Efficiency Calculation for Mechanical System with Incomplete Information

[TyErd]

Homework Statement

Hey I've attached the question. My main problem is figuring out how to start.

Homework Equations

not exactly sure, PE=mgh?
efficiency= used power/total power *100 = used power/ 2950 * 100...

The Attempt at a Solution

i calculated the potential energy 70*30*9.81*7.5=154507.5 J
I thought about finding the angle of elevation but don't have enough information.

 

[PeterO]

Homework Statement

Hey I've attached the question. My main problem is figuring out how to start.

Homework Equations

not exactly sure, PE=mgh?
efficiency= used power/total power *100 = used power/ 2950 * 100...

The Attempt at a Solution

i calculated the potential energy 70*30*9.81*7.5=154507.5 J
I thought about finding the angle of elevation but don't have enough information.

You will need to match up your time units for a start.

Watts is Joules per second. but the load is 30 people per minute.

 

[ehild]

The work done by the elevator raised the people; increased their potential energy. Is that useful work?
That happened in 1 minute. What was the power ?

ehild

 

[TyErd]

okay I've matched up time units. 2950 J/s and 0.5 people/ second.
potential for half a person? = 35*9.81*7.5 = 2575.125 J/s is that right??

 

[ehild]

It is right.ehild

 

[TyErd]

so then to find efficiency i just do 2575.125/2950 * 100 = 87.3%?

 

[ehild]

so then to find efficiency i just do 2575.125/2950 * 100 = 87.3%?

Yes.

ehild