# Applied Physics: Weight-Lifting Work

by rustythevibeguy
Tags: applied, physics, weightlifting, work
 P: 1 OK, sorry, this may be in the wrong place.... but where would you post something like this? This may be too simple, but I cannot get my mind wrapped around this. I need to lose weight and I want to be able to 'quantify' the calories I burn. If I am bench pressing, and I raise 100 lbs. through a distance of 24 inches, how much 'work' have I done, in terms of calories burned? We can neglect all the variables such as efficiency, speed of movement, work done in 'lowering' the weight, etc. I just want to know what the minimum work (change in potential energy) is, in terms of calories required. My goal is to be able to say, "I want to burn X calories per day. How much weight do I have to move per day (through the 24" distance) to burn at least that many calories?" Thanks in advance for your help.
 Sci Advisor P: 5,095 In the basic sense, you can calculate the work and then convert over to calories. Just remember that a food calorie is actually 1 kcal in engineering units (4317 J). You do realize though that this is an extremely inaccurate way to do this calculation...
 Mentor P: 22,303 Just to amplify - the reason this is in accurate is because in a physics sense, you should be absorbing work when you bring the weights back down, but you still actually expend energy.
 P: 70 Applied Physics: Weight-Lifting Work russ, I think that would be true if you aren't fighting gravity. For example, I push weights up, gravity does negative work and I do positive work. I let the weights fall, I do no work, gravity does positive work. However, if I SLOWLY bring the weights down, while gravity wants to bring the weights down faster, I apply a force (not great enough to accelerate the weights up) bringing the weights down slowly. So gravity does positive work, and I do negative work. But all the same, I'm STILL doing work.
Mentor
P: 22,303
 Quote by Da-Force russ, I think that would be true if you aren't fighting gravity. For example, I push weights up, gravity does negative work and I do positive work. I let the weights fall, I do no work, gravity does positive work. However, if I SLOWLY bring the weights down, while gravity wants to bring the weights down faster, I apply a force (not great enough to accelerate the weights up) bringing the weights down slowly. So gravity does positive work, and I do negative work. But all the same, I'm STILL doing work.
I know - I think you may have misread my post.

Btw, when lifting weights, you do lower the weights slowly for precisely that reason.
Mentor
P: 41,477
 Quote by Da-Force For example, I push weights up, gravity does negative work and I do positive work. I let the weights fall, I do no work, gravity does positive work. However, if I SLOWLY bring the weights down, while gravity wants to bring the weights down faster, I apply a force (not great enough to accelerate the weights up) bringing the weights down slowly. So gravity does positive work, and I do negative work. But all the same, I'm STILL doing work.
Careful here. When lifting the weight, you do positive work against gravity: you add mechanical energy to the system (barbell + earth). When lowering the weight slowly, gravity is doing positive work. The system (barbell + earth) loses mechanical energy, which you gain. You gain the energy that the barbell loses! Of course, that "lost" energy gets transformed into an increase in your internal energy--your muscles heat up.

(Just saying you "do work", albeit negative, is a bit misleading. It's like saying, after someone gives you $10, that you gave them negative$10. Either way you are the one getting the \$10.)

The real problem, as Crosson points out, is that we are talking about a biological system here, not just a barbell. It requires chemical energy (fuel = calories) to just hold the barbell in position, never mind lift it up or down. If you examine the muscle fibers in action, they are continually contracting and relaxing, doing positive work just to maintain overall tension. You expend chemical energy (fuel) both when lifting and lowering the weight.

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