- #1
sankalpmittal
- 785
- 26
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 ?
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 ?