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| Acceleration |
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| All of us know that a car moving on road does not have a uniform velocity. Either the speed or the direction changes. Whenever a vehicle is speeding i.e., when the speed is increased we say that the vehicle is accelerating. |
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| To get an idea of acceleration, let us
study the change in velocity of a train moving from station A to station B. When the train, which was initially at rest starts moving, its velocity slowly increases and after a certain time interval it attains a constant velocity. As the next station approaches its velocity gradually decreases and finally the train comes to rest. |
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| When the train starts from rest its speed
increases from zero and we say that the train is accelerating. After
sometime the speed becomes uniform and we say that it is moving with uniform
speed that means the train is not accelerating. But as the train is nearing
B it slows down, which means the train is accelerating in negative
direction. Again the train stops accelerating when it comes to a halt at B. |
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| Thus, it is clear from the above example that acceleration need not always mean that the speed of a moving body has to increase always, it can also decrease, remain the same or become zero. |
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| In general, acceleration is defined as the rate of change of velocity of a moving body with time. |
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| This change could be a change in the speed of the object or its direction of motion or both. |
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| Now let us get a mathematical formula for calculating acceleration. |
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| Let an object moving with an initial velocity 'u' attain a final velocity 'v' in time 't', then acceleration 'a' produced in the object is |
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| Acceleration = Rate of change of velocity with time |
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| The SI unit of velocity is m/s and time
is s |
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| Acceleration is a vector quantity. |
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| From the above example, it is very clear that acceleration is of different types depending on the change in velocity. |
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| Positive acceleration |
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| If the velocity of an object increases then the object is said to be moving with positive acceleration. |
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| Example: A ball rolling down on an inclined plane. |
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| Negative acceleration |
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| If the velocity of an object decreases then the object is said to be moving with negative acceleration. Negative acceleration is also known as retardation or deceleration. |
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| Example: |
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| (1) A ball moving up an inclined plane. |
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| (2) A ball thrown vertically upwards is moving with a negative acceleration as the velocity decreases with time. |
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| Zero acceleration |
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| If the change in velocity is zero, i.e., either the object is at rest or moving with uniform velocity, then the object is said to have zero acceleration. |
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| Example: A car parked in the
basement of an apartment or a train moving with a constant speed of 90
miles/h. |
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| Uniform acceleration |
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| If the change in velocity in equal intervals of time is always the same, then the object is said to be moving with uniform acceleration. |
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| Example: A body falling from a height towards the surface of the earth. |
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| Non-uniform or variable acceleration |
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| If the change in velocity in equal intervals of time is not the same, then the object is said to be moving with variable acceleration. |
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