# Work energy and power problem

1. Aug 11, 2010

### holy_kamote

1. The problem statement, all variables and given/known data
a motor operates at its rated load of 10 hp for 8.0 h a day. its efficiency is 87 percent. what is the daily cost of operation if electric energy costs 5 cents/kWh?

2. Relevant equations

3. The attempt at a solution
i convert 10 hp to watts
10 x 746=7460 watts
then i multiply the 7460 with 5 cents and i get 37.3cents/daily cost...
is my answer correct??? if not plsss post some idea on how i can solve that ty very much...

2. Aug 11, 2010

### holy_kamote

help me plsssss. coz we will have a boardwork tomorrow ..

3. Aug 11, 2010

### PhanthomJay

You are missing a couple of things. The motor is operating for 8 hours each day...you didn't factor that in. Also, you didn't factor in the efficiency rating. If the motor is rated at 10 hp , that's how much power you get out of it....but more power has to go into it due to the inefficiency losses within the motor, and that's what you pay for.

4. Aug 11, 2010

### holy_kamote

what formula will i use to solve this prob?... thx sir jay for posting in my topic.

5. Aug 11, 2010

### PhanthomJay

Please show an attempt at a solution. If the motor is 87% efficient, the power in to the motor is greater than 10 hp. First calculate that number. Then once you get that power, how would you convert it to energy used (kWh) in 8 hours? We can't be of much more help unless you show some attempt. Thanks.