Gravitation

5. GRAVITATION

When a body is held in hand and then released, the body falls vertically downwards. Let us think as to why the body falls towards the earth? Why is it not going up? It was Sir Issac Newton who posed this question and he himself answered it.

It is said that when Newton was sitting under a tree, an apple fell on him. The fall of the apple made Newton think. He thought that if the earth can attract an apple can it not attract the moon? Is the force the same in both cases? He conjectured that the same type of force is responsible in both the cases.

Newton was sitting under an apple tree, when an apple fell on him. He thought that the apple fell due to downward pull of the Earth on the apple.

When a body is thrown up, it reaches a certain height and then falls down. The downward pull of the Earth, on the body, decreases its velocity in the upward direction to zero at some height. The body cannot rise further. This is, therefore, the maximum height attained by that body. The same downward pull of the Earth on the body makes it fall downwards from the maximum height.

Consider a piece of stone tied to one end of a string. Hold the other end of the string in hand and whirl it around. The stone moves in a circular path with a certain speed, but its direction of motion changes continuously. The change in direction of motion involves change in velocity or acceleration of the stone. The external force F that causes this acceleration and keeps the stone moving uniformly along the circular path is acting towards the centre of the circular path. This is called centripetal force, i.e., centre-seeking force. At any instant, if we release the string, the stone flies along the tangent to the circular path at that instant. This is because, the moment the string is released, centripetal force is no longer provided. The stone is free to fly off along the tangent. Based on this activity, we can perceive that the motion of the Moon around the Earth is due to centripetal force, provided by force of attraction of the Earth on the Moon.

Gravitation is the phenomenon of attraction between any two objects in the universe. The objects may be terrestrial (which are on the Earth) or celestial (which are in outer space such as stars, planets, satellite etc.). Further, the objects may be of any size, shape or mass and they may be any distance apart (small or large), with any medium between them. The force gravitation is always the force of attraction and it is never repulsive.

NEWTON’S UNIVERSAL LAW OF GRAVITATIONS:

Newton gave the universal law that gave the relationship between the force of attraction between any two bodies lying at certain distance. According to Newton’s universal law of gravitation:

“Every object in the universe attracts every other object with a force. This force of attraction between any two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them”.

The accepted value of G is 6.673 × 10⁻¹¹ Nm²/kg².

As the value of G is extremely small, therefore, gravitational force between any two ordinary objects will be really weak. However, the gravitational force becomes appreciable when even when one of the objects has a large mass.

IMPORTANCE OF THE UNIVERSAL LAW OF GRAVITATION

The universal law of gravitation successfully explained several phenomena which were believed to be unconnected.

1. It is the gravitational force between the sun and the earth which keeps the earth in uniform motion around the sun.

2. The gravitational force between the earth and the moon makes the moon revolve at uniform speed around the earth. Thus the gravitational force is responsible for the existence of our solar system.

3. The tides in the sea formed by the rising and falling of water level in the sea, are due to the force of attraction which the sun and the moon exert on the water surface in the sea.

4. The gravitational force of the earth is responsible for holding the atmosphere above the earth.

5. It is also responsible for rain falling on the earth and for the flow of rivers.

6. It is also the gravitational force of the earth which keeps us firmly on the ground.

GRAVITATIONAL FORCE BETWEEN LIGHT OBJECTS AND HEAVY OBJECTS

The formula applied for calculating gravitational force between light objects and heavy objects is the same, i.e., . Let us take three cases:

1. When two bodies of mass 1 kg each are 1 metre apart.

Taking we obtain gravitational force of attraction,

This is extremely small. Hence, we conclude that though every pair of two objects exerts gravitational pull on each other, yet they cannot move towards each other because this gravitational pull is too weak.

2. When a body of mass 1 kg is held on the surface of Earth.

It means that the Earth exerts a gravitational force of 9.8 N on a body of mass 1 kg. This force is much larger as compared to the force when both the bodies are lighter. That is why when a body is dropped from a height, it falls to the Earth.

This is really large. It is this large gravitational force exerted by Earth on Moon, which makes the Moon revolve around the Earth.

GRAVITATION AND NEWTON'S THIRD LAW OF MOTION

As we have read Newton's third law of motion states that, to every action, there is always an equal and opposite reaction. It means if an object A exerts some force on another object B, then the object B exerts an equal and opposite force on the object A at the same instant. This law is applied to the force of gravitation also.

We say that an apple falls towards the Earth due to gravitational pull of the Earth on it. As a reaction, the apple also exerts an equal force of attraction on Earth in the opposite direction (i.e., towards the apple). But then why do we not see the Earth rising towards the apple? The answer to this question is provided by Newton's second law of motion, according to which

force = mass × acceleration

i.e., for a given force, acceleration produced varies inversely as the mass. Now, force on Earth (exerted by apple) = force on apple (exerted by Earth)

But mass of Earth is too large compared to the mass of apple. Therefore, acceleration produced in Earth is too small to be seen. That is why the Earth is not seen rising towards the apple.

We know that acceleration produced in a body due to gravitational pull of Earth on it is 9.8 m/s². As this acceleration is very large, we can see the body falling towards Earth. We shall show that when gravitational pull of same magnitude acts on Earth (whose mass is ), the acceleration produced in Earth is . As this value of acceleration is too small, we cannot see the Earth moving towards the falling body.

KEPLER’S LAWS OF PLANETARY MOTION:

Johannes Kepler derived three laws, which govern the motion of planets. These are called Kepler’s laws. These are:

1. The orbit of a planet is an ellipse with the sun at one of the foci, as shown in the figure given below. In this figure O is the position of the sun.

2. The line joining the planet and the sun sweeps equal areas in equal intervals of time. Thus, if the time of travel from A to B is the same as that from C to D, then the areas OAB and OCD are equal.

3. The cube of the mean distance of a planet from the Sun is proportional to the square of its orbital period T. Or, r³ / T² = constant.

SOLVED EXAMPLE

Question: Why is Newton’s law of gravitation called as universal law of gravitation?

Solution: The law of gravitation is universal in the sense that it is applicable to all the bodies, whether the bodies are big or small, whether they are celestial or terrestrial.

Question: If the distance between two objects is increased by a factor of 6, how is the force altered?

Solution: Saying that F is inversely proportional to the square of d means that if d gets bigger by a factor of 6, F becomes times (smaller).

Question: The mass of the earth is and that of the moon is . If the distance between the earth and the moon is , calculate the force exerted by the earth on the moon. []

Question: Calculate the force of gravitation due to a child of mass 25 kg on his mother of mass 75 kg standing at a distance of 1 m from each other.

(Given: )

Question: A mass of 50 kg attracted by a mass of 20 kg lying at a distance of 2m with a force of . Find the value of G.

Question: Calculate the force of gravitation due to earth on a child weighing 10 kg standing on the ground. (Mass of earth = ; Radius of earth = & )

FREE FALL:

When we drop a body say a stone, we observe that its speed increases as it falls towards the earth. We have already seen that the body falls towards earth due to gravitational force between body and the earth. Whenever objects fall towards the earth under this force alone, we say that the objects are under free fall. While falling, there is no change in the direction of motion of the objects. But due to the earth’s attraction, there will be a change in the magnitude of the velocity. Any change in velocity involves acceleration.

The question is now whether we can measure its acceleration? Will this acceleration be more for heavier bodies?

If we drop stone and paper from a top of a house, we find that stone reaches the earth earlier than the paper. It was therefore thought that the acceleration for heavier bodies is more than lighter bodies.

Galileo decided to test this common belief. He dropped two stones of different masses from the top of leaning Tower of Pisa. It was found that both reached the earth in almost same time. He concluded that all the bodies fall towards the earth with equal acceleration. The acceleration with which the bodies fall towards the earth is independent of the masses of the bodies. Reason for slowing down of lighter bodies was attributed to the resistance or friction offered by the air.

Robert Boyle took a vacuum pump to remove air from a tube containing a heavy coin and a piece of paper. When the tube was inverted, both the coin and the paper hit the bottle at the same time. Thus, Galileo’s prediction that all bodies fall with the same acceleration towards earth stands confirmed.

GRAVITY:

As we have read that gravitation is the phenomenon of attraction between any two bodies of the universe. Gravity is one particular case of gravitation when one of the two attracting bodies is Earth. Hence, gravity is the phenomenon of attraction between Earth and any other body. The force of attraction exerted by Earth on the body is called force of gravity.

The force of gravity is calculated using Universal Law of Gravitation given by Newton as,

Where m = mass of the body

M = mass of earth

r = distance of the body from the centre of Earth

F = force of gravity exerted by Earth on the body

ACCELERATION DUE TO GRAVITY

When a body is dropped from a certain height, it falls with a constant acceleration.

This uniform acceleration produced in a freely falling body due to the gravitational pull of the earth is known as acceleration due to gravity and it is denoted by the letter ‘g’.

Although g varies very slightly from place to place but its average value is taken to be 9.8 m/s².

This means that the velocity of a body increases by 9.8 m/s every second. Say, if the body is dropped with zero velocity, its velocity becomes 9.8 m/s after 1s; 19.6 m/s after 2s; 27.4 m/s after 3s and so on. Similarly, if a body is projected upwards, its velocity decreases by 9.8 m/s after every second.

RELATION BETWEEN ‘g’ AND ‘G’

Case I: If we drop a body (say, a stone) of mass ‘m’ from a distance ‘d’ from the center of the earth of mass M, then the force exerted by the earth on the stone is given by Newton’s law of gravitation.

We also know from the second law of motion that force is the product of mass and acceleration. We already know that there is acceleration involved in falling objects due to the gravitational force and is denoted by g.

Therefore, the magnitude of the gravitational force F will be equal to the product of mass and acceleration due to the gravitational force, that is

Case-II

Let an object be on or near the surface of the earth. The distance will be equal to, the radius of the earth. Thus, for objects on or near the surface of the earth,

The earth is not a perfect sphere. As the radius of the earth increases from the poles to the equator, the value of ‘g’ becomes greater at the poles than at the equator.

VALUE OF g ON EARTH

To calculator the value of g, we should put the values of ; mass of the earth and radius of the earth (R) , in the formula :

Thus, value of acceleration due to gravity of the earth, .

We observe from this relation that value of g depends on mass of Earth and radius of Earth. Mass of the body (m) is nowhere involved in this relation. It means that the value of acceleration due to gravity does not depend upon mass of the body. Hence, all bodies when free will falls towards the centre of earth with the same acceleration – whatever be their masses. It means that a big stone and a small stone when dropped from a particular height will move with the same acceleration due to gravity. Both will strike the ground at the same time whenever they fall under gravitational pull alone (vacuum).

VARIATION IN ACCELERATION DUE TO GRAVITY (g)

The relation between g and G is

(a) Effect of shape of Earth on 'g'

As is known, Earth is flattened at the poles and bulged out at the equator. Therefore, polar radius of Earth = is minimum, and equatorial radius of Earth = is maximum.

Acceleration due to gravity at poles, = maximum, as is minimum

Acceleration due to gravity at equator, = minimum, as is maximum

(b) Effect of height above the surface of Earth on 'g'

As we move above the surface of the Earth, the distance (r ) from the centre of earth increases. As , therefore, value of acceleration due to gravity decreases with height above the surface of Earth.

(c) Effect of depth below the surface of Earth on 'g'

Most of you might be thinking that as we go down the surface of Earth, the value of radius of Earth R decreases. Therefore, g must be increasing. But it is not true. You will learn in higher classes that the value of acceleration due to gravity decreases as we go down inside the Earth. So much so that at the centre of Earth, g = 0.

Hence, we conclude that value of acceleration due to gravity is maximum at the surface of Earth. It decreases as we move above the surface of Earth or go inside the surface of Earth.

SOLVED EXAMPLE

Question: A planet whose mass and radius both are half as that of the earth, the acceleration due to gravity at the planet's surface would be

(a) 19.6 m/s (b) 9.8 (c) 4.9 (4) 2.45

Question: If a planet existed whose mass was twice as that of the earth and whose radius 3times greater, a 10 kg mass on its surface will weigh:

(a) 21.7 N (b) 4.4 N (c) 6.7 N (d) 13.3 N

Choose the correct answer.

EQUATIONS OF MOTION OF FREELY FALLING BODIES:

When the bodies are falling under influence of gravity, they experience acceleration g i.e. 9.8 . However, when these are going up against gravity they move with retardation of 9.8 .

MASS AND WEIGHT:

The similarities and differences between mass and weight are discussed as follows :

MASS

The mass of a body is the quantity of matter contained in it.

Mass is a scalar quantity. The unit of mass is kilogram.

A body contains the same quantity of matter whether it be on the earth, moon or even in outer space. Thus, the mass of a body is constant and does not change from place to place.

Mass of a body is usually denoted by the small letter ‘m’.

Mass of a body is a measure of inertia of the body and hence it is also known as inertial mass.

The mass of a body cannot be zero.

WEIGHT

We know that the earth attracts every object with a certain force and this force depends on the mass (m) of the object and the acceleration due to gravity (g).

The weight of an object is the force with which it is attracted towards the earth.

We know that,

That is,

The force of attraction of the earth on an object is known as the weight of the object. It is denoted by W.

So we have,

As the weight of an object is the force with which it is attracted towards the earth, the S.I. unit of weight is the same as that of force i.e. Newton (N).

The weight is a force acting vertically downwards; it has both magnitude and direction, so it is a vector quantity.

The value of g is constant at a given place. Therefore at a given place, the weight of an object is directly proportional to the mass, say m, of the object, that is, . It is due to this reason that at a given place, we can use the weight of an object as a measure of its mass.

The mass of an object remains the same everywhere, that is, on the earth or on any planet whereas its weight depends on its location.

Weight of a freely falling body

Let us suppose that a body is placed on a lift, the weighing machine will show the weight of the body on its scale. Now, the lift is made to fall freely due to gravity, both the weighing machine as well as the body will fall with same acceleration i.e., with g in the downward direction. The body will, therefore, not press the weighing machine with any force and hence show zero weight. Thus a body is weightless during free fall.

Weightlessness in space

Consider an astronaut in a space ship orbiting the earth about 1000 km above its surface. At that distance from the earth, the force of gravity of earth is still quite strong. Since the acceleration due to gravity is not zero, the weight of astronaut in the space ship certainly cannot be zero. But we all have seen them on T.V., floating is a space ship and believe that in this situation they are weightless. This can be explained as follows:

When an astronaut in the space ship is orbiting the earth, then both, the astronaut and the spaceship are in a continuing state of free fall towards the earth with the same acceleration due to gravity. Since the downward acceleration of the astronaut is the same as that of the spaceship he does not exert any force on the sides of the space ship and a weighing machine kept in the space vehicle will show his weight to be zero. Though the free fall of a body produces a feeling of weightlessness but a true weightlessness can be experienced by a spaceship in a region of outer space where the acceleration due to gravity ‘g’ is zero.

Weight of an object on the moon

We have learnt that the weight of an object on the earth is the force with which the earth attracts the object. In the same way, the weight of an object on the moon is the force with which the moon attracts that object.

The mass of the moon is less than that of the earth. Due to this, the moon exerts lesser force of attraction on objects. Let the mass of an object be m. Let its weight on the moon be W_m. Let the mass of the moon be Mm and its radius be R_m.

THRUST AND PRESSURE:

The force acting on an object perpendicular to the surface is called thrust. Let us understand the meaning of thrust and pressure practically.

Situation-1:

To fix a poster on a notice board and while doing so you need to press drawing pins with your thumb. So pressing drawing pins means applying force on the surface area of the head of the pin. This force is directed perpendicular to the surface area of the board.

Situation-2:

When you stand on loose sand your feet go deep into the sand. But when you lie down on the sand, you will find that your body will not go deep on the sand.

This is because when you stand on loose sand, the force i.e., the weight of your body is acting on an area equal to area of your feet. When you lie down, the same force acts on an area equal to the contact area of your whole body. Which is larger than the area of your feet

In honour of scientist Blaise Pascal, the S.I. unit of pressure is called pascal, denoted as .

Pressure depends on two factors :

(i) Force applied (ii) Area over which force acts.

SOLVED EXAMPLE

Question: Mass of an object is 10 kg. What is its weight on the earth?

Question: What is the mass of object whose weight is 49 N?

Question: A force of 100 N is applied to an object of area . Calculate the pressure.

Question: A man weighs 600N on the earth, what is its mass? If it was taken to the moon, his weight would be 100 N. What is his mass on moon? What is his accelerations due to gravity on the moon.

Solution: (i) Let m be the mass of body on earth. We know,

(ii) Mass will be same on Moon i.e., 60 kg

Weight on moon = mass × acceleration due to gravity on moon.

Question: A car falls off a ledge and drops to the ground in 0.5s. Let .

(i) What is its speed on striking the ground?

(ii) What is its average speed during the 0.5 s?

(iii) How high is the ledge from the ground?

Solutions: Time, second

Initial velocity,

Acceleration due to gravity,

Acceleration of the car, (downward)

(i) Speed

(ii) Average speed

(iii) Distance traveled

Question: An object is thrown vertically upwards and rises to a height of 10m. Calculate

(i) the velocity with which the object was thrown upwards and

(ii) the time taken by the object to reach the highest point.

Question: To estimate the height of a bridge over a river, a stone is dropped freely in the river from the bridge. The stone takes 2 seconds to touch the water surface in the river. Calculate the height of the bridge from the water level .

Question: A block of wood is kept on a table top. The mass of wooden block is 5 kg and its dimensions are 40 cm × 20 cm × 10 cm. Find the pressure exerted by the wooden block on the table top if it is made to lie on the table top with its sides of dimensions

(a) 20 cm × 10 cm (b) 40 cm × 20 cm

Question: When a ball is thrown vertically upwards, it goes through a distance of 19.6 m. Find the initial velocity of the ball and the time taken by it to rise to the highest point. (Acceleration due to gravity, )

Question: A ball is thrown up with a speed of 15 m/s. How high will it go before it begins to fall? .

Question: Give reason for the following :

(a) Why school bags have wide straps?

(b) Why a sharp knife cuts better than a blunt knife?

(c) Why a nail has a pointed tip?

(d) Why buildings have wide foundations?

Solution: (a) A school bag has wide straps made of thick cloth so that the weight of bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder. Due to less pressure, it would be more comfortable to carry the heavy school bag. On the other hand, if the school bag has a straps made of thin strings, then the weight of school bag will fall over a small area of the shoulder. This will produce a large pressure on the shoulder of the child and it will become very painful to carry the heavy school bag.

(b) A sharp knife has a very thin edge of its blade. A sharp knife cuts objects better because due to its very thin edge, the force of our hand falls over a very small area of the object producing a large pressure. And this large pressure cuts the object easily. On the other hand, a blunt knife has a thicker edge. A blunt knife does not cut an object easily because due to its thicker edge, the force of our hand falls over a large area of the object and produces lesser pressure. This lesser pressure cuts the object with difficulty.

(c) A nail has a painted tip, so that when it is hammered the force of the hammer falls on a very small area of the wood or the wall creating a large pressure which pushes the nail into the wood or the wall.

(d) The foundation of buildings and dams are laid on a larger area of ground so that the weight of the building or dam produces less pressure on ground and they may not sink into the ground.

PRESSURE AND FLUIDS:

All liquids and gases are called as fluids. A solid exerts pressure on a surface due to its weight.

Similarly, fluids have weight, and they also exert pressure on the base and walls of the container in which they are enclosed. A fluid exerts pressure in all directions even upwards.

BUOYANCY

To understand the term buoyancy, let us perform an activity :

Take an empty plastic bottle. Close the mouth of the bottle with an air tight stopper. Put it in a bucket filled with water.

• We will see that the bottle floats.

• Now push the bottle into the water. We will feel an upward push. If we push it further down, we will find it difficult to push deeper and deeper.

• This indicates that water exerts a force on the bottle in the upward direction.

• This upward force exerted by the water goes on increasing as the bottle is pushed deeper till it is completely immersed.

• If we release the bottle, it bounces back to the surface.

Explanation

The force due to the gravitational attraction of the earth acts on the bottle in the downward direction. So the bottle is pulled downwards. But the water exerts an upward force on the bottle. Thus, the bottle is pushed upwards.

The weight of an object is the force due to gravitational attraction of the earth. When the bottle is immersed, the upward force exerted by the water on the bottle is greater than its weight therefore it rises up, when released.

Check Out- CBSE Class 9 Science Notes

To keep the bottle completely immersed the upward force on the bottle due to water must be balanced. This can be achieved by an externally applied force acting downwards. This force must at least be equal to the difference of upward force and the weight of the bottle.

The upward force acting on an object immersed in a liquid is called buoyant force or upthrust and this phenomenon of exerting buoyant force is called buoyancy.

FACTORS AFFECTING BUOYANT FORCE

The magnitude of buoyant force acting on an object immersed in a liquid depends on two factors :

(i) Volume of the object immersed in the liquid.

The buoyant force exerted by a liquid depends on the volume of the solid object immersed in the liquid. As the volume of solid object immersed inside the liquid increases, the upward buoyant force also increases. And when the object is completely immersed in the liquid, the buoyant force becomes the maximum and remains constant. Also, the magnitude of buoyant force acting on a solid object does not depend on the nature of the solid object.

It depends only on its volume. For e.g. If two balls made of different metals having different weights but equal volumes are fully immersed is a liquid, they will experience an equal upward ‘buoyant force’ i.e. equal loss in weight.

(ii) Density of the liquid.

The buoyant force exerted by a liquid depends on the density of the liquid in which the object is immersed. As the density of liquid increases, the buoyant force exerted by it also increases, for example sea water has higher density then fresh water therefore sea water will exert more buoyant force on an object immersed in it than the fresh water. Therefore it is easier to swim in sea water because sea water exerts a greater buoyant force on the swimmer due to its higher density.

Archimedes’ principle

Archimedes was a Greek scientist. He discovered the principle, subsequently named after him, after noticing that the water in a bath tub overflowed when he stepped into it. He ran through the streets shouting “Eureka”! which means “I have got it”. This knowledge helped him to determine the purity of the gold in the crown made for the king. This work in the field of geometry and mechanics made him famous. His understanding of levers, pulleys, wheels–axle helped the Greek army in its war with roman army.

“Archimedes’ principle states that when a body is partially or wholly immersed in a liquid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Activity to understand Archimedes’ principle

(a) Take a piece of stone and tie it to one end of a rubber string or a spring balance.

(b) Suspend the stone by holding the balance or the string as shown in figure below.

(c) Note the elongation of the string or the reading on the spring balance due to the weight of the stone.

(d) Now, slowly dip the stone in the water in a container. You will find that the elongation of the string or the reading of the balance decreases as the stone is gradually lowered in the water.

(e) However no further change is observed once the stone gets fully immersed in the water.

Explanation:

The elongation is produced in the string or the spring balance due to the weight of the stone. Since the extension decreases once the stone is lowered in water, it means that some force acts on the stone in upward direction. As a result, the net force on the string decreases and hence the elongation also decreases. This upward force exerted by water is known as the force of buoyancy.

Applications of Archimedes’ principle

(a) Archimedes’ principle is used in determining the relative density of a substance.

(b) The hydrometers used for determining the density of liquids are based on Archimedes’ principle.

(c) The lactometers used for determining the purity of milk are based on Archimedes’ principle.

(d) Archimedes’ principle is used in designing ships and submarines.

Why objects float or sink in a liquid?

When an object is put in a liquid, then two forces act on it :

• Weight of object acting downwards due to the gravitational pull of the earth on the object.

• Buoyant force acting upwards which tends to push the object up.

(a) Sinking of an object in water

If we place an iron nail on the surface of water in a beaker then the nail sinks. The force due to the gravitational attraction of the earth on the iron nail pulls it downwards. There is an upthrust of water on the nail, which pushes it upwards. But the downward force acting on the nail is greater than the upthrust of water on the nail, so it sinks.

(b) Floating of an object in water

If we place a piece of cork on the surface of water in a beaker, then the cork floats. This happens because of the difference in their densities. The density of a cork is less than the density of water. This means that the upthrust of water on the cork is greater than the weight of the cork. So, it floats.

Therefore, objects of density less than that of a liquid float on the liquid. The objects of density greater than that of a liquid sink in the liquid.

DENSITY:

We describe the lightness or heaviness of different substances by using the word density.

The density of a substance is defined as mass of the substance per unit volume. That is,

RELATIVE DENSITY:

The relative density of a substance is the ratio of its density to that of water.

Relative density of a substance

Since, the relative density is a ratio of similar quantities, it has no unit.

The relative density of a substance expresses the heaviness of the substance in comparison to water. For example, the relative density of iron is 7.8. This means that iron is 7.8 times as heavy as an equal volume of water.

The relative density of water is 1. Now if the relative density of a substance is more than 1, then it will be heavier than water and hence it will sink in water. On the other hand, if the relative density of a substance is less than 1, it will be lighter than water and hence will float in water.