physics 4.....................................................

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Romesa’s notes for class matric
CHAPTER NO. 4

PHYSICS NOTES

CLASS MATRIC

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Q: define force.
Ans:
FORCE
Force is an agent which changes or tends to
change the state
of rest or of uniform motion of a body.
OR
Force acting on a body is equal to the product of the
mass and acceleration produced in the body.
• Force can accelerate or decelerate a
body.
• Force is a vector quantity.
FORMULA
F = ma
UNITS OF FORCE
(i) NEWTON (N) in S.I system
(ii) DYNE in C.G.S system
(iii) POUND (Lb) in BRITISH ENGINEERING SYSTEM (F.P.S)
Q: define newton.
NEWTON

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Newton is the unit of force and can be defined as:
"The amount of force that produces an acceleration of
1 m/s2 in a body of mass 1-kg is equal to 1 NEWTON."
1 N = 1 kg x 1m/s2
[ N = kg m/s2]
Q: state newton’s first law of motion.
Ans:
Newton’s First Law Of Motion
“Every body continues its state of
rest or of uniform motion unless it
is acted upon by some external
force which changes its state of
rest or of uniform motion”
EXAMPLE:
First law of motion consists of two parts:
PART NO 1
The first part states that a body at rest
remains at rest unless an external force act
upon it.
This part is in accordance with our
common experience for example, a book
lying on a table remains at rest unless it is
lifted or pushed by an external force.

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PART NO 2
Second part states that a body in motion
remains in motion with uniform velocity
unless an external force act upon it.
This part is not self-evident because a ball
pushed once does not continue its motion
forever. A little consideration however,
shows that there is an opposing force like
ground friction and air friction acting in this case. These frictional
forces are responsible to stop the ball. If we eliminate these
opposing forces, a body in motion will continue its motion forever.
Q: state and explain Newton’s second law of motion.
Ans:
NEWTON’S SECOND LAW OF MOTION.
It can be stated as
“When a force acts on an object it produces
an acceleration, which is directly proportion
to the amount of the force and inversely
proportional to the product of mass”
MATHEMATICALLY

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Combining (i) and (ii)
....

a = k (F)(1)
m
here
constant k=1
a= (1)(F)(1)
m
a=F
m
ma= F
we get,

EXPLANATION
The net force acting on a body is equal to the product of the
mass of body and the acceleration produced in it.
EXAMPLE

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A motorcycle is accelerating due to the
force in the engine the greater the force
in the engine the greater the
acceleration is produced whereas the
greater the mass is loaded on the bike /
motor cycle the lesser the acceleration is produced.
Q: state and explain Newton’s third law of motion.
Ans:
NEWTON’S THIRD LAW OF MOTION
It can be stated as
“To every action there is an equal and
opposite reaction”
This Photo by

EXAMPLES
1. Motion of rocket: fuel burns rapidly, exerts force
in downward direction and rocket moves upward
as a reaction.
2. Book lying on a table: weight of the book on
the surface is action and the force exerted
by the surface (R) is the reaction.
R = -W
3.Walking on a street
4.Motion of helicopter

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Q: define inertia.
Ans:
INERTIA
Inertia is the tendency of a body to resist a change in
its state.
EXAMPLE
• If a driver suddenly applies the brakes in a fastmoving car the passengers will move forward due
to inertia
• If a boy will jump from a fast-moving bus he will
also run in the direction of bus after the jump due
to inertia
Q: define mass.
Ans:

MASS

The quantity of matter in a body is called its mass.
• Mass is a scalar quantity.
• Mass of a moving body is m=F/a.
• Unit of mass in S.I system is KILOGRAM (kg)

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Q: define weight.
Ans:
WEIGHT
Weight is the force by which the earth attracts a body towards
its center.
• Weight is a vector quantity
• Weight of a body vary place to place and
become zero on the center of earth and far away
from the surface of earth
• Weight of a body is W = mg.
• Unit of weight in S.I system is NEWTON (N).
Q: define tension.
Ans:
TENSION
The force experienced by a string or a rope when it
is stretched is called tension
• It is denoted by (T)
• Its SI unit is newton (N)
Q: derive an expression for (tension) in a string and
acceleration produced in bodies when:
BOTH BODIES HANG VERTICALLY
Consideration:

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Consider two bodies of unequal masses m1 and
m2 connected by the ends of a string, which
passes over a frictionless pulley as shown in
the diagram.
If m1>m2, the body ‘A’ will move downward with
acceleration ‘a’ and the body ‘B’ will move up
with same acceleration. Here we have to find
the value of ‘a’ and tension ‘T’.

FORCES ACTING ON BODY A
There are two forces acting on A
(i)Weight of body: w1 = m1g
(ii) Tension in the string = T
The net force acting on the body is
F= m1g – T
Net force acting on body 'A' is given by Newton’s 2nd law as m1a.
Thus we have the equation for the motion of body "A" as:
m1g – T = m1a --------- (i)

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FORCES ACTING ON BODY B
There are also two forces acting on B
(i)Weight of body: w2 = m2g
(ii) Tension in the string = T
Since body "B" is moving up, the net force acting on body is
F= T – m2g
T – m2g = m2a---------- (ii)
Adding (i) & (ii)

(m1 – m2)g = (m1 + m2)a

putting the value of 'a' in equation (ii) to find the magnitude of T
T – m2g = m2a
T – m2g = m2

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Case 2
ONE BODY MOVES VERTICALLY AND OTHER MOVES
HORIZONTALLY
Two bodies A & B of masses m1 and m2 are attached to the ends
of a string, which passes over a frictionless pulley as shown in
the figure. The body "A" moves vertically downward with
an acceleration equal to "a" and the body "B" moves on a
smooth horizontal plane towards the pulley with the same
acceleration.

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Consider the motion of body A
Forces acting on the body A
1. Weight of the body: w1 = m1g
2. Tension in the string = T
The net force acting on the body A
m1g – T
the resultant force acting on it is equal to m1a
Therefore,
m1g – T = m1a --------- (i)
Now Consider the motion of body B
There are three forces acting on it.

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1. The tension (T) in the string, which acts horizontally
towards the pulley.
2. Its weight w2 = m2g which acts vertically downward
3. Reaction of the surface (R) on the body which acts
vertically upward.
As there is no motion of body "B" in the vertical direction.
Therefore, weight and normal reaction cancel each other
For latest information,
Thus, the net horizontal force acting upon body B is T
T = m2a --------(ii)
Putting the value of T in equation (i), we get
m1g – m2a = m1a
m1g = m1a + m2a
m1g = a (m1 + m2)

Putting the value of "a" in equation (ii), we get
T = m2a --------(ii)
T = m2

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Q: define momentum and describe law of conservation of
momentum.
Ans:
MOMENTUM
The quantity or quality of motion is called
momentum and it is denoted by P
OR
It is the product of mass and velocity.
MATHEMATICALLY
P = mV
where:
p is the momentum
m is the mass
v the velocity
LAW OF CONSERVATION OF MOMENTUM
STATEMENT
it states that

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“The total momentum of an isolated
system always remains constant.”
DERIVATION
Consideration
Consider two bodies A and B of mass m1 and
m2 moving in the same direction with velocity
U1 and U2 respectively such that U1 is greater
than U2. Suppose the ball acquire velocity V1
and V2 respectively
after collision
Momentum of the system before collision = m1U1 + m2U2
Momentum of the system after collision = m1V1 + m2V2
According to the law of conservation of
momentum:
Total momentum of the system before collision = Total
momentum of the system after collision

=>m1U1 + m2U2 = m1V1 + m2V2
Q: define friction.
Ans:
FRICTION

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When a body moves over the surface
of another body then the opposing
force is produced and this opposing
force is called force of friction
Or
The force which opposes the motion of a
body while in contract with the other body
is known as friction.
Example
Suppose a wooden block is placed on a table and a
spring balance is attached on it. If we apply a very
small force of magnitude F by pulling the spring
gradually and increase it, we observe that the block
does not move until the applied force has reached a
critical value. If F is less then critical value, the
block does not move. According to Newton’s Third
Law of motion an opposite force balance the force.
This opposing force is known as the force of friction
or friction.
Q: define static friction.
Ans:
STATIC FRICTION

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the resisting force between two surfaces before the motion start
is called static friction
Q: define limiting friction.
Ans:
Limiting Friction
the maximum value of static friction
within which the motion of the body is
about to start is called limiting friction.
Or
When an external force is applied against the force of friction, the
force of friction also increases by the same amount. Therefore, it
adjusts itself in such a way that it is equal and opposite to the
external force. It has a maximum value just before the motion
starts. So, friction is a self-adjusting force. The maximum force of
friction that stops the body from moving is called LIMITING
FRICTION.

It is denoted by Fs.
LIMITING FRICTION is directly proportional to the surface
reaction.
Limiting friction Fs is:

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Where R = normal
reaction
but R = W
and R = mg

Where

= constant known as coefficient of friction.

Q define rolling friction.
Ans:
Rolling friction
When a body rolls over a surface, the
force of friction is called ROLLING
FRICTION. When a sphere rolls over a
surface it experiences an opposing force
called ROLLING FRICTION.
Rolling friction is much less than the This Photo by Unknown Author is
licensed under CC BY-NC-ND
sliding friction because in case of rolling
contact area of two surfaces is very small as compared to sliding.

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Q: define kinetic friction.
Ans:
KINETIC FRICTION
The force of friction which is produced during the motion is called
kinetic friction it is slightly less than the limiting friction.
Q: define coefficient of friction.
Ans:
Coefficient Of Friction
Coefficient of friction is the ratio of LIMITING FRICTION to the
NORMAL REACTION.
Coefficient of friction is constant for a given pair of surfaces but
different for different pairs
Unit of

:

Since it is a ratio of two similar quantities, therefore it has no
unit.
Q: define Causes of Friction.
Causes of Friction
If we see the surface of material bodies through microscope, we
observe that they are not smooth. Even the most polished

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surfaces are uneven. When one surface is placed over another,
the elevations of one get interlocked with the depression of the
other. Thus they oppose relative motion. The opposition is known
as friction.
Q: define Factors on which Friction Depends.
Factors on which Friction Depends
The force of friction depends upon the following factors:
1. Normal Reaction (R)
Force of friction is directly proportional to normal reaction (R),
which act upon the body in upward direction against the weight
of the body sliding on the surface.
2. Nature of Surfaces
Force of friction also depends
upon the nature of the two
surfaces. It is denoted as u and
has constant values for every
surface. It is different for the two
surfaces in contact.

Q describe advantages and disadvantages of friction.
ADVANTAGES OF FRICTION

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• We could not walk without the friction
between our shoes and the ground. As
we try to step forward, we push your
foot backward. Friction holds our shoe to
the ground, allowing you to walk.
• Writing with a pencil requires friction. we
could not hold a pencil in our hand without
friction.
• A nail stays in wood due to friction
• Nut and bold call hold due to friction
DISADVANTAGES OF FRICTION
• In any type of vehicle–such as a car,
boat or airplane–excess friction means
that extra fuel must be used to power
the vehicle. In other words, fuel or
energy is being wasted because of the
friction.
• Due to the friction a machine has less
frequency 100%
• Due to friction machine catch fire.

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• Due to friction wear and tear occurs in shoes and
tires etc
Q: describe methods to remove friction.
Ans:
Methods to remove friction
Friction can be removed or reduced by the following methods:
1. The various parts of the machines that are moving over one
another are properly lubricated.
2. In machines, the sliding of various parts is usually replaced by
rolling. This is done by using ball bearings.
3. Where sliding is unavoidable, a thick layer of greasing material
is used between the sliding surfaces.
4. The front of the fast-moving objects, e.g. cars, aero planes are
made oblong to decrease air friction.

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