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Old Aug 30th 2011, 06:22 AM   #1
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chair-lift at constant speed: confusion over passengers per minute, power, etc.

A chair-lift runs at constant speed. Each passenger starts from rest at the lower station, and is delivered to the upper station with the speed of the lift. The lift raises 30 passengers a minute, of average mass 75 kg. The top of the lift is 300 metres higher than the bottom, and the ride takes three minutes. What power do the motors produce?

Firstly this exercise confuses me as it is in the middle of (objectively) much simpler problems such as 'a cyclist maintains a steady speed of x ms^1 when opposed by a force of y newtons, what power is he producing?'; so I'm guessing there is some generalisation or simplification I'm supposed to make, but I'm just missing it.
I'm not even sure of how to formulate the function of the chair-lift exactly, as in the '30 passengers per minute' and the 'ride takes 3 minutes'.

I just need any suggestion of how to tackle this problem, I'm not looking for a full answer or even an overly-detailed explanation.
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Old Aug 30th 2011, 11:10 AM   #2
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It means that at any instant, you have 30+30+30 = 90 passengers on the lift. The maximum you can get is when one group is at the top, another two in the middle and the last at the bottom. The lift is no more lifting the top one, but is lifting both the middle ones and the bottom one.

Just like here: http://sarahforreal.files.wordpress....r-lift-jl1.jpg

But with 30 people on a chair instead of 4 (maximum)

Can you give that a try now?
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Last edited by Unknown008; Aug 31st 2011 at 02:25 AM. Reason: Forgot something :)
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