- Forums
###### Mechanics 2. Center of masses of bodies. A levels

Watch

1 year ago

So, I am studying mechanics for my A levels and came across an interesting exercise, which got me thinking, is it possible for a body to be in equilibrium if the vector sum of all forces is not equal to 0? The book doesn't seem to mention anything like that.

I attached a screenshot of the exercise. How come the exercise says that the body is in equilibrium even tho the horizontal sum of all the vectors are not 0? What am I missing?

Original post by DFranklin

No, it's not possible.

Original post by MrCarving

I attached a screenshot of the exercise. How come the exercise says that the body is in equilibrium even tho the horizontal sum of all the vectors are not 0? What am I missing?

There may be something else in the question, but just from the diagram, the body would not be in equilibrium as the resolved T2 would move the body to the right as there is no force to stop/balance it.

(edited 1 year ago)

Thank you for you reply. If you don't mind, here is the full question. Do you have ideas why it is in equilibrium?

Original post by MrCarving

Thank you for you reply. If you don't mind, here is the full question. Do you have ideas why it is in equilibrium?

I don't see anything wrong with the question? In the original model you aren't told it's in equilibrium, just that it's hanging in that position at that particular moment.

Frictional forces could also be large enough to resist motion provided by the horizontal component of T2, this isn't mentioned anywhere in the question but could be an assumption made.

In part b it moves to equilibrium but that offending string has snapped so isn't a problem any more.

(edited 1 year ago)

Reply 7

1 year ago

Original post by mqb2766

Can only presume its wrong. Do you have the solutions / which book is it from?

The question is from Edexcel (2017 Spec) FM2 Ex 2G. The model working for part (a) ignores any consideration of equilibrium, dealing instead in terms of locating the CoM then calculating moments about two points on the framework.

My suggestion to the OP would be to locate the CoM and then move on to part (b), ignoring the nonsense about tensions in part (a).

Agreed. Id found the question and it looks like a cut and paste from the preivous question where they had two vertical strings and it makes sense. Id guess (was 1/2 way through working) they equated vertical and 1 moment to get the two "solutions" 7W/18 and 11W/9, but its nonsense (horizontally).

Original post by Skiwi

I don't see anything wrong with the question? In the original model you aren't told it's in equilibrium, just that it's hanging in that position at that particular moment.

Frictional forces could also be large enough to resist motion provided by the horizontal component of T2, this isn't mentioned anywhere in the question but could be an assumption made.

In part b it moves to equilibrium but that offending string has snapped so isn't a problem any more.

Frictional forces could also be large enough to resist motion provided by the horizontal component of T2, this isn't mentioned anywhere in the question but could be an assumption made.

In part b it moves to equilibrium but that offending string has snapped so isn't a problem any more.

There is no force acting towards "the plane producing friction" and even if there was, friction would also act vertically so the two tensions would not sum (resolved) to W (which they do), so there is no friction. The question and the previous one seem to assume equilibrium in the first part, even though its not explicitly stated (obviously not good).

Original post by mqb2766

Can only presume its wrong. Do you have the solutions / which book is it from?

It is from Edexcel Mechanics 2. (not physics, my bad)

(edited 1 year ago)

Original post by old_engineer

The question is from Edexcel (2017 Spec) FM2 Ex 2G. The model working for part (a) ignores any consideration of equilibrium, dealing instead in terms of locating the CoM then calculating moments about two points on the framework.

My suggestion to the OP would be to locate the CoM and then move on to part (b), ignoring the nonsense about tensions in part (a).

My suggestion to the OP would be to locate the CoM and then move on to part (b), ignoring the nonsense about tensions in part (a).

Wait a second. If the body is not in equilibrium then I can't say that T1 plus the vertical component of T2 equals to the weight of the object and therefore I can't find T2. I can find T1 by taking moments around D (assuming i found the center of mass), but how do i find T2 after that without using Newton's second law?

Ive got it in Further Maths/Mech 2, Pearson, but the examples and other questions in the section (called "Frameworks in Equilibrium") are generally based on vertical strings so I can only assume they've simply messed up with ;setting T2 to be diagonal and have not thought about resolving horizontally, so its wrong.

(edited 1 year ago)

Original post by mqb2766

There is no force acting towards "the plane producing friction" and even if there was, friction would also act vertically so the two tensions would not sum (resolved) to W (which they do), so there is no friction. The question and the previous one seem to assume equilibrium in the first part, even though its not explicitly stated (obviously not good).

Looking at the mark scheme for the question it does seem they have just disregarded horizontal component completely.

A lot of questions in the fm2 textbook are almost copy and paste from previous ones. The dynamics and kinematics chapters are especially bad for this and led to a couple of questions with bizzare scenarios that didn't really make sense. I found it was best to just solve it how I was going to have to in the exam and to try to ignore anything else.

I don't really understand why you say friction must be upwards, if you tried to move the framework through a horizontal force wouldn't you also have a horizontal frictional force due to air resistance acting against it? ( I haven't worked through the question so that may be why I'm missing something)

Original post by Skiwi

Looking at the mark scheme for the question it does seem they have just disregarded horizontal component completely.

A lot of questions in the fm2 textbook are almost copy and paste from previous ones. The dynamics and kinematics chapters are especially bad for this and led to a couple of questions with bizzare scenarios that didn't really make sense. I found it was best to just solve it how I was going to have to in the exam and to try to ignore anything else.

I don't really understand why you say friction must be upwards, if you tried to move the framework through a horizontal force wouldn't you also have a horizontal frictional force due to air resistance acting against it? ( I haven't worked through the question so that may be why I'm missing something)

A lot of questions in the fm2 textbook are almost copy and paste from previous ones. The dynamics and kinematics chapters are especially bad for this and led to a couple of questions with bizzare scenarios that didn't really make sense. I found it was best to just solve it how I was going to have to in the exam and to try to ignore anything else.

I don't really understand why you say friction must be upwards, if you tried to move the framework through a horizontal force wouldn't you also have a horizontal frictional force due to air resistance acting against it? ( I haven't worked through the question so that may be why I'm missing something)

Im assuming that youre saying there could be a vertical plane "behind" the framework which is producing friction? If so, there would also have to be a force acting into it so there would be friction produced between the body and the plane and there is no reason to assume the resulting friction would only be horizontal, rather a component would likely act upwards (against weight) and the tension in the strings reduced. But Ive not crunched the numbers to work it through (neither am I going to).

But there is no plane and no force acting into it so there is no friction, diagonal, horizontal or whatever.

(edited 1 year ago)

Original post by mqb2766

Im assuming that youre saying there could be a vertical plane "behind" the framework which is producing friction? If so, there would also have to be a force acting into it so there would be friction produced between the body and the plane and there is no reason to assume the resulting friction would only be horizontal, rather a component would likely act upwards (against weight) and the tension in the strings reduced. But Ive not crunched the numbers to work it through (neither am I going to).

But there is no plane and no force acting into it so there is no friction, diagonal, horizontal or whatever.

But there is no plane and no force acting into it so there is no friction, diagonal, horizontal or whatever.

One of the modelling assumptions for a rod is that it has no thickness, completely forgot about that. So you're correct that there would be no friction.

Thank you

Reply 16

1 year ago

Original post by MrCarving

Wait a second. If the body is not in equilibrium then I can't say that T1 plus the vertical component of T2 equals to the weight of the object and therefore I can't find T2. I can find T1 by taking moments around D (assuming i found the center of mass), but how do i find T2 after that without using Newton's second law?

That's why my suggestion was to ignore the nonsense about tensions and move on to part (b).

- AQA A Level Further Mathematics 7367/3M - 15 Jun 2022 [Exam Chat]
- Center of mass
- Mechanics problem
- Circular motion
- Mechanics
- Mechanics as and a level question
- Can someone help me understand these definitions in mechanics?
- 7.4.4 A Rocket 1 - Isaac Physics Question - Completely stuck!
- AS Mechanics
- Mechanics
- pulley
- How to be good at A-Level Maths Mechanics?
- Ial m1 oct 22 q
- mechanics 2 question help
- Mechanics 1 question on connected particles
- Edexcel A Level Further Maths Paper 4C: Further Mechanics 2 9FM0 4C -27 Jun 2022 Chat
- AS physics
- a level biology mitosis question
- Imperial JMC offer
- resonance question

- Uni decision - Master econ and finance
- Help! Failed module... will I still get my First Class Honours?
- Is it weird for me (female 15) to date a 17yr old
- Public Health ST1 Programme 2024 Entry Thread
- DWP Work Coach Interview - how long to hear
- what do you do in freshers week?
- Oxford Postgraduate Offer Holders 2024
- [Official thread] Durham University 2024 Applicants
- Official: Anglia Ruskin University A100 2024 Entry
- Official: Edge Hill University A100 2024 Entry Applicants
- MPhil in finance 2024-25 intake- Cambridge
- Civil Service Fast Stream 2024 - Applicants thread
- November 2023 Illegal Migration Intake Unit - Intake Response Team - Immigration Offi
- offers for Leeds Uni
- Coleraine Primary PGCE 2024/25 story
- MPhil in Machine Learning and Machine Intelligence at Cambridge 2024
- BBC Apprenticeship 2024
- Replacement Tenant websites
- UCL Developmental Psychology and Clinical Practice
- Interviews CSM 2024

- tips for first year of uni
- LSE september 2024 grad intake
- Physician associate at Surrey University
- Santander Graduate Scheme 2024
- Haven't heard back from LSE
- LSE Economics
- Official: Hull York Medical School A100 2024 Entry Applicants
- How to Apply for Catholic University Slovakia
- Medicine apprenticeships
- Student Finance for non-UK/EEA spouse of UK National
- Is it normal to be paranoid about this?
- Bbc apprenticeships 2024
- The Apprentice 2024
- Amazon Degree Apprenticeships 2024 - Assessment Centre
- Official Veterinary Medicine Applicants thread 2024 entry
- cardiff uni mpharm
- Dissertation Survey Help!
- Sciences Po | Admissions 24
- Official: University of Bristol A100 2024 Entry Applicants
- Still waiting on an interview offer for pharmacy at Cardiff?

- GCSE Mathematics Study Group 2023-2024
- A-level Mathematics Study Group 2023-2024
- Mock set 4 paper 2 q14 a level maths (4 distinct points)
- UKMT Intermediate Math Challenge 2024 - Discussion
- Help with complex summation further maths a levels
- Could I have some help with this suvat question?
- MAT practice
- Alevel Maths Question
- weird cosine question
- Series function not differentiable at a point

- hyperbolic function catenary problem
- can anyone answer this A level vectors question (very challenging)
- STEP foundation module help pls
- Ukmt IMC 2024
- ukmt imc
- This maths question is driving me crazy
- Maths Mechanics
- Confused on surds question
- HNC MATHS A2 Task 3 (Radio Transmitters)
- Senior Maths Challenge 2023

- GCSE Mathematics Study Group 2023-2024
- A-level Mathematics Study Group 2023-2024
- Mock set 4 paper 2 q14 a level maths (4 distinct points)
- UKMT Intermediate Math Challenge 2024 - Discussion
- Help with complex summation further maths a levels
- Could I have some help with this suvat question?
- MAT practice
- Alevel Maths Question
- weird cosine question
- Series function not differentiable at a point

- hyperbolic function catenary problem
- can anyone answer this A level vectors question (very challenging)
- STEP foundation module help pls
- Ukmt IMC 2024
- ukmt imc
- This maths question is driving me crazy
- Maths Mechanics
- Confused on surds question
- HNC MATHS A2 Task 3 (Radio Transmitters)
- Senior Maths Challenge 2023

The Student Room and The Uni Guide are both part of The Student Room Group.

© Copyright The Student Room 2024 all rights reserved

The Student Room and The Uni Guide are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: Imperial House, 2nd Floor, 40-42 Queens Road, Brighton, East Sussex, BN1 3XB