Series resistance to step down 120V to 54V for a 140W motor

[yuvlevental]

Homework Statement

Here's another motor--this one operates on 54 V and does work at a rate of 140 W. How much resistance must be connected in series to this motor, if you wish to plug it into a 120 V outlet?

Homework Equations

I'm not sure, but:
P = (V^2)/R
Where P is power, V is voltage, and R is resistance.

The Attempt at a Solution

Am I supposed to subtract the motor voltage from the outlet voltage?

((120-54)^2)/140 = 31.114 ohm

I know that this is wrong...

 

[quantumdude]

Am I supposed to subtract the motor voltage from the outlet voltage?

Yes, but you're not supposed to just throw that difference into the power formula.

Try to answer the following questions in order.

1.) Given that you know the voltage and power ratings of the motor, what current will it draw?

2.) Given that you know the voltage of the voltage rating of the motor, what voltage drop do you require across the resistor? (This is where you subtract the voltages).

3.) Given that you know the current through the resistor and the voltage across it, what is the required resistance?

 

[yuvlevental]

thanks, i got it