The Student Room Group

A level - Equilibrium

I know this question has been asked before, but the answer in that closed thread doesn’t seem right.

In the last question of the M1 equilibrium paper in madasmaths. The question asks specifically to minimize tension. It doesn’t specify whether the string is taught or whether we are trying to minimize both force P and tension. Why can’t force P be applied directly upwards against the weight such that the tension is 0. Doesn’t that minimize tension?

https://madasmaths.com/archive/maths_booklets/mechanics/m1_equilibrium.pdf

Scroll to the last question in the page from the above link

Reply 1

Original post
by Jbbhhhز😐😐
I know this question has been asked before, but the answer in that closed thread doesn’t seem right.
In the last question of the M1 equilibrium paper in madasmaths. The question asks specifically to minimize tension. It doesn’t specify whether the string is taught or whether we are trying to minimize both force P and tension. Why can’t force P be applied directly upwards against the weight such that the tension is 0. Doesn’t that minimize tension?
https://madasmaths.com/archive/maths_booklets/mechanics/m1_equilibrium.pdf
Scroll to the last question in the page from the above link

Not seen the other thread (could you link it?), but Id guess youre right the way the question is posed. Id presume it should have asked for minimum (magnitude of) P (rather than minimum T) and the ans would be ok, using horizontal/vertical/harmonic. Minimum T (0) would correspond to a vertical P.

Edit - A simple geometric justification (sketch) of minimum P could be done by adding the resultant tension, T_res, to the end of the weight (tip to tail) and noting that for equilibrium, P must be equal and opposite. The minimum value will therefore be when W + T_res is closest to the origin and this must be when its 30 to the vertical or 60 to the horizontal, so its just a simple 30-60-90 right triangle. Obv the minimum T is when the added T_res is zero. Its basically just madas argument in the pdf but without jumping straight to the perpendicular argument.

Reply 2

For some reason, the previous reply has disappeared. It may be being moderated for whatever reason.

As a placeholder, youre correct. Tension would be minimised (0) when P is vertical (W). P is minimised at 60 degrees as per the solution and you can derive it by horizontal/vertical/harmonic identity or a simple sketch/justification similar to madas solution. Id guess the question should really say P is minimum, rather than T.

Reply 3

Original post
by mqb2766
For some reason, the previous reply has disappeared. It may be being moderated for whatever reason.
As a placeholder, youre correct. Tension would be minimised (0) when P is vertical (W). P is minimised at 60 degrees as per the solution and you can derive it by horizontal/vertical/harmonic identity or a simple sketch/justification similar to madas solution. Id guess the question should really say P is minimum, rather than T.
Ya i got a notification for ur reply but couldn’t find it. I guess the solution is more of a minimum of both P and T.

Reply 4

Original post
by Jbbhhhز😐😐
Ya i got a notification for ur reply but couldn’t find it. I guess the solution is more of a minimum of both P and T.

No, the madas solution is minimum P. Minimum T, which is zero, corresponds to vertical P as you say in the op.
(edited 9 months ago)

Quick Reply

How The Student Room is moderated

To keep The Student Room safe for everyone, we moderate posts that are added to the site.