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Help with this hard M1 question please

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So I used F=MA equation, 12000 - 300g = n x 75g(a), where n stands for number of people. Then I divided the equation by 75g(3) to get n on its own, which got 4 people (1 s.f.) but the answer is 8. Can someone tell me what I'm doing wrong? Thanks.

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Reply 1
On the left hand side, 12000 is the upwards force you want. But is the downwards force just 300g if there are n people in the lift?
Could it be that the lift starts from rest, so its average acceleration is (0+3)/2?

n=(12000-300g)/(75g(0+3)/2)=8
Reply 3
Original post by AlexBakSik
Could it be that the lift starts from rest, so its average acceleration is (0+3)/2?

n=(12000-300g)/(75g(0+3)/2)=8


So you half the acceleration?
Original post by ℓove
So you half the acceleration?


I'm assuming so (I dropped AS Physics and haven't got to this yet in M1 maths, I'm a bit rusty! XD)
Reply 5
Original post by ℓove
So you half the acceleration?


No - you need to consider what will happen at the maximum acceleration, as this will correspond to the maximum tension in the cable, which you mustn't exceed.
no, you forgot to include the mass of the people
so 1200 - (300+75n)g
Reply 7
Original post by 123Master321
no, you forgot to include the mass of the people
so 1200 - (300+75n)g


Well, that's what I was trying to say without giving the game away...
Original post by Pangol
Well, that's what I was trying to say without giving the game away...

yh but she didnt seem to take it in so i said it again
@the bear your high quality services are required Monsieur
Original post by RDKGames
Okay, let's do this the way it's supposed to be done.

You know that T<12000T<12000

T(300+75n)g<12000(300+75n)g\Rightarrow T-(300+75n)g < 12000-(300+75n)g

ma<9060735g\Rightarrow ma<9060-735g

Net force (ma) increases proportionally to the increase of acceleration, and since acceleration has an upper bound, take it to be the maximum in order to work out the maximum amount of people when the acceleration is maximum. So a=3a=3.

Plug that into the inequality, plug the total mass in, solve the inequality for n.

dont post full solutions
Original post by 123Master321
dont post full solutions


That's not a full solution.
Original post by RDKGames
That's not a full solution.


practically is
Original post by 123Master321
practically is


A full solution would require me to finish off the question completely, clearly indicating each step, and doing it all for her with the correct answer at the end. I simply pushed her in the right direction by providing a decent start and hopefully making her understand where it all comes from and why. The other half of the solution is up to her to complete.
Original post by RDKGames
A full solution would require me to finish off the question completely, clearly indicating each step, and doing it all for her with the correct answer at the end. I simply pushed her in the right direction by providing a decent start and hopefully making her understand where it all comes from and why. The other half of the solution is up to her to complete.


pushed her to the point of simply needing to substitute values which you explicitly mentioned was necessary.
(edited 7 years ago)
Original post by 123Master321
pushed her to the point of simply needing to substitute values which you explicitly mentioned was necessary.


Considering she went with a completely different method, it is safe to assume that throwing her into my method without much information wouldn't be beneficial whatsoever. I pushed her in the right direction by telling her what she needs to do, and showing half of it. Substituting values is the obvious part that I mentioned for the sake of understanding.
Original post by RDKGames
Considering she went with a completely different method, it is safe to assume that throwing her into my method without much information wouldn't be beneficial whatsoever. I pushed her in the right direction by telling her what she needs to do, and showing half of it. Substituting values is the obvious part that I mentioned for the sake of understanding.


What do you mean the other HALf, more like the other 1/100, literally called me out yesterday just so you could do the same thing, nice work
Original post by 123Master321
What do you mean the other HALf, more like the other 1/10, literally called me out yesterday just so you could do the same thing, nice work


I've seen a lot of solutions around this forum to see what counts as a full solution and what does not under different circumstances and contexts, hence why I called your solution out.

If you feel like it is too much information, then report it.
Original post by RDKGames
I've seen enough around this site to see what counts as a full solution and what does not under different circumstances and contexts, hence why I called your solution out.

If you feel like it is too much information, then report it.


why am i even doing this lol, I literally dont care
Reply 19
Original post by RDKGames
Okay, let's do this the way it's supposed to be done.

You know that T<12000T<12000

T(300+75n)g<12000(300+75n)g\Rightarrow T-(300+75n)g < 12000-(300+75n)g

ma<9060735g\Rightarrow ma<9060-735g

Net force (ma) increases proportionally to the increase of acceleration, and since acceleration has an upper bound, take it to be the maximum in order to work out the maximum amount of people when the acceleration is maximum. So a=3a=3.

Plug that into the inequality, plug the total mass in, solve the inequality for n.


What am I doing wrong here? (12000-300g+9060+735g)/ -735 ?
(edited 7 years ago)

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