Subject: How would you calculate the time needed to lift an object?

Here is our problem.

A 5hp electric motor is used to operate a small elevator at a 
construction site. The motor, which is 60% efficient, lifts a load of 
15 kN (including the platform) to a height of 20m.

a. How much work is done?
	w = F * s = 15kN * 20m = 300,000 J

b. How much electrical energy must be supplied to the motor?
	E = output / input          
							.6 = 300,000 / x
							x = 500,000 J

c. How long does it take to reach the 20 m height?
* This is where the problem occured. 
	Which energy do you use 300,000 J or 500,000 J
	
	If you use 300000 J (that's what we(students used))
	P = W / t = 300000/t = 3730 
				t= 80.4 sec

	If you use 500000 J (that's what the teacher used)
				t= 134 sec

Re: How would you calculate the time needed to lift an object?