Forces are present everywhere in the universe and can be both large and small. Forces can be contact forces, which require physical contact to act, or non-contact forces like gravity and magnetism. Pressure is defined as force per unit area and is determined by both the magnitude of the applied force and the area over which it acts. Greater pressure results from either increased force or decreased area of contact. Pressure is important in applications like cutting, carrying loads, and building foundations.

1. Force & Pressure
By Janhavi
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2. Introduction
• Force is one of the most important parts of physics. Physics studies the interaction of objects and forces in space
over time. Physicists from all parts of the world through the ages have been studying forces that are present
everywhere in the universe.
• A force can be so large that it can pull big objects like stars and galaxies.
• It can also be very tiny such that it can bind a nucleus with electrons in an atom.
• Forces are ubiquitous and are present everywhere at all times in the universe. This chapter deals with the concept
of force and it also deals with pressure, which is closely associated with force.
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4. Force- a push & pull
• In day-to-day life in order to accomplish various tasks, one either pushes an object or pulls it. While playing football, a player pushes a ball very
hard by kicking it to score a goal. Similarly, in order to keep an unruly dog away from moving vehicles, we pull the chain on the dog.
• In both the above instances, force is being applied on the football and the dog. Therefore, a push and a pull are related to applied force. To put
it another way, a push.
• The amount of we did not apply a force on the force differs from small to large. For example, a broken car, a large force is required. We push
the car harder and harder in the forward direction.
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5. Contact and Non-contact Forces
• Forces act on objects when they are in direct or
indirect physical contact with the object. In some
instances, forces act on objects without any physical
contact as shown in the Fig 11.2.
• The given diagram shows, a coconut falling from the
coconut tree. The coconut falls because the Earth
attracts objects downwards due to the force of
gravity.
• Similarly, a magnet attracts iron nails towards itself
by applying the magnetic force.
• Depending upon such interactions between the force
and the object, forces are classified as contact
forces and non-contact forces.
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6. Contact Forces
A force which can be applied only when it is in contact with an object is called a contact force.
Following are some examples of contact forces:
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8. Muscular Force & Frictional Force
Muscular Force
A force that results due to the action of muscles is known as muscular force. It can be applied to the objects only when it is in direct or indirect
contact with the objects. It is used to perform different activities like walking, running, catching, pushing, pulling and lifting certain objects.
The muscular force is used with different hand tools to enhance the efficiency of various activities such as using hammers to fix nails, knives to cut
vegetables and fruits, saws to cut trees. Besides our muscular force, the muscular force of domesticated animals are also used with different
instruments to accomplish a variety of tasks or activities.
Following are a few examples where muscular force of animals are used by humans to perform different activities:
• Buffaloes are used to plough farms for agriculture.
• Horses and buffaloes are used to run carts to carry loads.
• Elephants are used to carry big and heavy loads like the luggage of travelers, heavy logs, etc.
• Camels, donkeys and horses are used to carry luggage in remote areas.
Frictional Force
A force that acts along the two surfaces in contact and opposes the motion of one object over another is called the force of friction. A force of
friction always acts on the objects that are in motion. It is always applied at the point of contact of two bodies.
It arises due to the contact between two I surfaces. A ball rolling on the ground gradually slows down and finally comes to rest due to the force of
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10. Non-Contact Forces
A force which can be applied without any contact with an object is called a non-contact force. Following are a few examples of non-contact forces:
Magnetic Force
• A force that is exerted by a magnet on another magnet or any other magnetic material is called a magnetic force. It is a non-contact force
because there is no physical contact between
• the materials. This force is due to the magnetic field developed around the magnet. A magnet has two poles called the South Pole and North
Pole. When two magnets are brought together, then like poles repel each other while unlike poles attract each other without any contact
between these two magnets.
• A magnet attracts objects made up of cobalt, nickel, iron and their alloys.
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11. Let us do the following activity to know about non-contact magnetic force.
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12. Electrostatic Force
A force exerted by a charged body on
another charged or uncharged body is
known as an electrostatic force. Like
the magnetic force this force also
exists without any contact between
the two bodies. When a comb is
rubbed on dry hair, then the comb can
attract small pieces of papers. Let us
do the following activity to know about
non-contact electrostatic force.
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13. Gravitational Force
• The force that pulls all objects towards the centre of Earth is called the gravitational force of the Earth or the force of gravity. Gravitational force is
the attraction between an object and the Earth, even though they do not have any direct and indirect physical contact with each other.
• This force is always attractive, for instance, when a body is dropped from a height it moves in the downwards direction towards the Earth with
increasing speed at a constant rate.
• Gravitational force or gravity is responsible for keeping all the planets in their respective orbits around the Sun. The measure of the gravitational force
with which the Earth pulls an object towards itself is measured in terms of a quantity called weight.
• In other words the weight of an object is a force with which the Earth pulls it downwards.
• The scale of a spring balance uses this fact to measure the weight of different objects. When an object is hung on a spring balance, the gravitational
force of the Earth pulls the object in the downward direction. Due to this the pointer attached with the spring balance indicates the corresponding
weight of the object on the scale of the spring balance.
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14. Balanced & Un-Balanced Force
• A force has both magnitude and direction, which makes it a vector quantity. The magnitude of a force is strength of that force which can be
calculated in the numbers. Direction of the force is the direction of pull (towards) or push (away) due to the force.
• Two forces with the same magnitude, but different directions are not equal forces. The SI unit of force is newton (N) which is named after the
famous scientist, Sir Isaac Newton. Weight is also a type of force due to the gravity of the Earth, hence its SI unit also newton.
• In the real world, an object is always acted upon by more than one force at the same time. Therefore, while dealing with forces the concept of
equilibrium or balance becomes essential.
• A single resultant force which is a sum or combination of all forces acting on an object is called net force. When the net force acting on an
object is zero, then the object is said to be in equilibrium because there is no force which is acting on the object.
• For example, in the game of tug of war, when the pull of the force is equal on the two sides, the rope does not move. Here, the two forces are in
equilibrium or balanced. However, when the force on one side increases, the rope moves in the direction of greater force and net forces become
unbalanced.
• Thus, an unbalanced force changes the motion of an object.
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16. Effects of Force
As force results due to the interaction of different objects, it shows some effect on the interacting objects as well. The various effects of force are
as follows:
 Force can make stationary objects move or make moving objects move faster
• A stationary object can be set into motion when a force is applied on it. For instance, a stationary football can be made to move by kicking it.
Similarly, a motionless hand cart starts moving, when it is pushed by a force.
• When an object is already in motion, a force can make it move faster by pushing it in the direction in which it is already moving. In spite of this,
it is not necessary that a force always makes a stationary object move.For example, if an average human tries to push a truck, it will not move.
This because the applied force is not sufficient.
 Force can stop moving objects or slow them down
• A moving object can be stopped by applying force in the direction opposite to the motion of the moving object. For example, a goalkeeper in the
football game stops the football from hitting the goal post by applying a force.
• When applied force is not sufficient to stop the object, the applied force slows down the moving object. For example, a bicycle can be stopped
or slowed down by applying a force on the brakes.
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17. Force changes shapes and sizes
• A force can change the internal arrangement of molecules of an object and depending upon the magnitude of the force, the size or shape of the
object can be changed.
• A compressive (pushing) force compresses a spring, similarly a pulling force stretches the spring as shown in the figure. In both the cases, the
force causes a change in the size of the spring.
• In a similar fashion, the shape of an object can be changed, like the shape of a rubber band changes when a force is applied on it.
• Let us carry out the following activity to see how force affects the shape and size of an object.
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Activity 5
Aim: To study the effect on different objects.
Materials: A lump of dough, a rubber band, a plastic ruler, a spring, a lump of clay
Procedure:
• Apply a force on all the given objects using your hands by giving a push and a pull.
• Observe the effects of the force on different objects and record your observations in the observation table.
Observation:
Conclusion:
• It is observed that force has different effects on different objects.
Sr.no Objects
Effects of the Force on objects
Change of Shape Change in Size
Yes No Yes No
1 A lump of dough
2 A rubber band
3 A plastic ruler
4 A spring
5 A lump of clay

20. Pressure
• Force and pressure are related but they are not the same. Suppose there are two board pins, one with a sharp pointed tip and the
other with a flat blunt tip. Press them on a soft board with equal force. The sharp pin will pierce the board, but the flat pin will not
pierce the board. This is because the area of contact between the tip of the pins and the board is different in both cases. The area of
contact is much smaller in the case of the sharp pin. As a result, the force applied is concentrated in a small area. On the contrary,
the pin with the flat tip has a wider area of contact. Therefore, the force applied is distributed over a large area. Thus the
effectiveness of the applied force reduces.
This effectiveness of the applied force is a quantity which is known as pressure. Thus, pressure is defined as the force exerted
perpendicularly on a unit area. It is a scalar quantity.
Pressure=
𝐹𝑜𝑟𝑐𝑒
𝐴𝑟𝑒𝑎 𝑜𝑛 𝑤ℎ𝑖𝑐ℎ 𝑡ℎ𝑒 𝑓𝑜𝑟𝑐𝑒 𝑖𝑠 𝑎𝑝𝑝𝑙𝑖𝑒𝑑
The following details can be concluded from the above formula.
1. Pressure is directly proportional to force. Hence, greater the force, the larger the pressure exerted.
2. Pressure is inversely proportional to the area of contact, the larger the pressure exerted.
As it is known, the SI unit of force is the newton (N) and the area is measured in 𝑀2
Therefore,
Si unit of pressure=
𝑆𝐼 𝑢𝑛𝑖𝑡 𝑜𝑓 𝑓𝑜𝑟𝑐𝑒
𝑆𝑖 𝑢𝑛𝑖𝑡 𝑜𝑓 𝐴𝑟𝑒𝑎
=
𝑁
𝑚2 =pascal(Pa)
Thus, the SI unit of pressure is 𝑁 𝑚2
. It is known as Pascal (Pa).
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21. Activity 6
• Aim: To study the relation between pressure, force and area.
• Materials: a very thin wooden plank, two boxes, 4 thick books
• Procedure:
• Take a very thin wooden plank and place it on the boxes which are kept at a certain distance from
each other as shown in the figure.
• Take four thick books and place them on the plank at equal distances from one another and observe
the plank.
• Now, pile up all books at the middle of the plank and observe the plank.
• Observation:
• First case: When books are placed on the plank at equal distances, the plank bends a little.
• Second case: When books are piled up at the middle of the plank, the plank bends deeply.
• Conclusion:
• First case: The wooden plank bends less due to less pressure, because the force applied by the books
is spread over a large area.
• Second case: The wooden plank bends deeply due to more pressure, because the force applied by
the books is concentrated over a small area.
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22. Some the applications of pressure are given in the following
examples.
• It is easy to cut vegetable, fruits, paper and different items with instruments like a sharp knife or scissors. Such sharp edge instruments have
lesser area of contact. Thus, a small applied force is enough to generate a large amount of pressure in order to cut a variety of objects.
• A porter puts a piece of cloth on his head to carry heavy loads. The cloth increases the area of contact between the head and the load. In this
way, the porter reduces the pressure on his/her head which helps him/her to carry the load with more ease.
• It is very difficult to walk on sand, but camels walk easily on it. The bottom surface of the camels’ feet is broad. This increases the area of
contact of its feet to the ground, thereby the pressure reduces. Thus, the camel is able to walk on sand without sinking in.
• Foundations of buildings are made broader to reduce the pressure on the ground.
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23. Let us solve some numerical problems based on force and pressure.
1. A force of 2400 N acts on the surface of area 20 cm normally. What would be the 2 pressure on the surface?
Ans. Given: Force = 2400 N and area =20 𝑐𝑚2
= 20 x 10−4
𝑚2
= 0.002 𝑚2
To find out: Pressure = ?
Solution: In order to obtain pressure the following formula is needed.
Pressure=
𝐹𝑜𝑟𝑐𝑒
𝐴𝑟𝑒𝑎
Substituting given values in the above formula, we get
Pressure =
2400
0.002
= 1200000 𝑁 𝑚2
= 1.2 x 106
Pa
Thus , applied pressure on the given surface would be 𝟏. 𝟐 x 𝟏𝟎𝟔
Pa.
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24. 2. A solid object that weighs 200N is placed on a wooden plank and a pressure of 40 pa is applied. Calculate the area of
contact between the object and the wooden plank.
Ans. Given: Force (weight) = 200N and Pressure = 40 Pa
To find out: Area = ?
Solution: As we are aware that,
Pressure=
𝐹𝑜𝑟𝑐𝑒
𝐴𝑟𝑒𝑎
∴ Area =
𝐹𝑜𝑟𝑐𝑒
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
Substituting given values in the above formula, we get
Pressure =
200
40
= 5𝑚2
Thus, the area of contact between the object and the wooden plank is 5m .
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25. Pressure of Liquids & Gases
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26. Pressure of liquids
• All of us know solids exert pressure. Similarly, liquids also exert pressure, although they do not have a definite shape like solids. Liquids acquire the shape of containers in
which they are stored.
• Liquids exert pressure on the walls and the base of the containers. In short, they exert pressure all round them, unlike solids which can exert pressure on a surface only.
• Thus, pressure exerted by a liquid at the bottom of the container depends on the height of its column.It means pressure applied by a liquid is not same at all depths.
• The pressure increases as an object goes deeper into a liquid. In other words, the amount of pressure applied by a liquid is small under the surface of a liquid, but it is
greatest at the bottom of the liquid.
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28. Some the applications of concept of liquid pressure are given in the
following examples.
• As water pressure increases with depth, walls of dams are made thicker at the bottom. This is because thicker walls can withstand a greater
pressure exerted by the water at depth.
• The bodies of submarines are made up of stronger and thicker materials. So that, they can endure the tremendous pressure exerted by water in
deep seas.
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29. Pressure of Gases
Like liquids, gases also exert pressure on the walls of the container in which they are enclosed. Also,
all enclosed gases in containers exert pressure equally in all directions.
When a balloon is inflated by filling air, the air exerts pressure from within on the walls of the
balloon. When a hole is created in an inflated balloon, air leaks out of the balloon. If someone places
a finger in front of the hole, the gush of the air moving out can be felt.
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