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| Inertia |
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| It is clear from Galileo's experiments that all objects have a tendency to continue in their state of rest or of uniform motion until an external force acts on it. The following examples will help to understand the observations of Galileo's experiment. |
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| Place a cardboard on an empty tumbler and a coin on the cardboard as shown in the figure. |
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| Cardboard and a Coin placed on an Empty Tumbler |
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| Flick the cardboard with the finger. What do you observe? The coin drops into the tumbler. When we flick the cardboard the cardboard moves fast whereas the coin continues in its state of rest and hence drops into the tumbler. |
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| Coin Drops into the Tumbler as the Cardboard is Flicked |
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| A passenger standing in a moving bus leans forward when the brakes are applied all of a sudden. This is because the body of the passenger is in motion along with the bus. When the bus stops all of a sudden, the lower part of his body comes to rest along with the bus whereas the upper part of the body continues to move forward. |
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| From the above examples it is clear that objects continue to remain in their state of rest or of uniform motion until an external force is applied. This tendency of an object to resist any change in its state of rest or of uniform motion is called inertia. |
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| Inertia can be defined as the property of matter by virtue of which it opposes any change in its state of rest or of uniform motion along a straight line. |
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| Inertia is Classified as: |
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 Inertia of rest |
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 Inertia of motion |
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 Inertia of direction |
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| Examples of Inertia of Rest |
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 A passenger standing in a bus leans backwards when the bus starts all of a sudden |
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 Fruits fall down when the branches of a tree are shaken |
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 Dust particles on a carpet falls when we beat the carpet with a stick |
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| Examples of Inertia of Motion |
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 A passenger standing in a moving bus leans forward when the bus stops all of a sudden |
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 A man carelessly alighting from a moving train leans forward |
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| Example of Inertia of Direction |
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 The water particles sticking to the cycle tyre are found to fly off tangentially |
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 Whenever a driver is negotiating a curve, the passengers experience a force acting away from the centre of the curve |
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| Inertia of a body depends upon its mass. That is, massive objects possess more inertia than lighter ones. |
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