Wednesday, 18 October 2017

homework and exercises - Why do we experience the normal force in an elevator and not the net force?


In a book I saw that the time period of a pendulum inside a elevator moving up is, T=2πLg+a I was curious as to why we use (g+a) as we know inside an elevator,


Fnet=Fnmg


or,manet=Fnmg


or,Fn=m(anet+g)


So my question is shouldn't we use anet instead of the acceleration due to the normal force?



Answer



The net force on the pendulum is:



Fnet=mgFn


so


manet=mgman


dividing both sides by m:


anet=gan


where both g and an point downwards. You state instead that (and I think that this is the source of confusion) Fn, and thus an points upwards, which is the case for the lift, but the normal reaction force (which the pendulum experiences), which is equal in magnitude to the acceleration of the lift a is pointing downwards. So the magnitude of the net acceleration is g+an, pointing downwards, which we obviously can't use as an.


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