Credits: Pixabay.com Newtons First law of motion defines force and inertia. According to Newtons first law of motion, an external force must act on a body to bring about a change in the state of rest or uniform motion in a straight line. This leads us to define force. Force is that which changes or tends to change the state of rest or of uniform motion of a body in a straight line. From first law itvis clear that a material body by itself is unable to change its state of rest or of uniform motion in a straight line. This inability of a body to change by itself its state of rest or state of uniform motion along a straight line is called Inertia of the body. Newton's first law defines inertia and hence it is described as law of inertia. Quantitatively, mass is the measure of inertia. Print Page <<PREVIOUS Newtons 1st Law of motion