MOMENTUM
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| Momentum |
The momentum of a body
is defined as the product of its mass and its velocity
Momentum=Mass✕Velocity
p=m✕v
Momentum's direction is
the same as the direction of velocity of the body.
Momentum is a vector
quantity.Momentum's SI unit is kgm/s.
Concept of momentum was
introduced by Newton.
In the absence of
external forces, the total momentum of the system is conserved.
APPLICATIONS OF CONSERVATION OF MOMENTUM
When a bullet is fired
from a gun, the gun recoils or gives a sharp pull in back ward direction.
While firing a bullet,
the gun must be held tight to the shoulder.
When a man jump from a
boat to the shore, the boat slightly moves away from the shore.
Rocket works on the principle
of conservation of momentum.
If someone left on a
frictionless floor desires to get out of it,he can do so by blowing air out of
his mouth.
IMPULSE
An impulse is a large
force acting on a body for a short time to produce a finite change in momentum.
Impulse =Force✕ Time
=Change in momentum
i.e.,
I= F✕ t
Impulse's SI unit is N-s
or kg-m/s.
EXAMPLES OF MOMENTUM AND IMPULSE
In catching a ball, a
player by drawing his hands backwards increases the time of contact.
An athlete is advised to
come to stop slowly after finishing a fast race.
The layer of bricks is
broken by player.
FRICTION
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| FRICTION |
Friction
is the opposing force that is set-up between the surface of contact, when one
body slides or rolls or tends to do so, on the surface of another body.
The
maximum value of the force of friction which comes into play before a body just
begins to slide over the surface of another body is called limiting
friction.
The force
of friction that comes into play between the surfaces of two bodies before the
body actually starts moving is called static friction.
The force
of friction that opposes relative motion between two surfaces in contact is
called kinetic or sliding friction.
The
frictional force developed when a body rolls over a surface, is known as
the rolling friction.
Static
friction is a self adjusting force and it adjusts itself so that it became
equal to the applied force.
LAWS OF FRICTION
Friction
acts in a direction opposite to the direction of motion of the objects.Friction
depends upon the nature of the two surfaces in contact.
Friction
is independent of the area of contact of the two surfaces.Rolling friction is
less than sliding friction.
Force of
friction (F) is directly proportional to the normal reaction (R).
i.e., F∝R ⟹F=𝝁R
where, 𝝁 = coefficient of friction.
Also
𝝁 = tan ɑ, where ɑ is angle of friction.
Angle of
repose = Angle of friction.
ADVANTAGES
Due to
friction, we are able to move on the surface of earth.
The
fibres of thread are held together due to force of friction.
The
brakes applied in automobiles work only due to friction.
Sledges
are used in arctic region as friction is very low on the surface of ice.
DISADVANTAGES
A lot of
energy is wasted in the form of heat that causes wear and tear of the moving
parts.
Due to
friction, speed of automobiles cannot be increased beyond a certain limit.
METHODS FOR REDUCING FRICTION
By using
lubricants e.g., grease,oil etc.
By using
ball or roller bearings.
By
changing sliding to rolling.
By using
soap solution.
By using
powder.
The tyres
are threading which also increases the friction between the tyres and the road.
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