Hello , I am in class XI now and I am 15. Ok , so I have a doubt : Majority (no all !!) of our textbooks state Newton's first law as follows : "Every object remains in its original state of rest or uniform motion unless it is acted upon by a net external force." But our teacher told us that the definition in each and every book (nearly) is incomplete. He told us that correct definition will be to say : "If observation is being made from an inertial or non accelerating frame of reference , then every object remains in its original state of rest or uniform motion unless it is acted upon by a net external force." He then gave us the following examples : Suppose a block is kept at rest on a floor. An observer observes this standing beside the block and thus concludes that the block is at rest. So |a| =0 Or |F| =0. So he says that Newton's first law holds. Another observer in a car is moving with a velocity towards the block and observes the acceleration in the block as a. So F = ma So he says that the newton's first law is wrong , as there is an external force being applied and object was at rest! ____________________________________________________________________ Now here are my views regarding this : I think that the teacher is confusing relative acceleration or relative force with frame of references. For example another observer in a car is moving towards the block with velocity v. The velocity of block with respect to car is given by : vb/c = -v If we differentiate this , we get : dvb/c/dt = -dv/dt "ab/c = -a" Also relative momentum of block w.r.t car is : pb/c = -p If we again differentiate this , we get : Fb/c = -F Thus the above expression gives force on block w.r.t car. Also , I think that inertial frame means that newton's first law isn't applicable inside the frame. I think that every textbook just can't be incorrect. Can someone clarify please ?