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

Date: Tue Oct 6 09:17:33 1998
Posted by Jake Good
School: Burlington High School
City: Burlington State/Province: IA
Country: No country entered.
Area of science: Physics
ID: 907683453.Ph
Message:
```
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?

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