A very simple moments question

[physicals]

Homework Statement

This is a past paper question that has appeared on the IAS level physics exam in the January series.

Relevant Equations

moments=fx

Hey there,
Before i introduce you to the question, i would like to clarify that this is not a home work question so i encourage all support that can be given without holding back, marking this as "cheating" (this has sadly happened to me in the past).

This question was part of the recent Edexcel IAS level physics unit 1 paper.
I had completed this paper myself, and its results were released just a few days ago. My marks (as always) has been exemplary but I am still skeptical about one question.

For some context, A level exam boards have strict mark schemes that deem to be a 100% correct. However, i believe, i have found a potential mistake in the mark scheme and would like some backup verification to my explanation.

Please refer to the question image below and its corresponding mark scheme explanation.
Though answer stated in the mark scheme is full correct, I disagree with the explanation given (which many students sadly loose marks for).

Here is my explanation:
P here is the thrust force in the diagonal rod
I believe that force F doesn't contribute to moments of its own since it must be balanced by a horizontal force in the opposite direction from the diagonal bar to maintain horizontal equilibrium. This means we can equate F=sin55*P. Now to find F we just need to find the value of P. We can construct another equation based on the principles of moments, 0.16*cos55*P=0.28*34, to find P. Using this we can easily find F.

The mark scheme, however, seems to ignore P and only focus on the "imagninary" moments due to F. I call this imaginary since the mark scheme seems to ignore the true moment effects of P is cancelled by the horizontal component of P.

Please share your thoughts on this problem.

.

 

[Steve4Physics]

For some context, A level exam boards have strict mark schemes that deem to be a 100% correct.

Pre-retirement I marked physics exam' papers for Edexcel (and for others).

Occasionally examination questions are found to be ambiguous/faulty. Similarly for mark schemes. It shouldn’t happen but it does.

Any problems are quickly discovered by the markers once the marking starts. Each problem is reported and guidance notices are then issued to the markers.

However, for the question you posted, there is nothing wrong with the mark scheme!

However, i believe, i have found a potential mistake in the mark scheme and would like some backup verification to my explanation.

Recent papers and mark-schems are only available to certain personnel (e.g. teachers at registered centres) - not to students. So it is not clear how you would have obtained the information.

P here is the thrust force in the diagonal rod

'P' is already used for a point on the diagram. It can cause great confusion to use the same symbol for different things. Please don't do it!

Let's use 'D' for the diagonal rod's compression. Then each end of the diagonal rod exerts an outwards force, of magnitude D, on whatever it is connected to.

I believe that force F doesn't contribute to moments of its own since it must be balanced by a horizontal force in the opposite direction from the diagonal bar to maintain horizontal equilibrium.

That's wrong. You can take moments about ANY point. If an object is in equilibrium the net moment will be zero (moments balance) whatever point you choose.

Choosing different points leads to different (correct) equations, possibly involving different forces.

We usually choose the point so that an unknown force passes through that point, therefore that force has a zero moment about the point. Then we can set-up a moments equation without worrying about that particular unknown force.

The reason the question instructs you to take moment about P is so that you can avoid having to find the compressive force (D) in the diagonal. You should always follow the instructions!

This means we can equate F=sin55*P. Now to find F we just need to find the value of P. We can construct another equation based on the principles of moments, 0.16*cos55*P=0.28*34, to find P. Using this we can easily find F.

You have come up with an alternative solution - but did not follow the instructions. If correct, you might be awarded 2 out of 4 marks.

The mark scheme, however, seems to ignore P and only focus on the "imagninary" moments due to F.

The moment of F about point P is very real!

I call this imaginary since the mark scheme seems to ignore the true moment effects of P is cancelled by the horizontal component of P.

Edit: Force P (what I call D) passes through point P. so it has zero moment about point P. The only forces (acting on the horizontal rod) which have moments about point P, are F and the 34N weight.

 

[Herman Trivilino]

I believe that force F doesn't contribute to moments of its own since it must be balanced by a horizontal force in the opposite direction from the diagonal bar to maintain horizontal equilibrium.

It's an equilibrium situation so all the forces are "balanced" by other forces. That's what it means to have a net force of zero.

Do you have any idea where your belief comes from? Learning involves examining your own cognitive processes.