MSBSHSE Class 8 Science Chapter 3 Force and Pressure

Question 1: Write the word in the blank space.

a. The SI unit of force is_________ (Dyne, Newton, Joule).

b. The air pressure on our body is equal to __________ pressure (Atmospheric, sea bottom, space).

c. For a given object, the buoyant force in liquids of different ________ is__________ (the same, density, different, area).

d. The SI unit of pressure is_______ (N/m³ , N/m² , kg/m² , Pa/m² ).

Answer:

a. The SI unit of force is Newton .

b. The air pressure on our body is equal to atmospheric pressure.

c. For a given object, the buoyant force in liquids of different density is different .

d. The SI unit of pressure is N/m² .

Question 2: Make a match:

A group	B group
1. Fluid	a. Higher pressure
2. Blunt knife	b. Atmospheric pressure
3. Sharp needle	c. Specific gravity
4. Relative density	d. Lower pressure
5. Hecto Pascal	e. Same pressure in all directions

Answer:

A group	B group
1. Fluid	a. Same pressure in all directions
2. Blunt knife	b. Lower pressure
3. Sharp needle	c. Higher pressure
4. Relative density	d. Specific gravity
5. Hecto Pascal	e. Atmospheric pressure

Question 3: Answer the following questions in brief.

a. A plastic cube is released in water. Will it sink or come to the surface of water?

b. Why do the load carrying heavy vehicles have a large number of wheels?

c. How much pressure do we carry on our heads? Why don ’t we feel it?

Answer: (a) As the density of the plastic cube is less than that of water, it will float on the surface of water.

(b) We know,

Pressure = Force / Area

When the area of contact between two surfaces is greater, the pressure will be less too. Hence, the load carrying heavy vehicles usually have greater number of wheels as there is a reduction in pressure on the road due to greater contact area. This happens as the load is equally shared among the tyres which ensures that the force caused is equally distributed among all the tyres and no single tyre is under tremendous pressure.

(c) It is said that human beings can carry an atmospheric pressure of 10⁵ Pa (approx.) on their heads. We don ’t feel the pressure because there is a balance of atmospheric pressure created by the air and the blood running inside our body.

Question 4: Why does it happen?

a. A ship dips to a larger depth in fresh water as compared to marine water.

b. Fruits can easily be cut with a sharp knife.

c. The wall of a dam is broad at its base.

d. If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.

Answer: a) The density of fresh water is less than marine water due to the buoyant force on the ship that floats on marine water and is greater than the fresh water. For this reason, the ship dips to a larger depth in fresh water as compared to marine water.

b) Fruits can be easily cut with a sharp knife because of acute pressure. The sharp edged knife makes less contact with the fruits that are meant to be cut, hence the pressure exerted by it on the fruit is immense.

c) We are aware that pressure of liquid increases along with its depth. So, the walls of a dam are made broader at its base so that it can bear the heavy amount of pressure that is exerted by the river water.

d) If a stationary bus suddenly speeds up, the passengers are thrown in the backward direction because initially the passenger’s whole body was in the state of rest inside the bus. However, when the bus suddenly starts speeding up, the lower part of the passenger ’s body comes in motion towards the forward direction but the upper part of the body still remains at rest particularly due to inertia of rest. This is the reason why the passengers are thrown in a backward direction when a stationary bus suddenly increases its speed.

Question 5: Complete the following tables.

Mass(Kg)	Volume (m3 )	Density (kg/m3 )
350	175	–
–	190	4

Density of Metal (kg/m3 )	Density of water (kg/m3 )	Relative Density
	103	5
8.5×103	103	–

Weight (N)	Area (m2 )	Pressure(Nm−2 )
–	0.04	20,000
1500	500	–

Answer:

We know,

Density = Mass / Volume

By using the above formula, we can find the quantity of one substance if the other two quantities are furnished:

Mass (kg)	Volume (m3 )	Density (kg/m3 )
350	175	2
760	190	4

We know,

Relative density of substance = Density of substance / Density of water

By using the above formula, we can find the quantity of one substance if the other two quantities are furnished:

Density of Metal (kg/m3 )	Density of water (kg/m3 )	Relative Density
5×103	103	5
8.5×103	103	8.5

We know,

Pressure = Force / Area

By using the above formula, we can find the quantity of one substance if the other two quantities are furnished:

Weight (N)	Area (m2 )	Pressure (Nm−2 )
800	0.04	20,000
1500	500	3

Question 6: The density of a metal is 10.8×10³ kg/m³ . Find the relative density of the metal.

Answer:

Given: Density of metal = 10.8×10³ kg/m³

We have density of water = 1000 kg/m³

Relative density of substance =Density of substance / Density of water

Relative density of substance = 10.8×10³ / 1000 = 10.8

Question 7: Volume of an object is 20 cm³ and the mass is 50 g. Density of water is 1 gcm^-3 . Will the object float on water or sink in water?

Answer:

Given: Volume of object = 20 cm³

Mass of object = 50 g

Density of object = Mass of object / Volume of object = 50 / 20 = 2.5 g /cm³

Now, we know density of water = 1 g/cm³

Since, density of object >density of water, therefore, the object will sink in water.

Question 8: The volume of a plastic covered sealed box is 350 cm³ and the box has a mass 500 g. Will the box float on water or sink in water? What will be the mass of water displaced by the box?

Answer:

Given: Volume of box = 350 cm³

Mass of box = 500 g

Density of object = Mass of object / Volume of object = 500 / 350 = 1.43 g/cm³

We know, density of water = 1 g/cm³

Since, density of box >density of water, therefore, the object will sink in water.

Now, volume of liquid displaced = Volume of the object = 350 cm³

⇒Mass of liquid displaced / Density of liquid displaced = 350 cm³

⇒Mass of liquid= 350×1 = 350 g

MSBSHSE Class 8 Science Chapter 3 Additional Questions

Question 1: Define a force.

Answer: A force is an interaction, which when unopposed, changes the motion of an object. It can cause an object with mass to change or accelerate its velocity. Force can also be defined intuitively as a push or a pull. It is expressed in magnitude and direction as a vector quantity.

Question 2: What is Newton’s first law of motion?

Answer: When no force is applied to a stationary object, it remains still or stationary. A moving object in motion continues to move with the same intensity, speed and direction when no force is acting on that particular object. This is Newton’s first law of motion.

Question 3: What is Contact force?

Answer: When a force is seen to act through a direct contact of the objects or through any other

object. Such a force is called ‘Contact force’.

Question 4: Define Non contact force.

Answer: When a force is applied between two objects even if the two objects are not in contact with each other, such a force is known as ‘Non contact force’.

Question 5: Give examples of Contact force.

Answer: A fine example of contact force is muscular force, when it is applied to objects with the help of our muscles. It is applied in some cases such as lifting, pushing, pulling. For example, when a man applies force from behind to a car so that it moves in the forward direction.

Question 6: Give examples of Non contact force.

Answer: When force is applied to an object through some magnetic force, gravitational force, electrostatic force act without any direct contact between the objects. These are the examples of non contact forces. For example, when a comb is brushed against hair, then small pieces of paper kept on a table get attracted to the comb. This is because the comb has an electrostatic charge and there is an induced opposite charge on the pieces of paper and the pieces stick to the comb.

Question 7: What is friction?

Answer: Friction is the force, which resists the relative motion of solid surfaces or material elements which slide against each other. There are various types of friction. Dry friction is that force by which it opposes the relative lateral motion of two solid surfaces which are in contact with each other.

Question 8: What happens in the absence of frictional force?

Answer: Frictional force is very useful in our daily lives. In the absence of frictional force, the object would remain in motion. It would continue to move and would not be in a stationary position.

Question 9: What happens due to reduced frictional force? Explain with examples.

Answer: When we walk on the road, we exert pressure on the ground with our feet. In the absence of friction, we will slip and will fall down immediately. For instance, when we slip over banana peel on the street or slip due to mud on the road. These examples occur due to reduced frictional force.

Question 10: Define electrostatic force.

Answer: Electrostatic force is the force that exists between electrically charged particles or some objects at rest. It is a non contact force.

Question 11: What are balanced forces?

Answer: When two forces that are of equal size but are acting in opposite directions, these are called balanced forces. For example, when two pans with equal masses are connected to a box kept on a table, they have equal gravitational force as they are in a balanced state with effective force equal to zero as both are acting in opposite directions.

Question 12: What is unbalanced force?

Answer: Forces that cause a change in the motion of a particular object are called unbalanced forces. When unequal forces are applied to the box on a table on both the two sides, an unbalanced force acts on the box which results in imparting motion to the box.

Question 13: Give a good example of unbalanced force.

Answer: When children play tug of war at school, they pull the rope in their respective directions. If the pull of the force is equal on the two sides, the rope does not move. If the force is more on one side, the rope moves towards that direction. This means that initially, the two forces are balanced; the rope moves in the direction of higher force when the forces become unbalanced.

Question 14: What happens when force is applied to a stationary object?

Answer: When a force is applied on a stationary object it gradually starts to move and its speed and direction also changes.

Question 15: Define inertia.

Answer: The tendency of an object to remain in its existing state is called its inertia. Due to this reason, a stationary object remains in the similar state and an object in motion remains in the state of motion in the absence of an external force.

Question 16: What are the types of inertia?

Answer: There are three types of inertia which are discussed below:

1. Inertia of the state of rest: An object in the stationary state which is at rest cannot change its state of rest due to its innate property. This property is known as the inertia of the state of rest.

2. Inertia of motion: The innate property of an object due to which its state of motion cannot change, is known as its inertia of motion. For example, an electric fan continues to revolve even after it is switched off, when passengers are sitting in a running bus get a sudden jerk in the forward direction if the bus tends to stop suddenly.

3. Directional inertia: The innate property of an object due to which the object cannot change the direction of its motion, is called directional inertia. For instance, if a vehicle in motion which is travelling along a straight line suddenly turns, then the passengers sitting in it are thrown in the opposite direction of the turning.

Question 17: Define Pressure.

Answer: Pressure is the force exerted on a unit area perpendicularly.

Pressure = Force / Area on which the force in applied

Question 18: What is the unit of pressure?

Answer: The SI unit of force is Newton (N). The area is measured in m² . Hence, the SI unit of pressure is N/m² . It is also known as Pascal (Pa).

Pressure is a scalar quantity. In atmospheric science, the unit for pressure is bar i.e. 1 bar = 10⁵ Pa.

Question 19: What happens to the pressure of a force when area increases?

Answer: When area increases, pressure reduces for the same force and vice versa.

Question 20: Why doesn’t a camel’s feet penetrate into the sand while walking on it?

Answer: As the bottom surfaces of a camel’s feet are broad and are adapted to the sand in the desert. Therefore, the camel’s weight is exerted on a large area on the sand and the pressure is reduced on the sand. Due to this reason, the camel’s feet do not penetrate into the sand and it becomes easy for it to walk on the sand.

Question 21: What is pressure on solids?

Answer: There is air pressure in all objects, which are kept in air. Likewise, when some weight is kept on a solid, pressure is exerted on it. This pressure is totally dependent on the value of the weight and the contact area between the two.

Question 22: Where do fluids in a container exert pressure from?

Answer: Fluids in a container usually exert pressure from inside it especially on the walls and the bottom of the particular container. When a fluid is enclosed in a container, it exerts pressure in all directions equally at a point within the fluid.

Question 23: What is atmospheric pressure?

Answer: We are surrounded by air from all directions. This layer of air is known as atmosphere. It exists to about 16 km in height and further stretches up to about 400 km in a dilute form. The pressure created due to the air is known as atmospheric pressure.

Question 24: What happens to atmospheric pressure at sea level?

Answer: The air pressure at the sea level is called 1 Atmosphere pressure. The air pressure tends to decrease as we go up in height from the sea level.

1 Atmosphere = 101 x 10³ Pa = 1 bar = 10³ mbar

1 mbar ≈ 10² Pa (hecto pascal)

Question 25: What is buoyant force?

Answer: The buoyant force is the upward force exerted on an object that is fully or partially immersed in a fluid. This upward force is also called Upthrust. Due to this reason, the buoyant force of a body submerged partially or fully in a fluid appears to lose its weight i.e. appears to be lighter.

Question 26: What are the two factors of buoyancy?

Answer: Buoyant force is dependent on two factors:

1. Volume of the object – The buoyant force is usually more if the volume of the object which is dipped is more.

2. Density of liquid – With the increase in the density of the liquid, the force of buoyancy also increases.

Question 27: Who was Archimedes?

Answer: Archimedes was a Greek scientist and sharp mathematician who found out the value of π by numerical calculations. His knowledge of various pulleys, levers, wheels in Physics proved useful to the Greek army and helped them to fight the Roman army. He was particularly famous for his geometry and mechanics works.

There is a famous incident, while taking a bath in a bathtub, he discovered the Archimedes principle by observing the water overflowing out of the tub. He came out of the tub in the same state shouting ‘Eureka’, ‘Eureka’, which meant ‘I found it’, ‘I found it.’

Question 28: Write about Archimedes principle.

Answer: Archimedes principle states that “When an object is partially or fully immersed in a fluid, a force of buoyancy acts on it in the upward direction. This force is equal to the weight of the fluid displaced by the object”.

Question 29: Identify some uses of Archimedes principle.

Answer: The Archimedes principle has been used in the construction of ships and submarines. Instruments such as the hygrometer, lactometer are based on this principle.

Question 30: What is Density of a substance? Explain with a formula.

Answer: Density of a substance is usually defined as the mass per unit volume of the substance.

The SI unit of density is kg/m³ .

Density = Mass / Volume

Question 31: What is Relative density?

Answer: The relative density of a substance is expressed with reference to the density of water.

Relative density = Density of substance / Density of water

This particularly implies the ratio of two equal physical quantities that has no unit. Relative density of a substance is known as its ‘specific gravity.’

Question 32: Give an example of gas pressure.

Answer: When a balloon is inflated on all sides by filling it with air from mouth and a pin hole is created in the balloon, then the air leaks out of it and it does not inflate fully. This implies that gas exerts pressure on the wall of the respective container in which it is enclosed from all sides.

We hope that the above mentioned solutions of “MSBSHSE Class 8 Science Chapter 3 Force and Pressure ” will help students to build a strong and solid foundation of the different concepts mentioned in the chapter.