as per the icse syllabus

1. PHYSICS
CLASS: VIII
CHAPTER: force and pressure

2. INTRODUCTION

5. SOME FACTS ABOUT FORCE
 THE MASS OF THE BODY REMAINS UNEFFECTED ON APPLYING
THE FORCE .
 THE FORCE APPLIED ON A BODY CANNOT BE SEEN
ALTHOUGH WE CAN FEEL OR SEE THE EFFECTS CAUSED BY IT.
 FORCE HAS BOTH MAGNITUDE AND DIRECTION.
 FORCE IS REPRESENTED BY AN ARROW ( ) WHOSE
LENGTH GIVES THE MAGNITUDE AND OF THE FORCE AND
ARROW SHOWS THE DIRECTION.

6. EFFECTS OF FORCE

8. TURNING EFFECT
OF FORCE

9. TURNING EFFECT OF FORCE
When we paddle a pivoted wheel, it only
rotate about its axis but does not move in a
straight line.
Turning effect can be observed when a force is
applied on an object which is not free to move
in a straight line, but is pivoted at a point about
which it can turn.

10. TURNING EFFECT OF FORCE
Turning effect of force is called moment of force.
The turning effect of force around a fixed point or fixed axis is called
moment.
A point around which the rotation occur is called a pivot.
Example: a. a door opening around a fixed hinge.
Moment of force is determined by a product of force and distance arm.
M = F x d
 The SI unit of moment is newton metre
( Nm)

11. Factors affecting turning effect of force
Magnitude of applied force:
Larger the magnitude of applied force larger is
the turning effect.
Perpendicular distance between force
and the pivoted point:
Larger the perpendicular distance larger is the
turning effect.

12. Examples of turning force
Handles are provided near
the free end of the door.
Reason: larger the
perpendicular distance from
the pivot point, greater is the
Turning effect.
Driver applies force near
the rim of the steering
wheel.
Reason: larger the
perpendicular distance
from the pivot point,
greater is the Turning
effect.
Spanner and wrench used
to tighten or loosen a nut
and bolt has large handle.
Reason : larger the
perpendicular distance
from the pivot point,
greater is the turning effect
Children sitting on see
saw’s edges moves up and
down easily.
Reason : larger the
perpendicular distance
from the pivot point,
greater is the turning effect

13. TYPES OF MOMENT OF FORCE
When a force causes an object to
turn in clockwise direction.
When a force causes an
object to turn in
anticlockwise direction
Example:
Clockwise moment
Anticlockwise
moment

14. PRESSURE

15. HAVE YOU EVER WONDERED….
Why a camel can run and walk in the
sand easily?
Why the area of the tyres in
heavy vehicles is kept broad?
Why is it easy to fix a sharp pin on a
board?
Observation:
Effect of force of the same magnitude on different area is different.
Lesser the area, more the effect of force.
Thus,
The physical quantity which explains the dependence of force on area is the PRESSURE.
 Pressure decreases when contact area increases
 Pressure increases when contact area decreases

16. PRESSURE
THE FORCE ACTING ON A UNIT SURFACE AREA OF AN OBJECT IS PRESSURE
Pressure = force
area
The SI unit of force is newton(N)
The SI unit of area is metre square (m2)
The SI unit of pressure = F/A = N/ m2 = PASCALS = ‘Pa’
BLAISE PASCAL

17. ROLE OF PRESSURE IN DAY TO DAY LIFE

18. LIQUID PRESSURE

19. Do you know…….
Why are the dams constructed on river
thicker at the bottom?
Why do deep sea divers wear diving
suit?
Why does the water flows at the
faster rate from the tap located on
the ground floor than the taps
located on the first floor?
First
floor
Ground
floor
Lets understand the reason for
these….

20.  When a person swims under water :
 Water exerts pressure on his body from all the sides.
 The liquid pressure increases with the increase in depth
Thus ,
 The divers wear the special dress to protect themselves from getting crushed.
 The dams are thicker at the bottom as the pressure increases with depth and withstand the pressure .
 Since the pressure decreases with the increase in height therefore water from the taps located on the
on the first floor flows at a slower rate.

21. Liquid pressure
 Liquid exert pressure on the surface of the body immersed in it and the container in which it is
placed.
DERIVATION:
Let liquid mass be m
Density = ρ (rho)
Be taken in a cylindrical container
Area of the base of the container = A
Height of the liquid level in the
container from bottom= h
Volume occupied by the liquid = v

22. The liquid exerts a force equal to its weight towards the bottom of the container.
The weight of the liquid in the container is
W = m x g
Where g is acceleration due to gravity
Mass = density x volume
So,
W = ρ x A x h x g
Hence, the force exerted by liquid is
F = W = ρ x A x h x g
We know that , pressure
p = F/ A
So ,
p = ρ x A x h x g = h ρg = h ρ
Since acceleration due to gravity is constant at a given place , the pressure exerted by a liquid depends on the
height of the liquid and density of a liquid
A

23. Properties of liquid pressure
 Pressure exerted by a liquid increases with increase in depth from the liquid surface
 Pressure exerted by a liquid at the same depth will be the same.
 Liquid exerts pressure in all directions.

24. Measuring liquid pressure
(manometer)
o A manometer is an instrument used to measure pressure in
fluids.
Description:
o It consists of a U-shaped tube containing a liquid like water.
o One of the end of the tube is left open and another is
connected to a rubber tube which is immersed in the trough
containing another liquid.
Working:
 The level of the water in both the arms of the U –tube is same.
 The liquid pressure forces the liquid level in the rubber tube
immersed in other liquid to go down.
 Liquid level in another arm goes up.
 If h is the difference in both the arms then liquid pressure p = h ρ g

25. ARCHIMEDES’
PRINCIPLE
https://www.youtube.com/watch?v=05WkCPORlj4

26. ARCHIMEDES’ PRINCIPLE
Archimedes principle states that the apparent loss of weight of a body fully or partially immersed in a
liquid is equal to the weight of the liquid displaced by the body.
The weight of the body in liquid acts downwards (w).
An upward force called upthrust.
This upthrust on a body immersed in a liquid is called buoyancy or
buoyant force (Wb).
The apparent weight of the body is the difference between the two
forces. Wapparent = W - Wb
Floatation:
When the upthrust = the weight , the object floats below the surface.
When upthrust > weight of the object , floats on the surface
When upthrust < weight of the object , the object sinks.

27. Applications of Archimedes’ principle
ships submarine hydrometer
The weight of the water displaced by
the immersed part of the ship is equal
to the weight of the ship itself
The water is filled in its ballast
tank. If weight increases it sinks
and by letting the water out it
floats.
It helps to measure the density of
liquids. Hydrometer floats in
liquid due to liquid displaced .

28. PRESSURE
EXERTED DUE TO
GAS