Module Objectives Uniform circular motion Centripetal force
Centrifugal reaction Centrifugal force 10th - Physics
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Introduction The simplest type of motion is motion along a
straight line. In every day life, we encounter motion along curved
paths more frequently than the straight line paths. Example: Curved
train tracks, curved roads, circular paths in athletics,
merry-go-round, giant wheel, etc. 10th - Physics
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Circular Motion Motion along a circular path with constant
speed is called uniform circular motion. Examples of circular
motion include: an artificial satellite orbiting the Earth at
constant height, a stone which is tied to a rope and is being swung
in circles, a car turning through a curve in a race track, an
electron moving perpendicular to a uniform magnetic field, etc.,
10th - Physics
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Newtons First law of Motion If there is no net force on an
object, then its velocity is constant. The object is either at rest
(if its velocity is equal to zero), or it moves with constant speed
in a single direction. Consequently, An object that is at rest will
stay at rest unless an external force acts upon it. An object that
is in motion will not change its velocity unless an external force
acts upon it. 10th - Physics
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Centripetal Force An accelerating object is an object which is
changing its velocity. A change in either the magnitude or the
direction constitutes a change in the velocity. An object moving in
a circle at constant speed is indeed accelerating. It is
accelerating because its velocity is changing its directions. This
acceleration is called centripetal acceleration or radial
acceleration. The force causing this acceleration, is called
centripetal force. Centripetal force is defined as the radial force
directed towards the center, acting on a body in circular motion.
The direction of the centripetal force, changes continuously, so
that it is always radial. 10th - Physics
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Centripetal Force - Example Consider the case of an stone tied
to a string moving in a circle about point C as shown in the
diagram below. In a time of t seconds, the object has moved from
point A to point B. In this time, the velocity has changed from vi
to vf. The process of subtracting vi from vf is shown in the vector
diagram; this process yields the change in velocity. Imagine that
the string snaps. This means that the centripetal force is
withdrawn. How would the stone move now? 10th - Physics
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Centripetal Force - Example Centripetal force is not a new
force. Any force on an object that keeps it undergoing uniform
circular motion is centripetal force. The orbit of the moon around
the earth and the orbit of the earth around the sun, are nearly
circular. The centripetal force in these cases is supplied by
gravitational force. 10th - Physics
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Centripetal Force - Example Friction provides the centripetal
force necessary for a car to travel around a flat circular race
track. However, this force is small. So the vehicle has to be
driven slowly, other wise it may skid. To overcome this difficulty,
the roadbed has to be inclined suitably by raising the outer edge
over the inner. This prevents skidding and damages to tyre. 10th -
Physics
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Centripetal Force - Example A runner while taking a sharp bend
leans inwards to obtain the necessary centripetal force. If the
roadbed is tilted properly, the inclination provides the
centripetal force. A cyclist going round a curve, leans towards the
center of the curve in order to get necessary centripetal force.
10th - Physics
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Centrifugal Reaction According to Newtons third law of motion,
the agent which exerts a centripetal force, is subjected to a
reaction called centrifugal reaction. It is equal and opposite to
the centripetal force and is directed away from the center of the
circular path. 10th - Physics
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Centrifugal Reaction - Example In the case of a stone whirled
round by a string in hand, the stone exerts an outward force namely
centrifugal reaction on the hand. In the case of a train taking a
turn along a curved track, the centrifugal reaction is the lateral
thrust of the wheels on the outer rails. 10th - Physics
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Centrifugal Force There is a common misconception that an
object moving in a circle has an outward force called centrifugal
force acting on it. Centrifugal force has no real existence.
Consider the case, when the string whirling a stone snaps. If the
centrifugal force exists, the stone will move outward. Since there
is no existence of centrifugal force, stone moves tangentially.
10th - Physics
Slide 14
Centrifugal Force - Example Consider you are riding in a car
going around a curve. Sitting on your dashboard is a cassette tape.
As you go around the curve, the tape moves to outside edge of the
car. Centrifugal force pushed the tape across the dashboard? Wrong.
The animation below shows both birds eye view and passenger view at
the same time. 10th - Physics
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Centrifugal Force - Example The car tires on the road have a
enough static friction to act as centripetal force which forces the
car to go around the curve. The tape on the slippery dashboard does
not have enough friction to act as a centripetal force, so in the
absence of a centripetal force the tape follows straight line
motion. If the car you are riding in has the windows rolled down,
then the tape will leave the car, as it follows its straight line
path. If the windows are rolled up, then the window will deliver a
centripetal force to the tape and keep it in a circular path. Any
time the word Centrifugal Force is used, what is really being
described is a Lack-of- Centripetal Force. 10th - Physics
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Centrifugal Force - Example CENTRIFUGE: A centrifuge is a piece
of equipment, generally driven by an electric motor that puts an
object in rotation around a fixed axis, applying a force
perpendicular to the axis. The centrifuge works using the
principle, where the centripetal acceleration causes denser
substances to separate out along the radial direction (the bottom
of the tube). A centrifuge is also used to separate the components
of blood in blood banks. If a dish containing mercury and water is
rotated about a vertical axis, water will be at the center with
mercury around. 10th - Physics
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Centrifugal Force - Example Centrifugal Drying Machine: It
consists of a cylindrical vessel with perforated walls and can spin
rapidly about its axis. Wet clothes are placed in the cylinder
which is set into rapid rotation. Water is forced out due to
centrifugal force and the clothes are dried. Centrifugal Governor:
The speed of an engine can be controlled by an automatic device
called governer devised by James Watt; the working of this device
depends on centrifugal force. Centrifugal Pump: Centrifugal pump,
used for transferring large quantities of liquids from low pressure
regions, make use of centrifugal force. Centrifugal pumps are also
used as blowers and exhaust fan. 10th - Physics
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Exercises You might have watched the sharpening of knives on a
rotating wheel. The sparks appear to be ejected from the rotating
wheel. What is the direction in which the sparks move? 10th -
Physics
