Question

An elevator filled to capacity has a mass of 1 200 kg. It goes up one floor, a distance of 3.6 m, at constant velocity in 3.0 s. What is the power of its motor?

(Disregard losses in energy)

Answer 1

The power is `=14112W`

Explanation:

The potential energy is

`PE=mgh`

The mass is `m=1200kg`

The distance is `h=3.6m`

So,

`PE=1200*3.6*9.8=42336J`

The time is `t=3.0s`

The Power is

`P=(PE)/t=42336/3=14112W`

Answer 2

The power is `14" kW"` as shown by others. Here a proper treatment of significant digits is included.

Explanation:

The input figures `3.6" m"`, `1200" kg"`, and `3.0" s"` all have two significant digits. In the time factor the zero after the decimal point also comes after a nonzero digit and so is significant.

Then carrying the multiplication and division properly to two significant digits:

`(1200" kg")(3.6" m")(9.8" m/s"^2)=42000" J"=42" kJ"`

`{42" kJ"}/{3.0" s"}=14" kW"`