Class 8 CBSE ( NCERT ) PPT

1. Presented by:-
Anil Kumar Yadav

2.  Push: To move an object away from our body.
Example : closing a door.
Pull: To move an object towards our body.
Example : opening a door.

3.  Pulling
 Pushing
 Hitting
 Kicking
 Lifting
 Shutting
 Opening
 Flicking

4.  Force is a pull or push.
 A force cannot be seen but can feel and
observe the effects of the force.
Unit of force
 SI Unit of force is Newton (N).

8.  An interaction of one object on another
object results in a force between the two
objects.
 At least two objects must interact for a force
to come into play.
Example: if a man stands near a ice cream
cart, the cart does not move. If he pushes the
ice cream cart, he applies force and cart begins
to move in the direction of the applied force.

9. 

12.  1) if force acts on a body in the same
direction , the net force is the sum of the
two forces.

13.  if force acts on a body in the opposite
direction , the net force is the difference of
the two forces.

14.  The strength of a force is usually expressed
by magnitude.
 Force have a direction.
 Example: 100N in Right direction.

15.  State of motion means objects are at Rest(stop) or in
motion(move).
 The state of motion of an object is described by its speed
and the direction of motion .
 Example: Running in north direction.
 The state of rest is said to be the state of zero speed.
 An object may be at rest or in motion; both are its state of
motion.

16. Force can move an object.
Example : when player kicks on football it begins to
move.

17. Force can stop an object.
Example: when force is applied by goalkeeper
on football, it stops in his hand.

18. Force can change speed of an object.
1) If force is applied on the object is in the direction of motion, the
speed of an object increases.
Example: pushing a moving bicycle.
1) If force is applied on the object is in the direction opposite to
the direction of motion, the speed of an object decreases.
Example: when brakes are applied to a car its speed
decreases.

19.  Force can change the direction of an object.
Example: when force is applied by
batsman on ball its direction changes.

20.  Many a time it is common experience that the
application of force does not result in change
in motion.
 Example: Pushing a wall

21. Force may change the shape and size of an object.
Example : pressing spring, stretching rubber band.

23. Contact forces: A force that acts when two objects are
physically in contact with each other is known as a
Contact force.
Means if you have to apply a force on an object , our body
has to be in contact with the object. It may be direct or
indirect contact.
Example: pushing a trolley, Lifting of books.

24.  Contact forces are two types:
1) muscular force,
2) friction force
1) Muscular force: the force exerted by the muscles of our body
is called muscular force.
The human beings exert muscular force for performing the
various activities like walking, jumping, running, climbing,
lifting , pushing, pulling ,stretching, squeezing, twisting and
pressing the objects.
Animals also make use of muscular force to carry various tasks.
Example: pulling a cart by animals.

26.  Ball rolling along the ground gradually slows
down and finally come to Rest.
 When you stop pedalling a bicycle, it
gradually slows down and finally come to a
stop.
 When engine of car or bike switched off , it
gradually slows down and finally come to
rest.

27.  Friction force: Friction force refers to
the force generated by two surfaces that
contacts and slide against each other.
 The force of friction always acts on all the
moving objects and its direction is always
opposite to direction of motion.


28. A non-contact force is a force which acts on an
object without coming physically in contact with
it.
Examples: Magnetic Force , Gravitational forces

29.  Magnetic Force: It is the force attraction or
repulsion that arises between two magnet.
 Like poles of two magnets
repel each other and unlike
poles attract each other.
 A magnet can exert a force
on another magnet without
being in contact with it so it
is example of a non-contact
force.

30.  Electrostatic force: the force exerted by a
charged body on another charged or
uncharged body.
Example: Take a comb and brush it on a hair
3-4 times and then take it near some
small bits of paper. Do not touch the
comb with the paper. The bits of paper
would get attracted to the comb.


31.  Gravitational force: Any falling object is
attracted towards earth. This is because earth
pulls them. The pulling force is gravitational
force or force of gravity.
 This force is an attractive force.
 This force not only exist on
earth but also every object in
universe, exerts a force on
every other object.

32.  Describes how a force is spread over an area.
 Definition: The force acting on a unit area of a
surface is called pressure.
 Pressure=
Force
Area
 unit of pressure is Pascal (Pa).
 If area is smaller the pressure is more on the
surface for the same force.

33.  Increasing the pressure by reducing the area.
Examples:1) A sharp knife has a very small surface
area on its cutting edge so that high
pressure can be exerted to cut the
apple.
2) When a force is applied to the head
of a nail, the pressure will drive its
sharp end into a piece of wood easily.

34.  The pressure exerted by a liquid kept in
container, at any point , is known as hydrostatic
(liquid) pressure.
 The liquid pushes on every surface in contact
with it.
 Pressure exerted by water at the bottom of the
container depends on the height of its column.
 A liquid exerts pressure on the walls of the
container.
 Liquids exert equal pressure at the same depth.

35.  Gases (Air) also exert pressure in all
directions on the walls of their container.
Example: When you make a hole on a fully
inflated balloon, then does the balloon not
break, thereby releasing all the air from
inside.

36.  Atmosphere is envelop of air all around us.
 Definition : The pressure exerted by air is known as atmospheric
pressure.
Atmospheric pressure is the force
of gravity on air in a column of unit
area.
Air column
Force of gravity is equal to weight of air .

37.  The force due to air in a column of the height
of the atmosphere and area 15 cm × 15 cm
nearly equal to the force of gravity on an
object of mass 225 kg (2250N). The reason
we are not crushed under this force of gravity
is that the pressure inside our bodies
is also equal to the atmospheric
pressure and balances the pressure
from outside.

38.  At high altitudes where the pressure is lower,
breathing is difficult. Nose bleeding may
occur as results from the pressure difference
between the body and the external pressure.

39. Thank you