Physics/Engineering question

[Harding91]

Hi all, firstly if this is posted in the wrong section feel free to move it but i couldnt really see anywhere suitable to post it so went with general?

I'm currently studying part time for a degree and im working on one of my assignments and i've confused myself rather a lot.

Hey there, currently working my way through my TMA02 for T174 and ive possibly stumped myself.

Anyone who's good with physics in general able to help? I've put the question below with different figures but the same exact wording and my thoughts/answer so far! Any help appreciated, id prefer it if im wrong just point me in the right direction id rather not have somebody answer it for me as im looking to learn it myself.

Thanks in advance!



Question 5.

g) when the lift is close to the bottom of the lift shaft, the cable has a length of 90m. Show that the mass of the lift fully loaded as decribed in (d) above stretches the cable by about 15mm (4 marks)



Deleted this working out crap because it's wrong now, what i think is the correct answer below!

Edited by Harding91 on Saturday 26th March 11:32

[Harding91]

nevermind im a fool, worked it out now

[Harding91]

Uploaded it as an image as getting it to format correctly on here was a nightmare.

But here's how i solved it i think, or at least it makes sense to me. Will find out when its handed in and marked i guess.

[Harding91]

V8LM said:

You can't use the 15 mm to calculate E and then use that E to calculate the 15 mm extension: you could show the extension was anything. There must be information you've been given previously - E, some value of the extension when the lift is further up the shaft, extension under a different load.

The unit of force is the newton (N), not newton per m.

Edited by V8LM on Saturday 26th March 07:51

Yeah using 15mm as the input before was the mistake i made, the second post i put where i got it isnt using that it's using youngs modulus to work it out.

And i know force is newtons as i worked the force out in an earlier question on the assignment but i converted it to newton per mm to input it into the equation to achieve the answers in mm. I should probably make this clear in my workings?

[Harding91]

Also forgot to mention when posting the second post with my solution i worked it out completely differently.

In a previous question they gave me a youngs modulus of the cable to answer that said question, in this question im tackiling now i assumed it would be the same which was a mistake so i had to re-work out the youngs modulus for this question ignoring what i was previously told. Which leads me to 15mm the figure they give me in the question so i'd guess it was right? They give me the 15mm figure in the question and ask me to show how it is 15mm.

I think it's almost impossible i use all the numbers ive gained previously and end up with 15mm if the working is incorrect? Especially considering how long some of the numbers are i doubt it's coincidence that i end up with the figure they've asked me to end up with.

After a nights sleep i re-worked it and looked at it again and im fairly sure my second solution there is correct.

Edit : i also made the mistake of trying to use hookes law initially instead of youngs modulus which was obviously the right way to do it, silly mistake really.

[Harding91]

2gins said:

Thanks

I'd be skeptical about using the given value of 15mm to work back to it, all the expressions you're using are linear so you can only ever end up back at 15 mm. Is there a way you can find the extension with the given information without using 15mm as an input?

Also, A = 0.45m, that's a meaty lift cable (0.4m radius or thereabouts). How big is this lift?

On these questions I tend to assume

1 mark for identifying the route to the solution
1 mark for each correct law/expression
1 mark for the answer

Above 4/5 marks I also assume a mark or two for setting out the structure neatly. Gives me an idea of what may be expected.

Thanks for the feedback. I think what i did above was working out the extension or 'stretch' without using the 15mm as the input

And the areas correct as per the figures they gave me of the cable having a circular cross-sectional with a diameter of 24mm so i half that to give me the radius and then it's just pi multiplied by the radius squared giving my area in metres squared which i then just convert to mm squared. It could just be a theoretical area given for the question hence why it seems bigger than it maybe should be, im not sure i just work off the figures im given.

And it'll be laid out neater on my final hand in this was mad scientist scrawlings on not much sleep plus i will probably hand write it instead of fighting with the pc to format it again.

[Harding91]

tapkaJohnD said:

Harding,

Have you seen this: https://www.youtube.com/watch?v=SRGaW3maK38

"Writing Math[s - it's American] Equations in Word"

John

I use libreoffice currently and it can also be done in that and i know how im just not quick nor efficient at it and i honestly prefer writing out maths work by hand anyway

But thank you for the link, much appreciated.

[Harding91]

V8LM said:

You're asked to show the increase in length is 15 mm. So all you need to show in your answer is the second half - state the equation for the modulus, rearrange to express as deltaL, plug in the numbers.

"Force per mm" makes no sense. Force per length has units of kg s-2, not newtons.

Just stick to the same units and scale (N, m).

Also learn to state only significant figures, not every decimal place your calculator gives.

See what you're saying, as for significant figures we're told to always put the entire number unless specified otherwise. Just following the instructions on that part.

[Harding91]

Ok so finally getting around to replying to this, i got a bit annoyed because everyone was poking holes in the logic of the question and such forth.

I appreciate every bit input truly and i thank you for it, some was helpful for sure.


As for the question, it is more than likely hypothetical, im in my first year of a degree and i was working everything out based on what i was given. It isnt my tutor who puts the questions there the assignments are set for the course so yeah thats out of my control but i can always give them feedback at the end of it.

I am confident that i have answered it in what i believe to be the correct way bar a few incorrect units which i've amended since.

I'm also aware many if not all of you have more experience and are more than likely smarter.

But im going to hand it in as it is currently because thats answered to the best of my own ability, not willing to take somebody elses knowledge and pass it off as my own defeats the object of me wanting to do this degree. if i dont get full marks or i dont get any i'll receive feedback as to why from my tutor and it'll be a learning experience

Give me a month or two and i'll post both how i submitted it and my marked result back for any that might be interested.

Dan

[Harding91]

Oops double post.

[Harding91]

V8FGO said:

For the cable area I suggest you convert to m first then calc the area. Think about it a 24mm has nearly 0.5m2 area. You need to divide by 10000 not 1000 going from mm to M

I've ammended bits here and there, i didnt make it clear i guess ive changed bits from my original and second post in the thread but only what i picked up myself after re-reading it, i will again re-read it before submitting it next week

Edited by Harding91 on Tuesday 29th March 13:42

[Harding91]

V8LM said:

Not quite. To convert from mm to m is divide by 1000. To convert from mm^2 to m^2 is what the spoiler says.

OP, it is a shame you didn't answer some of the questions asked as I for one, as someone trying to help, would like to see you learn from your asking. Hopefully your lecturer will give you sufficient feedback.

Good luck.

While like I said I really do appreciate the help and the questions/input given ive learned it in the way they supplied it to me and im going by what I think is correct. If it's wrong that's on me and I need to re-visit it but im somewhat confident that once I write it up later this week i'll be happy ive got all 4 marks for the question

However id honestly rather drop marks over a question that challenged me and learn from that than put somebodys elses wording/figures down without fully understanding them. I got 93% and 97% on my first two assignments so I have a little bit of faith in myself.

But I will update this with it or start a new thread when its all marked just for the curious people


And thank you, I appreciate that, It's also nice to know there are a few more experienced people on here with an interest as it gives me the option to question myself on here in the future

Goldenballs13 said:

OP, where are you doing your degree?

OU, so far im impressed with the course and look forward to the oncoming years.

Edited by Harding91 on Tuesday 29th March 13:44

[Harding91]

Goldenballs13 said:

Very brave! Not something I could tackle myself. My understanding is that its very much self taught? (more than a normal university) (?) Do you get to specialise your modules in later years?

I wouldn't say you couldn't tackle it, i'm sure you could. It is very much self taught but the content is put across in an overall interesting way that's easy to learn.

For reference i left school with next to nothing grade wise, went to college and dossed my way through mechanics courses with no real effort involved. I dont think i'm stupid i just know i put little to no effort in back then, I'm 24 now and wanting to change that, i think im doing well with the course and finding spare time is hard but its more than manageable, i keep my laptop and books on me mostly and if i find a spare 20+ minutes i sit and read or work on a problem.

Went into it with the logic that it's possible to learn anything, some things take a lot longer than others but if i fail i've only my lack of motivation to blame.

And i get to specialize a little bit in the coming years but overall the course is very generic and broad, however i plan to do my masters after through the OU and probably focus on something like renewable energy then, not that it's my main interest currently but call it future proofing. My interest is motorsport and the motor vehicle industry and probably always will be but an engineering degree will allow me to get somewhere i can climb and enjoy being.


I have actually just written a letter to McLaren asking them about possible careers etc, figure there's no harm in asking, if i can find a company i'd enjoy working for that would take me on knowing im working towards my degree and masters

V8LM said:

A great attitude. Well done. (I disagree completely with your lecturer's position on significant figures, but thats their problem not yours).

Thank you i appreciate it. And many people on the course disagree with plenty which i find stupid, at the end of the day its a qualification, you do whats asked to an acceptable standard and you earn it, after that you learn so much more within the field anyway. Even if i disagree with things they say/do i do it the way im asked for the sake of the course even if i know outside of the uni work i'd do it differently

Edited by Harding91 on Tuesday 29th March 18:38

[Harding91]

Assignment is due in next week just finished writing up the final draft.

In regards to this whole thread, this is how i've put the answer to the question.

The question :
When the lift is close to the bottom of the lift shaft, the cable has a length of 95 m. Show that the mass of the lift fully loaded as described in (d) above, stretches the cable by about 15 mm.
(4 marks)






I will again update this when i get it back marked, should be by the end of the month.

Again i appreciate everyones comments and input but im still glad i stuck with what i got to myself and if im wrong im happy to be wrong and learn from it!