Physics/Engineering question

[V8LM]

Two more tips - do a rough calculation in your head (no calculator) to get an approximate answer (even if only to a nearest order of magnitude) by round up here, down there, etc. And then do a sanity check of the number you get.

A lift cable with cross-sectional area of 0.45 m^2 and an elastic modulus of 0.1 GPa suggests two things wrong to me.


ETA: And as it appears you can get the correct answer using a length of 95 m when it is 90 m in the question you should get your lecturer to rethink their advice about significant figures!

Edited by V8LM on Sunday 27th March 11:52

[AW111]

Harding91 said:

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.

Cable dia 24mm
Area = pi x d^2 / 4 (same as pi x r^2)
So Area = 452 mm^2

1 mm = 1 x 10^-3 m, so 1 mm^2 = 1 x 10^-6 m^2

So area = 452 x 10^-6 m^2 (or 4.52e-4 m^2 if you prefer that notation, or 0.000452 m^2)



More to come, but I need to go back and read the question again

[battered]

motco said:

V8LM said:

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

Edited by V8LM on Saturday 26th March 07:51

Quite so - I was reluctant to comment but it did seem odd.

The OP's logic is sound enough but a rewiew of the notions of force, stress and strain wouldn't go amiss. Sorry if this is granny and eggs but Young's modulus stuff like this is elementary enough once you have worked out that stress is force per unit area and strain is the stretch resulting from that stress. If you haven't already, read JE Gordon's "Structures" which is a great popular science book. Even when you know this stuff the way that he demonstrates that it applies in the real world not only to bridges and aeroplanes but to plants, animals, and dressmaking cloth is superb. Thanks to "Huff" of this parish for lending me the copy I'm reading now. It's a long time since I studied it, after all.

[AW111]

Harding91 said:

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)

Force applied to the cable (in Newtons) is mass (in kg) x G (9.80665)

Stress is force / area, but you have neglected to tell us the mass of the lift - without that I can go no further.

Given the stress, you can work out the %age elongation using Young's modulus of steel.

The stretch is then cable length (90 m) x elongation.

[V8LM]

I missed the bit where you said the cable was 24 mm in diameter. So, as has been pointed out above, the radius is 12 mm so the area is pi 0.012^2 = 4.5 x10^-4 m^2 (this is not 0.45 m^2 as there are one million square mm in one square metre not one thousand).

If the load of 9310 N is correct (did you calculate this?) then the elastic modulus of the cable is 123 GPa to give the 15 mm extension.

90 x 9310 / (123 x 10^9 x 4.5 x 10^-4) = 1.5 x 10^-3 m = 15 mm

How did you calculate E?
Is the 9310 N correct?

Edited by V8LM on Sunday 27th March 17:23

[AW111]

9310 N gives a mass of 950 kg : sounds about right.

[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.

[V8FGO]

Harding91 said:

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.

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

[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

[AW111]

Congrats OP : that is a very good attitude!

[AW111]

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

You might want to rethink that...

1,000,000

[V8LM]

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.

[Goldenballs13]

OP, where are you doing your degree?

[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

[V8LM]

Harding91 said:

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

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

[Goldenballs13]

Harding91 said:

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

Edited by Harding91 on Tuesday 29th March 13:44

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?

[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

[Goldenballs13]

Harding91 said:

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

I see. I very much enjoy the lectures/tutorials/practicals I get at university, they definitely aid all the learning and is something I couldn't do without!

You might find while pinging off emails to companies like McLaren, that they'd say no until you have completed 2 years of university, as thats when most placements happen, and I never found them to take anything less than that. Sounds like you have the same general interests with your future as I do. Im specialising in engines/CFD with an aim to work in automotive/F1 industries.

[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!