IIT JEE , AIEEE Forum - Physics , Mechanics

umang

Its a very silly question , but it has been troubling me for a long time . Consider an inclined plane with inclination theta (Q) with a mass m on it . If we balance the forces , we get N = mgcosQ . And , if we do it the other way , we get NcosQ = mg . So , how do we decide which is correct ?????

arvind1990

i dont think we get NcosQ = mg . We get only mgcosq=N.

Wht do u mean by other way??

magiclko

it jst depends on how u chose the X-Y axes... if u choose them along and perpendicular to the incline... u'll get evrything wrt that axes... nd same for the standard one... hopefully u'll get it!!

aditya_arora04

Hi,

Look,

Both the concepts are right. The reason is as follows :

When you take N = mgcos

, the forces are in the planes perpendicular and and parallel to the inclined plane.

In, the second case, we see that the forces are in different planes. If you have some specific values then you will see that the answers for both cases would be same. Just the way of attempting it is different.

joyfrancis

magiclko is absolutely correct.In the chapter of laws of motion the first and foremost thing we do while solving a problem is, we choose the axes and make equations according to that axes.

Like in simple coordinate geometry concepts we use only the x,y and z axes(rarely), x and y are always fixed and always at right angles, similarly the axes that you choose should also be fixed during the whole problem and should be at right angles(although it can be at an angle other than 90 also but that axes are beyond our reach as mere XIthees...ha ha)

So, choose axes, make equations solve.

Simple!

kislay

Consider an inclined plane with inclination theta (Q) with a mass m on it . If we balance the forces , we get N = mgcosQ .I T IS ONLY CORRECT And ,NOT the other way , we get NcosQ = mg,becuse the body is in in equilibrium in the direction perpendicular to incline and not in the direction of mg.

pls RATE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!11

shubham_sachdeva

arrey dono hi correct hai its depend ki which type of qn are u doing

umang

Thank you all for answering this question . Just a little query - How do we get to know which axes is the question demanding ?????

kghedriu

Hii umang, try to choose those axes along which most of the forces that act are parallel or perpendicular.

This is done just for convenience, see, u will be having no component of a force along its perpendicular, rite, so, u can solve the problem without resolving many forces....

hope m clear...

hr

choose the axes along and perpendicular to the direction of resultant acc cz most forces will also be along that direction... makes life real simple....

rate plz...

subs

itll change with ur selection of axes

rik_mad

we must always resolve the forces along the direction of motion of body

in case of inclined plane choose axes in such a way that u hav resolved force acting along the direction of motion....

umang

Hey !

Thank you all for answering !!!!!

Arjun

u'll hav to choose the axis which is most easier to go about,
usually all the problems will go on easily wid choosing N=mgcostheta as the component!
hope u got it,thank u

Ask & Discuss Questions with Community & Experts