Force is a push or pull that can cause an object to be in motion or at rest by overcoming inertia. Forces act due to an interaction between two objects. The net force on an object is the sum of all forces acting on it and depends on both the magnitude and direction of individual forces. Pressure is the amount of force applied per unit area. Liquids and gases exert pressure due to the weight of the fluid above. Atmospheric pressure decreases with increasing altitude and temperature and decreasing humidity.

2. CHAPTER -11SESSION BY:MS. PREETI KHOSLA

3. CONTENTS

4. WHAT IS
FORCE????

5. FORCE
Force is the push or pull on an object.
Eg:- If an object has to be moved, it has to be pushed or pulled.
The SI unit of force is NEWTON (N).

6. CAUSE OF A FORCE
• Whenever a force is applied, there is interaction between
two bodies.
• In other words we can say that an interaction between at
least two bodies is necessary for application of a force.

7. FORCES ARE DUE TO INTERACTION :-
An interaction of one object on another object results in a force
between the two objects.
Eg :- If a man stands behind a car, the car does not move. If he
pushes the car, he applies force and the car begins to move in the
direction of the applied force.

8. FORCE DEPENDS ON:-
• When two or more forces are involved, the
strength of the force (net force) depends on
the direction of each of the forces as well as
the its magnitude.
• When forces are applied in same direction,
they get add together whereas when the
forces are in opposite direction, the net force
is difference of the forces.

9. RESULTANT (OR NET) FORCE
Case: 1

10. Case : 2

11. Case:3

12. EXPLORING FORCES :-
a) If two forces act on a body in the same direction, the net force is
the sum of the two forces.
F1
F2
Net force = F1 + F2
b) If two forces act on a body in opposite directions, the net force is
the difference between the two forces.
F1 F2
Net force = F2 – F1
The effect on the object depends on the net force acting on it.

13. EFFECTS OF FORCE ON AN OBJECT :-
i) Force can make an object to move from rest.
ii) Force can stop a moving object.
iii) Force can change the speed of an object if it is
moving.
iv) Force can change the direction of a motion of an
object.
v) Force can change the shape of an object.
Force may cause one or more of these effects.

14. WHY?????

15. Let’s Test our self!!!!!!
Q1. Which among the following can apply force?
Lion, Human, Wall, Water
A. Only lion and Human
B. Only lion, Human and Wall
C. Only lion, Human and Water
D. All

16. Q2. The picture shows a goalkeeper trying to
stop the goal. Which type of force is he
applying on the ball? (Assume that he does
not move his hand backwards).
A. Push force
B. Pull force
C. None of the above

17. Q3. Hitting a ball with a bat is an example of
push type of force.
True or False ?

18. Q4. In a game of tug of war, the fat person in
team B can apply 50 units of force. Three
people in team A can apply 10, 20 , 15 units of
force respectively. Which team is going to
win?
A. Team A
B. Team B
C. It’s a tie

19. Q5. Block A is pushed by two guys with 500 units
force each and Block B is pushed by a single
guy with 20 units force as shown in fig.
Which block experiences greater net force?
A. Block A
B. Block B
C. Both experiences equal force

21. Contact forces :- are forces exerted by one object on another when they
are in contact with each other.
Contact forces are of two types. They are :-
i) Muscular force : exerted by muscles of the body.
ii) Force of friction : slows things down or prevents things from moving. It
always opposes the motion.
Non contact forces :- are forces exerted by one object on another
without any contact with each other.
Non contact forces are of three types. They are :-
i) Magnetic force : exerted by magnet on certain metals like iron.
ii) Gravitational force : the force with which the earth pulls everything towards
itself.
iii) Electrostatic force : the force exerted by electrostatic charge (positive and
negative).

22. CONTACT FORCES :-
i) Muscular force :-
Muscular force is the force exerted by the muscles of our body.
We use muscular force for our various activities.
Animals like bullocks, horses, camels, donkeys, elephants also
use muscular force to perform various tasks.

23. ii) Force of friction :-
Force of friction is the force which opposes the motion of an object
over a surface.
Eg :- A ball rolling on ground gradually slows down and comes to
rest due to force of friction.
If we stop pedalling a bicycle, it gradually slows down and
comes to a stop due to force of friction.

24. NON CONTACT FORCES :-
i) Magnetic force :-
Magnetic force is the force exerted by magnets.
Eg :- Force of attraction and repulsion between two magnets.
Force exerted by a magnet on a piece of iron or steel.

25. ii) Gravitational force :-
Gravitational force is the force exerted by the earth on all objects. It pulls all
objects towards the earth.
Eg :- A coin or pen falls down when it slips off your hand.
A ball thrown upward falls down .
Leaves and fruits fall to the ground when they get detached
from the plant.
Water in rivers flow downwards.

26. iii) Electrostatic force :-
Electrostatic force is the force exerted by a charged body on another charged
or uncharged body.
Eg :- A plastic straw charged by rubbing with paper attracts a suspended
plastic straw.
A plastic straw charged by rubbing with paper repels a suspended plastic
charged by rubbing with paper.

27. Let’s Test our self!!!!!!
Q1. A child slides down in a park as shown.
Which force makes the child to slide?
A. Muscular force
B. Frictional force
C. Gravitational force
D. Magnetic force

28. Q2. An apple tree is shown on the right . In
which of the following apples is gravitational
force of earth acting?
I. Apples on the ground
II. Apple falling towards the ground
A. Only I
B. Only II
C. Both I & II
D. Neither I nor II

29. Q3. In the picture shown , a thief is carrying a metallic
bag filled with metal coins and a policeman is trying to
get back the bag with the help of a magnet. What all
forces are acting on the bag at that moment?
A. Only magnetic force
B. Only magnetic force and
gravitational force
C. Magnetic force, muscular force of
thief and gravitational force
D. Magnetic force, muscular force
of thief, muscular force of
Policeman and gravitational force

30. WHAT IS
PRESSURE???

31. Unit: Pascal (Pa) after the French scientist Blaise Pascal.
- The greater the force ,the greater is the pressure.
- The greater the area, the lesser is the
pressure.

33. Numerical
Q: Calculate the pressure exerted by a brick,
which applies a force of 2.5 N, when
(a) It is placed on its widest base.
(b)It is placed upright on the soil .
The length, breadth and height of the brick are
25cm, 10cm & 5 cm respectively.
25cm
10cm
5cm
10cm5cm
25cm

34. A. When the brick is placed on its widest base:
Area in contact with soil = (L X B) = 25cmX 10cm
(1m = 100 cm so 1cm = 1/100m)
= 25 m X 10 m = 0.025m2
100 100
Force = 2.5 N
Pressure exerted = F = 2.5 N
A 0.025m2
=100 N/m2 or Pa
Solution
25cm
10cm

35. B. When the brick is placed upright:
Area in contact with soil = (L X B)= 5cmX 10cm
= 5 m X 10 m = 0.005m2
100 100
Force = 2.5 N
Pressure exerted = F = 2.5 N
A 0.005m2
=500 N/m2 or Pa
5cm 10cm

37. tyres

38. Let’s Test our self!!!!!!
Q1. In which of the following case is the child exerting
least pressure on the ground?

39. Q2. A cockroach and a spider are crawling on a
desert. Both the insects have same weight
and same size legs. Looking at the number of
legs they have, which insect will leave deeper
footprints after crawling on the desert sand?
A. Cockroach
B. Spider
C. Both leave footprints of same depth.

40. DO LIQUIDS ALSO EXERT
PRESSURE?????

41. PRESSURE EXERTED BY LIQUIDS :-
 Liquids exert pressure on the walls of a container.
 Liquids exert pressure on the bottom of a container. It depends upon the
height of the liquid column.
 Liquids exert sideways pressure.
 Liquids exert equal pressure at the same height.
 Liquid exert pressure which increases with the depth.
Rubber balloon

42. 1. A liquid exerts pressure on the
walls of the container.

43. 2. Liquid exert pressure on the bottom of
its container which depends on the height
of its column

44. 3. Liquids exert equal pressure at the
same height.

45. 4. Water pressure increases with
depth.

46. MEASURING LIQUID PRESSURE
• An instrument used to measure liquid
pressure is called pressure gauge.
• The simplest form of pressure gauge is a
manometer, which measure pressure
difference.

47. Q1. Assume that four containers with different base
areas are filled with equal volumes of water. The
pressure at the base of the containers is
A. same for all the containers.
B. greatest in case of C.
C. greatest in case of A.
D. Can’t be said.
Let’s Test our self!!!!!!

48. ATMOSPHERIC PRESSURE
 Atmospheric pressure is the pressure of air around us.
 Atmospheric pressure is due to the weight of air.
 The weight of an air column of the height of the of the
atmosphere and area 10 cm x 10 cm is as large as 1000 kg.
WHY DO NOT WE GET CRUSHED
UNDER THIS PRESSURE?
REASON: The Pressure exerted by the fluids in our
body is equal and opposite to the atmospheric
pressure acting on our body and thus it cancels the
effect of the atmospheric pressure.
10 cm
10 cm
Atmospheric pressure is measured by barometer in mm of Hg.
Air column

49. AIR EXERT PRESSURE IN ALL
DIRECTIONS
The steam inside the
bottle has been condensed
by cold water, the bottle
now has very little air
inside it.
The atmospheric pressure
acting on the bottle from
outside is therefore much
larger than the pressure
exerted by the air inside it.
This caused to bottle to get
crushed.

50. FACTORS AFFECTING ATMOSPHERIC
PRESSURE
1. Altitude: As we go to higher altitudes, the air
becomes thinner and the atmospheric pressure
decreases.
2. Temperature: Air expands on heating and
becomes less dense. Therefore atmospheric
pressure decreases with increase in
temperature.
3. Humidity: Water vapor is lighter than air.
Therefore humid air is lighter than dry air. Hence
air pressure decreases as humidity increases.

51. APPLICATIONS OF AIR PRESSURE

54. Let’s Test our self!!!!!!
Q1. An inflated balloon is hanging from ceiling of
a room filled with air, as shown. What would
happen to the balloon if the air inside the
room is removed completely? (Assume no
leakage of air from balloon.)
A. The balloon would remain as it is.
B. The balloon would shrink.
C. The balloon would burst.

55. Q2. Consider the following statements
and identify the right ones.
i )Low temperature at poles cause air to
expand.
ii ) High temperatures along equator
cause air to contract.
a. i only
b. ii only
c. Both
d. None

56. Q3. Consider the following statements and
identify the right ones.
i) Air pressure decreases when air is dry.
ii) Air pressure at sea level is lower than at the
mountain top.
a. i only
b. ii only
c. Both
d. None