# Work and Energy Question

1. Nov 17, 2009

### paulsberardi

1. The problem statement, all variables and given/known data
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.

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

3. 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.

2. Nov 17, 2009

### cepheid

Staff Emeritus
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.

3. Nov 17, 2009

### paulsberardi

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

4. Nov 17, 2009

### cepheid

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

5. Nov 17, 2009

### 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.