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Question
State and explain the law of inertia (or Newton's first law of motion).
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Solution
Statement of Newton's first law: If a body is in a state of rest, it will remain in the state of rest, and if the body is in the state of motion, it will remain moving in the same direction with the same speed unless an external force is applied on it.
Explanation: Newton's first law can be explained in the following two parts:
(i) Definition of inertia: The 1st part of Newton's first law of motion gives the definition of inertia, according to which an object cannot change its state by itself.
Example: A book lying on a table will remain in its position unless it is displaced.
(ii) Definition of force: The 2nd part of Newton's first law defines force, according to which force is that external cause which can move a stationary object or which can stop a moving object.
Example: A book lying on a table is displaced from its place when it is pushed.
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