CHAPTER 5- FORCE AND MOTION
Dynamics:
The study of movement caused by force.
Inertia
– Natural characteristics of an object to oppose any attempted change
on its original state, whether at rest or in motion.
– Tendency of an object to remain at rest,or to keep moving at
constant speed in a straight line.
Newton's First Law of Motion
(Law of Inertia)
Statement:
"Every object in a state of uniform motion tends to remain in that
state of motion unless an external force is applied to it".
EXPLANATION
This law consists of a two parts
(a) When body is at rest
(b) When body is moving with uniform velocity
When Body is At Rest:
Newton's Law states that when a body is at rest, it continues its rest
unless we apply a force on it. When we apply a force, it changes its
state of rest and starts moving along a straight line.
When Body is in Motion
Newton's Law states that when a body is moving, it moves in a straight
line with uniform velocity, but when we apply an opposite force, it
changes its state of motion and come to rest.
Examples
*.A body riding a push-bike along a leveled road does not come to rest
immediately when we apply a force, it changes its state of rest and
starts moving along a straight line.
*.If a bus suddenly starts moving, the passengers standing in the bus
will fall in the backward direction. It is due to the reason that the
lower part of the passengers which is in contract with the floor of
the bus is carried forward by the motion of the bus, but the upperpart
of the body remains at rest due to inertia and so the passengers fall
in backward direction.
Newton's Second Law of Motion
The acceleration of a body, a ,is directly proportional to the net
force acting uponit, F, and inversely proportional to its mass,m
DERIVATION
According to the Newton's Second law ofmotion when a force acts on an
object it produces an acceleration which is directly proportion to the
amount of the force.
So, F= ma
Where:
F= force [N]
m= mass [kg]
a= acceleration caused by F[m s-2]
1.Force
Changes the size, shape, state of rest, velocity and/or direction of an object.
Force is a vector quantity.
2. MASS
The quantity of matter contained in a body is called mass.
FORMULA
F = ma
m = F/a
UNIT
The unit of mass in M.K.S system is Kilogram (kg)
3. WEIGHT
It is a force with which earth attracts towards its centre is called weight.
FORMULA
W = mg
UNIT
The unit of weight in M.K.S system is Newton (N).
DIFFERENCE BETWEEN MASS AND WEIGHT
Mass
1. The quantity of matter present in a body is called mass.
2. The mass of a body remains constant everywhere and does not change
by change in altitude.
3. Mass of a body possesses no direction. So it is a scalar quantity.
4. Mass can be determined by a physical balance.
Weight
1. The force with which the earth attractsa body towards its centre is
called the weight of the body.
2. The weight of a body is not constant. It is changed by altitude.
3. Weight of a body has a direction towards the centre of the earth.
So it is a vector quantity.
4. Weight can be determined by only a spring balance.
Newton's Third Law
For every action there is an equal and opposite reaction.
EXPLANATION
According to Newton's Law of Motion, we have:
F(action) = – F(reaction
The negative (-) sign indicates that the two forces are parallel but
in the opposite direction. If we consider one of the interacting
objects as A and the otheras B, then according to the third law of
motion:
F(AB) = – F(BA)
F(AB) represents the force exerted on A and F(BA) is the force exerted on B.
Examples
*.When we walk on the ground, we pushthe ground backward and as a
reaction the ground pushes us forward. Due to this reason we are able
to move on the ground.
*.If a book is placed on the table, it exertssome force on the table,
which is equal to the weight of the book. The table as a reaction
pushes the book upward. This is the reason that the book is stationary
on the table and it does not fall down.
Principle of conservation of momentum
In any collision or interaction between two or more objects in an
isolated system, the total momentum before collision is equal to the
total momentum after collision.
EXAMPLE
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
FRICTION
Definition
"When a body moves over the surface of another body then the opposing
force is produce and this opposing force is calledforce of friction"
Explanation
Suppose a wooden block is placed on a table and a spring balance is
attached onit. 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.
Causes of Friction
If we see the surface of material bodies through microscope, we
observe that they are not smooth. Even the most polished 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.
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 theweight 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.
COEFFICIENT OF FRICTION
The coefficient of friction is a number which represents the friction
between two surfaces. Between two equal surfaces, the coefficient of
friction will bethe same. The symbol usually used for the coefficient
of friction is U, where 0 ≤ U ≤ 1 .
The maximum frictional force (when a body is sliding or is in limiting
equilibrium) is equal to the coefficient offriction × the normal
reaction force.
F = UR
Where m is the coefficient of friction andR is the normal reaction force.
This frictional force, F, will act parallel to the surfaces in contact
and in a direction to oppose the motion that is taking/ trying to take
place.
ADVANTAGES OF FRICTION
1, We could not walk without the frictionbetween our shoes and the
ground. As we try to step forward, we push your foot backward.
Friction holds our shoe tothe ground, allowing you to walk.
2, Writing with a pencil requires friction. we could not hold a pencil
in our hand without friction.
3, A nail stays in wood due to friction
4, Nut and bold cal hold due to friction
DISADVANTAGES OF FRICTION
1, 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.
2, The Law of Conservation of Energy states that the amount of energy
remainsconstant. Thus, the energy that is "lost" to friction in trying
to move an object is really turned to heat energy. The friction of
parts rubbing together creates heat.
3, Due to the friction a machine has less frequency 100%
4, Due to friction machine catch fire.
Methods of Reducing Friction
Friction can be 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 iddone 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, airplanes are made
oblong to decrease air friction.
Law of Friction
Statement
The value of limiting friction increases proportionally with the
increase in normal reaction. Hence, liming friction F(s) is directly
proportional to the normalreaction.
F(s) < R (Here < represents the sign of proportionality dont' write it
in the examination paper.)
=> Fs = uR ……….. (i)
u = F(s)/R
u is the constant of proportionality, which depends upon the nature of
the surfaces of the two surfaces in contact. It is known as the
coefficient of friction. It is only a number without any unit. We know
that the normal reaction is directly proportional to the weight of the
block, therefore,
R = W = mg
Substituting the value of R in equation (i)
=>Fs = umg
Rolling Friction
If we set a heavy spherical ball rolling, it experiences an opposing
force called rolling friction. When a body rolls over a surface, the
force of friction is called rolling friction. Rolling friction is much
less than the sliding friction. This is because the surfaces in
contact are very much less.
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