# Mechanical efficiency problem

1. Mar 17, 2012

### TyErd

1. The problem statement, all variables and given/known data
Hey i've attached the question. My main problem is figuring out how to start.

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

3. 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.

#### Attached Files:

• ###### scan0001.jpg
File size:
18.9 KB
Views:
101
2. Mar 18, 2012

### PeterO

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.

3. Mar 18, 2012

### 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

4. Mar 18, 2012

### 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??

5. Mar 18, 2012

### ehild

It is right.

ehild

6. Mar 18, 2012

### TyErd

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

7. Mar 18, 2012

Yes.

ehild