Force and Pressure

M.
SRINIVASA RAO, SA(PS) GUDIVADA PH: 9848143855 Visit: srini science mind
8th class Physical Science Chapter-1: Force and Pressure
Identifying Actions as Push or Pull or both
M.SRINIVASA RAO, SA(PS) GUDIVADA PH: 9848143855 Visit: srini science mind
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Moving a book placed on a table
Opening or shutting a door
Drawing a bucket
of water from a
well
A football player taking a penalty kick
A cricket ball hit by a batsman
Moving a loaded
cart
Opening a drawer
Erasing black board
From above examples, A force is a push or a pull or both

FORCE
A force is a push or a pull or both.
A force cannot be seen but we can feel and observe the effects of the force.
Push Pull
A force to move an object away from our bodies A force to move an object towards our body
Combination of push and pull
Forces are due to an Interaction
If a man stands behind a car, the car does not move.
If a man pushes the car, he applies force and the car

begins to move in the direction of the applied force.
11 22 33
1. Both the girls appear to push each other .
2. Both the girls appear to pull each other.
3. The cow and the man appear to pull each other.
From this observations
At least two objects must interact for a force to come

into play.
An interaction of one object with another object

results in a force between the two objects.
Exploring Forces
If two forces acting on a body in the same direction, the net force is the
sum of the two forces.
Unbalanced force acting in the same direction combine by addition.
Exploring Forces
If two forces acting on a body in opposite direction, the net force is the difference
between the two forces and is exerted in the direction of the larger force.
Unbalanced forces acting in opposite direction combine by subtraction.
The strength of a force is usually expressed by its
magnitude.
 We have also to specify the direction in which a force
acts.
 If the direction or the magnitude of the applied force
changes, its effect also changes.
 Newton (N) is the S.I unit of force.
 Force is a vector quantity.
A Force can Change the State of Motion
Coming to rest
Moving from rest
from motion
Effects Change the Speed
of force
A Force can Change the Shape of an Object
A Force can Change the Direction of a

Motion of an Object
A Force can Change the State of Motion
A force applied on an object may change its speed. If the force applied on the
object is in the direction of its motion, the speed of the object increases. If the
force is applied in the direction opposite to the direction of motion, then it results
in a decrease in the speed of the object.
Force can Change the Shape of an Object
A force applied on an object may change its shape.
A Force can Change the Direction of a Motion of an Object
A force applied on an object may change its direction of a motion of an object
MUSCULAR FORCE
CONTACT FORCES
FORCE OF FRICTION
A force that can be applied only
when it is in contact with an
object is called a contact force.
FORCES MAGNETIC FORCE
NON-CONTACT ELECTROSTATIC
FORCES FORCE
A force that can be applied
without any contact between
two objects is called Non-contact
GRAVITATIONAL
force. FORCE
CONTACT FORCES: - Muscular force

The force resulting due to the action of muscles is known as the
muscular force.
We use muscular force for our various activities and animals
also.
CONTACT FORCES: - Force of friction

The force of friction always acts on all the moving objects and its
direction is always opposite to the direction of motion.
Force of friction is the force which opposes the motion of an object
over a surface.
Non-Contact forces: - Magnetic force
The force exerted by a magnet to pull/push a metallic object is

called magnetic force.
Non-Contact forces: - Electrostatic force
The force exerted by a charged body on another charged or

uncharged body is known as electrostatic force.
Non-Contact forces: - Gravitational force
The force exerted by the earth to pull the objects towards itself
is called as Gravitational force.
8th class Physical Science Ch-1: Force and Pressure
Pressure
Pushing a nail A porter carrying a Cutting vegetables with a

into a wooden heavy load blunt knife and then with a
plank sharp knife.
Pressure: The force acting on

a unit area of a surface is
called pressure.
𝟏
P (If same force is applied)
𝑨
P F (If same area)
 The smaller the area, larger the pressure on a surface for the
same force.
 Pressure depends on Force and Area.
 S.I unit of Pressure is N/m2 (or) Pascal
Increasing the Pressure by Reducing the Area
1 2 3
1. A Sharp knife has a very small surface area on its cutting edge, so that high pressure can be
exerted to cut the vegetables.
2. The high pressure on surface of the ice, so that the ice melts and allowing the ice skater to glide
smoothly.
3. The ends of the nails and pins are made sharp in order to minimize the area which in turn
increases the pressure.
Reducing the Pressure by Increasing the Area
The lorry have broad tyres, so there is less pressure

on ground and do not sinking to comparatively soft
field.
School bags have wide straps so the weight of bag may fall
over a large area of the shoulder of the students producing
less pressure on shoulder.
Pressure Exerted by Liquids
Pressure Exerted by Liquids
A liquid exerts pressure on the walls of the container
Liquids exert equal pressure at the same depth
Liquids pressure on the bottom of

the container does not depend on
the area of the bottom. It act
perpendicular to the base.
Liquids exert pressure on the walls
of the container.
Liquids exert equal pressure at the
same depth.
Liquid pressure increases with
depth.
 Gases exert pressure on the walls

of the container.
 Air exerts pressure in all direction.
 Air pressure arises due to the
constant collision of the tiny
molecules of the gases present in
the air with the walls of the
container in which it is enclosed.
The pressure exerted by this air is known as atmospheric pressure.
A unit area and a very

long cylinder standing on
it filled with air, then the
force of gravity on the air
in this cylinder is the
atmospheric pressure
When you press the

sucker, most of the air
between its cup and the
surface escapes out. The
sucker sticks to the
surface because the
pressure of atmosphere
acts on it. To pull the
sucker off the surface, the
applied force should be
A rubber sucker pressed on large enough to overcome
a surface the atmospheric pressure.
If the area of my head were 15 cm × 15 cm, how much force air will exert on my head?
Area of the head(A) = 15cm x 15cm = 225 cm² = 225 x 10⁻⁴ m²
Atmospheric pressure is (Average) (P) = 1.01 x 10⁵ N/m² or Pa
According to the formula, F = A x P

F = 225 x 10⁻⁴ x 1.01 x 10⁵
= 2279.8125 N
= 227.25 kg
Weight(force) of the air on the head (W) = 227.25 kg
M.Srinivasa Rao, SA(PS)

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M.

Identifying Actions as Push or Pull or both

Opening or shutting a door

A football player taking a penalty kick

A cricket ball hit by a batsman

Erasing black board

From above examples, A force is a push or a pull or both

Combination of push and pull

If a man stands behind a car, the car does not move.

If a man pushes the car, he applies force and the car

1. Both the girls appear to push each other .

2. Both the girls appear to pull each other.

3. The cow and the man appear to pull each other.

At least two objects must interact for a force to come

An interaction of one object with another object

Unbalanced forces acting in opposite direction combine by subtraction.

A Force can Change the State of Motion

Effects Change the Speed

A Force can Change the Direction of a

A force applied on an object may change its shape.

A force applied on an object may change its direction of a motion of an object

CONTACT FORCES: - Muscular force

CONTACT FORCES: - Force of friction

Non-Contact forces: - Magnetic force

The force exerted by a magnet to pull/push a metallic object is

Non-Contact forces: - Electrostatic force

The force exerted by a charged body on another charged or

Non-Contact forces: - Gravitational force

Pushing a nail A porter carrying a Cutting vegetables with a

Pressure: The force acting on

Increasing the Pressure by Reducing the Area

Reducing the Pressure by Increasing the Area

The lorry have broad tyres, so there is less pressure

Liquids exert equal pressure at the same depth

Liquids pressure on the bottom of

 Gases exert pressure on the walls

The pressure exerted by this air is known as atmospheric pressure.

A unit area and a very

When you press the

Area of the head(A) = 15cm x 15cm = 225 cm² = 225 x 10⁻⁴ m²

Atmospheric pressure is (Average) (P) = 1.01 x 10⁵ N/m² or Pa

According to the formula, F = A x P

Weight(force) of the air on the head (W) = 227.25 kg

M.Srinivasa Rao, SA(PS)

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