What is the Significance of Line of Action of Force?

[logearav]

Homework Statement

http://www.xamidea.in/cbse-examination-guide.aspx?q=3&c=67&t=10000638&info=title

Homework Equations

Dear members,
In this link, forces f2 and f4 are equal and opposite but they have same line of action
forces f1 and f3 are also equal and opposite but they have different line of action.
I don't understand how f1 and f3 have different line of action and f2 and f4 have same line of action, because both in both cases forces are equal and opposite
Please help me understand this concept of line of action of force, revered members

The Attempt at a Solution

 

[bp_psy]

The problem I think you have is because the first image is a little confusing since it doesn't give any feeling of 3d space. You must consider both figs and apply the LHR you can see that both F1 and F3 are parallel and both perpendicular to the B field but they are not doth on any single line perpendicular to the B field.They do not act on the same line except when the B field is perpendicular to the plane of the loop.

 

[logearav]

Sir, Thanks for the reply. I got it 80 % now, after re reading your reply. But one more doubt, that is, the second diagram shows F1 and F3 in upward and downward directions but in the first diagram, they are projecting outward to the sides of the loop. Why such a difference between the first and second diagram.

 

[logearav]

"but they are not DOTH on any single line perpendicular to the B field.They do not act on the same line except when the B field is perpendicular to the plane of the loop."

I don't understand this line of your reply sir. What do u mean by DOTH?

 

[bp_psy]

Sir, Thanks for the reply. I got it 80 % now, after re reading your reply. But one more doubt, that is, the second diagram shows F1 and F3 in upward and downward directions but in the first diagram, they are projecting outward to the sides of the loop. Why such a difference between the first and second diagram.

There isn't any difference. The first diagram shows you the loop from the side of the loop ,you are looking towards the yz plane. The second shows you the loop at the same time from above ,you are looking at the xy plane. The problem is that the first diagram makes F1 and F3 look like they are not perpendicular to the B field. This is not what happens, the forces are always perpendicular to the B field and the current.It is just a bad diagram with no axes. .

What do u mean by DOTH.

Sorry it was a typo,I meant "BOTH".

 

[logearav]

Thanks a lot sir. What a beautiful explanation! I wonder why my physics teacher does not know all these things. Thanks again sir.