A motor with an efficiency of 75% lifts a 120 kg load at a steady 1.25 m/s.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A motor with an efficiency of lifts a load at a steady . Taking , find the useful power and the power the motor takes from the supply.
Need a reminder? The key ideaShow
Power is the rate of doing work: , in watts. For a force at speed , . Efficiency useful power out power in. One kilowatt-hour is .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Useful power from , then divide by the efficiency.
Step 1. Useful. .
Step 2. Input. .
Answer: useful; taken in. The other heats the motor.
Check. . Good.
Watch out. Multiplying by instead of dividing gives a smaller input than output, which is impossible.
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