Work and Energy of weightlifter

[paulsberardi]

Homework Statement

A weightlifter works out at the gym each day. Part of her routine is to lie on her back and lift a 44 kg barbell straight up from chest height to full arm extension, a distance of 0.54 m.

Homework Equations

If the weightlifter does 24 repetitions a day, what total energy does she expend on lifting? Assume 25 % efficiency.

The Attempt at a Solution

I looked at it, and could only see that the only change in energy is work. Work is 230J per repetition. I tried multiplying 230J by 24 in order to get the answer, but it was not correct.

 

[cepheid]

What about the efficiency? Only 25% efficiency means that 3/4 of the work done is wasted, and only 1/4 of it goes to actually lifting the weight.

 

[paulsberardi]

Ya, I took the 230 J and multiplied it by 24, then by .25 but it won't accept the answer

 

[cepheid]

Ya, I took the 230 J and multiplied it by 24, then by .25 but it won't accept the answer

That's because multiplying it by 0.25 is not the right thing to do. You should read my post again, more carefully.

 

[paulsberardi]

Ok I got the answer, its 22,000 Joules.
Part 3 of the question: How many 480 Calorie donuts can she eat a day to supply that energy?
I thought I could just convert the joules to calories by dividing by 4.184, then divide again by 480 to get number of donuts.