- #1

pandatime

- 21

- 15

- Homework Statement
- A lift of mass 800kg carries 20 people from a hotel lobby to their floors. Half the people get out on the 10th floor, the remainder on the 15th. What is total work done, in kJ, by gravity moving the lift and passengers? (You should assume each passenger has a mass of 70kg, the lobby is floor 0 and each floor is 10m in height)

- Relevant Equations
- W = Fd

w = mg

Hi! So, I've actually already solved this problem.. for the most part.

I have split up the work into two sections, floor 0 to 10, and floor 10 to 15.

From floor 0 to 10, I did

## F_{elevator} = w_{pass.} + w_{elev.} ##

## F = (70)(20 (num. of pass.))(9.8) + (800)(9.8) ##

## F_{elev.} = 21560N ##

From floor 10 to 15, I do the same calculations ( but change the number of passengers) and get ## 14700N ##

If I add both of these numbers, I get a total of 2891 kJ of work done, which is the correct answer ish.

I have split up the work into two sections, floor 0 to 10, and floor 10 to 15.

From floor 0 to 10, I did

## F_{elevator} = w_{pass.} + w_{elev.} ##

## F = (70)(20 (num. of pass.))(9.8) + (800)(9.8) ##

## F_{elev.} = 21560N ##

From floor 10 to 15, I do the same calculations ( but change the number of passengers) and get ## 14700N ##

If I add both of these numbers, I get a total of 2891 kJ of work done, which is the correct answer ish.

**What I'm getting wrong is the sign of the answer.**I was under the impression that the work done would be positive since the elevator is moving up, and the force done by the elevator to move everybody and the lift up is also pointing upwards right? I'm struggling to grasp**why the answer would be negative instead?**