1. CIRCULAR MOTION
10th Grade – Physics
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2. Uniform circular motion Centripetal force Centrifugal reaction
Module Objectives
Uniform circular motion
Centripetal force
Centrifugal reaction
Centrifugal force
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3. 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.
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4. 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.,
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5. Newton’s 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.
Please play the video of second image.
An object that is in motion will not change its velocity unless an external force acts upon it.
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6. 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.
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7. 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?
Vi is the velocity at position A. Vf is the velocity at position B. The direction of velocity is always towards the centre.
Answer: If the string snaps, the stone will move tangentially.
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8. 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.
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9. 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.
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10. 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.
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11. Centrifugal Reaction
According to Newton’s 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.
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12. 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.
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13. 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.
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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 bird’s eye view and passenger view at the same time.
Please play the video.
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15. 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.
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16. 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.
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17. 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.
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18. 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?
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