EXPLORING FORCES: Detailed Notes 8TH CLASS SCIENCE CURIOSITY NOTES By : K Sandeep Swamy(M.Sc,B.Ed) For Online Classes (IITJEE & NEET foundation and academics) contact : 9491878325 Subscribe Samyans Eduhub youtube channel A force is fundamentally described as a push or pull applied on an object. Forces result from the interaction between two or more objects. At least two objects must interact for a force to come into play. The SI unit of force is newton (N). What is a Force? Definition A force is fundamentally described as a push or pull applied on an object. Interaction Forces result from the interaction between two or more objects. At least two objects must interact for a force to come into play. Example When you push a table, your hand and the table are interacting. When this interaction ceases, the force is no longer experienced. Measurement The SI unit of force is newton (N). What Can a Force Do to Objects? Forces have several observable effects on objects they are applied to: Make an object move from rest Change the speed of a moving object. This includes stopping or decreasing speed Change the direction of motion of an object Bring about a change in the shape of an object A force can cause some or all of these effects simultaneously. None of these changes (speed, direction, shape) can occur without the action of a force. If an object is at rest, it means the forces acting on it are balancing one another. Types of Forces Forces can be broadly categorised into two main types: contact forces and non-contact forces. Contact Forces These forces act only when there is physical contact between the objects. This contact can be direct (e.g., using hands) or indirect (e.g., using a stick or rope). Non-Contact Forces These forces can be experienced even if the objects are not in physical contact. Examples include magnetic force, electrostatic force, and gravitational force. Contact Forces Muscular Force This force is caused by the action of muscles in our body It is used in everyday physical activities like walking, running, lifting, pushing, jumping, or stretching Muscular force also plays a crucial role in internal bodily functions, such as chewing food, pushing it through the alimentary canal, and the expansion and contraction of heart muscles for blood circulation Historically, humans have used the muscular force of animals for various tasks Friction Friction is the force that comes into play when an object moves or tries to move over another surface It always acts in a direction opposite to the direction in which the object is moving or trying to move. For example, a rolling ball stops due to friction Friction arises due to the irregularities in the two surfaces in contact. Even seemingly smooth surfaces have minute irregularities that lock into each other, opposing motion The force of friction depends upon the nature of the surfaces in contact. It is greater on rough surfaces Friction also acts on objects

1. EXPLORING FORCES: Detailed Notes
8TH CLASS SCIENCE CURIOSITY NOTES
By : K Sandeep Swamy(M.Sc,B.Ed)
For Online Classes (IITJEE & NEET foundation and academics) contact : 9491878325
Subscribe Samyans Eduhub youtube channel
A force is fundamentally described as a push or pull applied on an object. Forces result from the interaction between two
or more objects. At least two objects must interact for a force to come into play. The SI unit of force is newton (N).

2. What is a Force?
Definition
A force is fundamentally described as a push or pull
applied on an object.
Interaction
Forces result from the interaction between two or
more objects. At least two objects must interact for a
force to come into play.
Example
When you push a table, your hand and the table are
interacting. When this interaction ceases, the force is
no longer experienced.
Measurement
The SI unit of force is newton (N).

3. What Can a Force Do to Objects?
Forces have several observable effects on objects they
are applied to:
Make an object move from rest
Change the speed of a moving object. This includes
stopping or decreasing speed
Change the direction of motion of an object
Bring about a change in the shape of an object
A force can cause some or all of these effects
simultaneously. None of these changes (speed, direction,
shape) can occur without the action of a force. If an
object is at rest, it means the forces acting on it are
balancing one another.

4. Types of Forces
Forces can be broadly categorised into two main types: contact forces and non-contact forces.
Contact Forces
These forces act only when there is physical contact
between the objects. This contact can be direct (e.g.,
using hands) or indirect (e.g., using a stick or rope).
Non-Contact Forces
These forces can be experienced even if the objects
are not in physical contact. Examples include magnetic
force, electrostatic force, and gravitational force.

5. Contact Forces
1
Muscular Force
This force is caused by the action of muscles in
our body
It is used in everyday physical activities like
walking, running, lifting, pushing, jumping, or
stretching
Muscular force also plays a crucial role in internal
bodily functions, such as chewing food, pushing it
through the alimentary canal, and the expansion
and contraction of heart muscles for blood
circulation
Historically, humans have used the muscular force
of animals for various tasks
2
Friction
Friction is the force that comes into play when an
object moves or tries to move over another
surface
It always acts in a direction opposite to the
direction in which the object is moving or trying
to move. For example, a rolling ball stops due to
friction
Friction arises due to the irregularities in the two
surfaces in contact. Even seemingly smooth
surfaces have minute irregularities that lock into
each other, opposing motion
The force of friction depends upon the nature of
the surfaces in contact. It is greater on rough
surfaces
Friction also acts on objects moving through
liquids and gases (e.g., air resistance, water
resistance). Objects like aeroplanes and ships are
designed with specific shapes to reduce this force

6. Non-Contact Forces
Magnetic Force
This is the force exerted by a
magnet on another magnet or
a magnetic material
Like poles (North3North,
South3South) repel each
other, while unlike poles
(North3South) attract each
other
A magnet can exert this force
from a distance, making it a
non-contact force
Electrostatic Force
This force is exerted by a
charged body on another
charged body or an
uncharged body
When certain materials are
rubbed together, electrical
static charges build up on
their surfaces
A charged object can attract
uncharged objects (e.g., a
rubbed plastic scale attracting
small paper pieces)
Similar (like) charges repel
each other, while opposite
(unlike) charges attract each
other
This force acts even when
bodies are not in contact,
making it a non-contact force
Gravitational Force
This is the force with which
the Earth attracts (pulls)
objects towards itself
It causes objects to fall
downwards
Gravitational force is always
an attractive force
It acts without physical
contact with the object it
attracts, therefore it is a non-
contact force
Under the influence of
gravitational force, an object
undergoes vertical motion:
when thrown upwards, it
slows down, stops
momentarily, and then falls
straight down; when dropped,
it takes a straight vertical path
downwards

7. Gravitational Force and Weight
Weight is defined as the force with which the Earth pulls
an object towards itself. It measures how strongly an
object is pulled by the Earth.
Since weight is a force, its SI unit is also newton (N).
The Earth pulls different objects with different forces,
meaning different objects have different weights.
1
Measurement of Weight
Weight can be measured using a spring balance
A spring balance consists of a spring fixed at one end and a hook at the other. When an object is hung from the
hook, the spring stretches
The amount of stretching indicates the weight of the object, which is read from a scale marked in newtons
Spring balances have a range (e.g., 0 to 10 N) and a smallest measurable value (e.g., 0.2 N for a 10 N balance
with 5 divisions per newton)

8. Difference between Weight and Mass
Mass Weight
The amount of matter in an object The force with which the Earth pulls an object
towards itself
Measured in grams (g) or kilograms (kg) Measured in newtons (N)
The value of mass remains the same at every place Can vary slightly from place to place on Earth and
significantly on different planets because gravitational
force differs
1 kg mass is always 1 kg everywhere 1 kg mass weighs 10 N on Earth, but only 1.6 N on the
Moon
While everyday language often uses "weight" (e.g., "10 kg of wheat") when referring to mass, in scientific use, it is
crucial to use the correct terms and units.

9. Floating and Sinking
When an object is immersed in a liquid, the liquid applies an upward force on it, known as upthrust or buoyant force.
Archimedes' Principle states that this upward force is equal to the weight of the liquid displaced by the object.
Sinking
An object sinks if its gravitational force (weight) is
more than the buoyant force. This occurs if the weight
of the liquid displaced is smaller than the weight of the
object.
Floating
An object floats if its gravitational force (weight) is
equal to or less than the buoyant force. This occurs if
the weight of the liquid displaced is equal to the weight
of the object.

10. Buoyant Force and Density
The buoyant force also depends on the density of the
liquid.
An example of a floating rock is Pumice, which is less
dense than water due to trapped air bubbles.
Objects immersed in water feel lighter because of the
upward buoyant force opposing their weight.
When you swim or float in water, you experience the buoyant force firsthand. This is why you feel lighter in
water than on land - the water is pushing you upward with a force nearly equal to your weight!