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 This law states that everybody continues in its state of rest or of uniform motion
in a straight line unless it is compelled by some external force to change that
state.
 Newton's first law gives qualitative definition of force.
 It is an extension of Galileo’s law of inertia.
 Newton's first law defines inertia. According to Newton's first law of motion,
everybody continues in its state of rest or uniform motion unless an external force
act upon it. This shows that a body, by itself, cannot change its state of rest or
uniform motion. This inability of a body to change its state of rest or of uniform
motion along a straight line is called Inertia of the body. Thus, first law defines
inertia and hence it is rightly called the law of inerta.