Newton first law states that a object which is in rest or in uniform motion does not change its state of rest or uniform motion unless an external force acts on it.


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Newton's first law : A body either remains at rest or moves in a constant velocity unless acted upon by an external force. 
Or if a body is at rest, it will remain at rest unless an external force acts on it. for example, if a book is kept on a table, it stays there unless someone applies a force on it. And if a body is moving at constant velocity, it will not change its velocity if no external force acts on it. for example, a block sliding on a frictionless surface.