An electric motor rotates 60 times per second if the alternating current source is 60 Hz. How many times will an electric motor rotate in one hour if the alternating current source is changed to 50 Hz?

180,000

Explanation:

Frequency is a quantity that is measured in Hertz [Hz] and it represents the number of rotations per second.

A motor with a frequency of 50 Hz will rotate 50 times per second.

Since we don't want to know how many times it rotates per second, but per hour. The first step is to find how many seconds there are in an hour and then multiply that amount by 50.

Seconds in an hour:

there are 60 seconds per minute, and 60 minutes per hour, thus there are

60*60 = 3,600 seconds in an hour

We know that the motor will rotate 50 times per second so to find the number of rotations in 1 hour =  3,600 seconds we multiply:

50*3,600 = 180,000 rotations

the answer is b, have negligible mass