# Chair-lift at constant speed: confusion over passengers per minute, power, etc.

1. Aug 30, 2011

### furor celtica

1. The problem statement, all variables and given/known data

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?

2. Relevant equations

3. The attempt at a solution

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.

2. Aug 30, 2011

### PeterO

When people leave a chairlift, they have the speed of the chair lift, so if they started at rest they will have been given a little kinetic energy [0.5 m v2] by the lift. There is enough information given to calculate what the speed is.

Each rider is also raised up quite a distance, so there will be quite a gain in potential energy [mgh].

Now that total amount of energy is given to each passenger, an a lot of them are carried up each minute.

It is quite straight forward to change "Joules per minutes" to "joules per second" and that will be Power!!