**

**How does the force of attraction between two objects change when the distance ****between them is reduced to half?**

**Solution**: Refer pdf.

**Question 2.**

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

**Solution**:

Gravitational force acts on all objects in proportion to their masses. But a heavy object does not fallfaster than a light object. This is because of the reason that

Force = acceleration x mass

As force is directly proportional to mass, acceleration is constant for a body if any mass.

**Question 3.**

**What is magnitude of the gravitational force between the earth and a 1kg object ****on its surface? (Mass of the earth is 6X1024 kg and radius of the earth is 6.4 x106 ****m)**

**Solution**: Refer pdf.

**Question 4.**

**The earth and the moon are attracted to each other by gravitational force. Does ****the earth attract the moon with a force that is greater or smaller or the same as ****the force with which the moon attracts the earth? Why?**

**Solution**:

The earth attracts the moon with a force equal to the force with which the moon attracts the earth. This is because as per Newton’s third law of motion, forces of action and reaction are always equal and opposite.

**Question 5.**

**If the moon attracts the earth, why does the earth not move towards the moon?**

**Solution**:

Both the earth and the moon attract each other with the same force. But according to Newton’s second law of motion, acceleration produced in a body by any force is inversely proportional to the mass of the body. Since, mass of the earth is negligible. As a result, it appears as if the earth does

not move towards the moon.

**Question 6.**

**What happens to the force between two objects if**

**(i) The mass of object is doubled?**

**(ii) The distance between the objects is doubled and tripled.**

**(iii) The masses of both objects are doubled**

**Solution**: Refer pdf.

**Question 7.**

**What is the importance of universal law of gravitation?**

**Solution**:

Universal law of gravitation is important as it accounts,

(a) For the existence of the solar system, i.e., motion of planets around the sun.

(b) For holding the atmosphere near the surface of the earth.

(c) For rainfall and snowfall.

(d) For occurrence of tides.

**Question 8.**

**What is the acceleration of free fall?**

**Solution**:

All objects moving towards earth on account of gravitational force of earth on them are said to be in free fall. This force produces a uniform acceleration in the object. This is acceleration of free fall and its value is 9.8 m/s2

**Question 9.**

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

**Solution**:

The gravitational force between the earth and an object is called the force of gravity or simply earth’s gravity.

**Question 10.**

**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**:

We know that the value of g is greater at the poles than at the equator. So the weight of gold at the equator will be less than the weight of gold at the poles. So it is obvious that the friend at equator will not agree with the weight of gold bought at poles.

**Question 11.**

**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 of a 10 Kg object ****on the moon and on the earth.**

**Solution**:Refer pdf.

**Question 12.**

**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.**

**Solution**: Refer pdf.

**Question 13.**

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

**Solution**: Refer pdf.

**Question 14.**

**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?**

**Solution**: Refer pdf.

**Question 15.**

**Calculate the force of gravitation between the earth and the sun, given that the ****mass of the earth = 6 x 1024 kg and of sun = 2 x1030 kg**

**Distance between the earth and the sun, R = 1.5×1011’m**

**Solution**: Refer pdf.

**Question 16.**

**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.**

**Solution**: Refer pdf.

**Question 17.**

**A ball thrown up vertically return to the thrower after 6 s. Find:**

**(a) The velocity with which it was thrown p,**

**(b) The maximum height it reaches, and**

**(c) Its position after 4 s.**

**Solution**: Refer pdf.

**Question 18.**

**In what direction does the buoyant {area on an object immersed in a liquid act?**

**Solution**:

If an object is immersed in a liquid then the buoyant force due to liquid acts on the object in vertically upward direction.

**Question 19.**

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

**water?**

**Solution**:

The buoyant force acting on the block of plastic is more than its weight. As a result of this, it comes up when released under water. The cause of this larger buoyant force on the block of plastic is due to its density being smaller than water.

**Question 21.**

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

**Solution**: Refer pdf.

**Question22.**

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

**Solution**: Refer pdf.