INTRODUCTION
OF FORCE- In our daily life we do a number of activities. These activities include
push and pull of anything. Lifting, picking, throwing, pushing, striking,
flicking, stopping, hoisting, shutting, opening, hitting, lowering, flying,
Sliding, smashing etc. activities include push or pull. A push or pull
acting on an object is known as force. Force is important to do every work. The
direction of force will be the direction of push or pull applied on the object.
The amount of force varies according to the type of work. The S.I. Unit of
Force is Newton (N). The unit of force was given in honour of Sir Isaac Newton.
FORCE –A PUSH OR
PULL- Lets understand force by a few more examples where push or pull is
applied.
Activity 11.1- To Identify the actions as
push or Pull Force
|
S.N. |
TYPE OF ACTION |
PUSH OR PULL FORCE |
|
1 |
Lifting a stone |
Pull |
|
2 |
Striking a coin of carom board |
Push |
|
3 |
Flicking a ball by hockey |
Push |
|
4 |
Opening the door of Car |
Pull |
|
5 |
Opening a drawer |
Pull |
So,
force of pull or push is needed in every kind of work.
 |
| Flicking the golf Ball (Push) |
 |
| Lifting the Car (Pull) |
FORCES ARE DUE TO AN INTERACTION- Suppose a man is standing behind a stationary car, the car will not move due to his presence. But, he pushes that car the car moves. It is due to applying a push force. Here, interaction between the car and man occurs.
Suppose a milkman is pulling
his cow by a rope toward himself then it is an example of pull force. Here,
interaction between the cow and the milkman occurs.
 |
| Pull Force is due to interaction Push |
EXPLORING FORCES-
Case I-When
two girls are pushing a big box in a particular direction, it becomes easier
for them to move the box. It is because both are applying the force in same
direction and their forces are added.
Case II-
When two girls are pushing the box from opposite side, the box will move in the
direction of comparatively greater magnitude. In this case lower force will be
deducted from greater force. So, deduction of forces occurs in this case.
The magnitude of force denotes its
strength. It is expressed as │5│N.
Here, magnitude of force in two vertical lines shows that strength is 5 Newton.
Direction of the force is also important.
In case I the total strength of force will
be F1 + F2 = F
In case II the total strength of force will
be F1 - F2 = F Where, F1 > F2
UNIT OF FORCE-The
S.I. Unit of Force is Newton (N). The smaller unit of force is dyne in CGS System. 105 dyne= 1 N
So, 1 dyne = 1 N /105 = 10 _5
N
We Know that, Force (F) = Mass of an Object
(m) x Acceleration (a)
Or, F = m. a
If we put the units of mass and
acceleration then the unit of force will be- Kg. m/s2. This in short
is called as 1 N (Newton).
A FORCE
CAN CHANGE STATE OF A MOTION- When a football player kicks a stopped
football, a force is applied to move in a particular direction. The state of
rest of football changed. When the goalkeeper stops the kicked football, a
force is applied. The state of motion of football changes to zero.
The speed of a moving vehicle
increases when a force is applied on it in the direction of its motion.
Similarly, the speed of moving vehicle decreases when the force is applied
opposite to the direction of motion. So, the state of motion either increases
or decreases due to applying the force.
A FORCE CAN CHANGE DIRECTION OF A MOTION-When
a goalkeeper dives to deflect the incoming ball, its direction changes. The
direction of a ball changes when it strikes on a wall with an angle. The change
in direction is opposite and with same angle.
A FORCE
CAN CHANGE SHAPE OF AN OBJECT-Pressing down a lump of dough on a plate,
pressing the spring, pulling an elastic rubber etc are examples of change in
shape of objects due to force.
 |
| Change in shape and size of elastic rubber band |
A force may bring following changes-
1. make an object move from
rest.
2. change the speed of a moving
object.
3. change the direction of a
moving object.
4. change the shape of an
object.
5. change some or all of the
above effects together.
The application of force to
bring change in shape, size, position or state of the object is not always
applicable. e.g.- Even if you apply maximum on a wall, it will not move.
TYPES
OF FORCES-Some forces act on an object by other object when they are in
contact with each other. Some forces act on an object by other object when they
are not in contact with each other. On this basis there are two types of
forces. Contact forces and non-contact forces.
A. CONTACT FORCES- Those
forces which develop between two objects in contact are called contact forces.
e.g.-
1. Muscular Force-The force
resulting due to the action of muscles with an object is known as the muscular
force.
e. g.-The drought animals like
oxen, horses, donkeys, camel, Yak etc. are used to perform a number of tasks
like carrying, ploughing etc by applying muscular force.
 |
| Muscular Force of Animals and Humans |
e. g.- The inhalation and exhalation of air from our
lungs is due to movement of muscles. etc.
2. Frictional Force- The
force which arises between two surfaces in contact is known as frictional
force. Out of two at least one object should be in motion. If both the surfaces
in contact are in motion then also friction will be there. For rough surface frictional force is greater than smooth surface.
The force of friction always
develops in between moving surfaces in contact. The direction of force is
opposite to the direction of motion.
 |
| Rough surface produces more friction than the smooth surface |
e. g. Stopping of a moving ball
on the ground by its own without any external force.
e.g. Stopping of a boat after
some time without sailing in still
water.
e.g.- Burning of matchsticks on
rough surface of match box. etc.
The frictional force produces
heat. It is useful as well as a harmful force. When we walk, our energy is
consumed more due to friction. But, friction helps us to not skid. A
football, cricket, tennis, sprinter like
players wear spiked shoes to increase his/her friction so that skidding during
running can be prevented.
B. NON-CONTACT FORCES- Those
forces which develop between two objects without any contact are called
non-contact forces. e.g.-
1.
Gravitation Force- The attractive force which develops between the earth
and any other object with or without contact on it is called gravitational
force or force of gravity or simply gravity. Isaac Newton has discovered this force.
All the objects have their own gravitational force. The gravity depends on the
mass of objects and distance between them. The weight of an object varies from
the Earth to the Moon. It is because of change in gravity.
 |
| Earth Attracts all objects towards it. |
2. Magnetic Force- The fore
exerted by a magnet on another magnet and magnetic substance is called magnetic
force. Similar poles repel each other while dissimilar poles attract each
other.
e. g. A magnetic force of
attraction is found between dissimilar poles of two magnets.
|
N |
|
S |
|
N |
|
S |
N – N = Repulsion
S – S =
Repulsion
N-S =
Attraction
e.g. A magnetic force of
attraction is found between a magnet and iron.
Loadstone is a natural mineral
that behaves like a magnet. Iron, cobalt and Nickel and their alloys are
ferromagnetic. They are attracted towards the magnets.
3. Electrostatic Force- The
force exerted by a charged body on another charged or uncharged body is called
electrostatic force.
 |
| Electrostatic force of attraction |
Electrostatic forces may be
attractive or repulsive. Similar charges attract each other and dissimilar
charges repel each other. Electroscope device is used to know the nature of
charge present on anybody. Electrostatic force is also called as Coulomb force
or Coulomb interaction.
PRESSURE- The force acting on a unit
area of a surface is called pressure.
Pressure= Force/Area on which it acts
Unit of Pressure- Newton (N)/m2
or N.m-2 Newton per square meter) = 1 Pascal
Pascal is S.I. derived unit. One Pascal is defined as one newton per
square meter. Pascal was named after Scientist Blaise Pascal.
e.g. When an iron nail is
hammered with its pointed end into a wooden plank, it gets inserted easily.
But, it is very difficult to insert that iron nail into wooden plank with its
head. It is because the surface area of head is more than the pointed end. More
is the force, more will be the pressure.
e.g. The porters place a round piece of cloth on their heads to
increase the surface area so that the pressure of luggage on a particular point
on head can be reduced and distributed to comparatively bigger area. It helps
the porters to carry luggage with better comfort due to reduced pressure.
PRESSURE
EXERTED BY LIQUIDS AND GASES- When any liquid is filled in a container,
the liquid puts pressure on its walls.
e.g. A rubber sheet is fixed at
one of the end of a transparent glass tube or plastic pipe. The pipe is kept in
vertical position. The rubber sheet bulges out on pouring water. The bulging is
more on pouring more water. So, it confirms that liquids exert pressure on the
walls of their containers.
e.g. The fountains of leaked
water in supply water pipes is because water is supplied with force. If any
leakage or hole is there, water comes out with pressure.
ATMOSPHERIC
PRESSURE- The pressure exerted by air is known as atmospheric pressure.
Atmospheric pressure is very large which is counter balanced by our internal
body pressure.
Let us imagine a unit area over
our head which is filled with air in a very long cylindrical air column. If the
area over our head is 15 c.m. X 15 c.m then an atmospheric pressure of 2250 N
or 225 kg will be there.
e.g.- The rubber sucker pressed
on a plane glass is very difficult to remove because the air inside it is
almost removed after pressing. The outer air puts atmospheric pressure.
 |
| Partial vacuum and low air -pressure inside and high atmospheric pressure outside |
SOLVED EXERCISE QUESTIONS
Q.1 Give two
examples each of the situations in which you push or pull to change the state of
motion of objects.
Ans- Examples of push to change state of motion-
1. Kicking a football
2.
Opening of doors
Examples
of push to change state of motion-
1. Opening a drawer
2. Drawing a bucket of water from a well
Q.2 Give two examples each of the situations in
which applied force causes a change in the shape of an object.
Ans- 1.The shape of seat changes on sitting.
2. The shape of lump of dough changes on pressing
Q.3 Fill in the blanks in the following statements.
(a) To draw water from a well we have to ………………………
at the rope.
(b) A charged body ………………………an uncharged body
towards it.
(c) To move a loaded trolley we have to …………………..
it.
(d) The north pole of a magnet ……………………….the north
pole of another magnet.
Ans- (a) pull (b) attracts (c) push or pull (d) repels
Q.4 An archer
stretches her bow while taking aim at the target. She then releases the arrow,
which begins to move towards the target. Based on these information fill up the
gaps in the following statements using the following terms.
muscular,
contact, non-contact, gravity, friction, shape, attraction
(a) To stretch
the bow, the archer applies a force that causes a change in its ………………
(b) The force
applied by the archer to stretch the bow is an example of ……….. force.
(c) The type of
force responsible for a change in the state of motion of the arrow is an
example of a …………….. force.
(d) While the
arrow moves towards its target, the forces acting on it are due to ………….
and that due to ………………of air.
Ans- (a) shape (b)
muscular (c) contact (d)
gravitational force (gravity) , friction
Q. 5 In the
following situations identify the agent exerting the force and the object on
which it acts. State the effect of the force in each case.
(a) Squeezing a
piece of lemon between the fingers to extract its juice.
(b) Taking out
paste from a toothpaste tube.
(c) A load
suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete
making a high jump to clear the bar at a certain height.
Ans-
|
Agent
exerting the force |
Object
on which it acts |
Effects
of the force |
|
Fingers |
Lemon |
Change in shape of lemon and juice comes out |
|
Fingers |
Toothpaste tube |
Change in shape of toothpaste
tube and paste comes out |
|
Load |
Spring |
Stretching of Spring |
|
Athlete’s muscles |
Pole |
Change in athlete’s state of
motion |
Q.6
A blacksmith hammers a hot piece of iron while making a tool. How does the
force due to hammering affect the piece of iron?
Ans- The muscular force of the blacksmith changes the shape of the
piece of iron.
Q.7
An inflated balloon was pressed against a wall after it has been rubbed with a
piece of synthetic cloth. It was found that the balloon sticks to the wall.
What force might be responsible for the attraction between the balloon and the
wall?
Ans- An electrostatic charge develops on inflated balloon due to
rubbing by synthetic cloth. The force between charged inflated balloon and the
wall is due to electrostatic force.
Q.8 Name the forces acting on a
plastic bucket containing water held above ground level in your hand. Discuss
why the forces acting on the bucket do not bring a change in its state of
motion?
Ans- In this case, two forces namely ‘muscular force’ and ‘gravitational force’ acting on the plastic bucket containing water and held above ground by the hand. Muscular force is acting upward and gravitational force is acting downward. So, due to equal and opposite forces there is no change in the state of motion of the bucket.
Q.9 A rocket has been fired upwards
to launch a satellite in its orbit. Name the two forces acting on the rocket
immediately after leaving the launching pad.
 |
| Gravity and Drag on Flying Rocket |
Ans- Two Forces acting on the rocket are-
1. Gravitational force
2. Atmospheric friction (drag)
Q.10
When we press the bulb of a dropper with its nozzle kept in water, air in the
dropper is seen to escape in the form of bubbles. Once we release the pressure
on the bulb, water gets filled in the dropper. The rise of water in the dropper
is due to
(a)
Pressure of water
(b)
Gravity of the earth
(c)
Shape of rubber bulb
(d)
Atmospheric pressure
Ans- (d) Atmospheric pressure
Thank you sir
ReplyDelete