Applied Physics: Weight-Lifting Work

In summary: You're doing work against the force of gravity, but if you stop halfway and let the object fall, you've stopped doing work against gravity and instead you're doing work against the natural tendency of the object to move forward. You're actually doing more work by stopping and starting than you are by just pushing the object the whole way. So if someone says "when you lift weights, you do negative work against gravity", that's a misleading way of saying what you actually mean is "when you lift weights, you're doing work against the force of gravity".)
  • #1
rustythevibeguy
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Applied Physics: Weight-Lifting "Work"

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.
 
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  • #2
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...
 
  • #3
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.
 
  • #4
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.
 
  • #5
I lift, I have thought about this before. Unfortunately physics alone can't supply the answer without biology.

If we just consider the work done against gravity it would take (one rep of) a 200 pound bench press to burn one human food calorie.

From here the issue comes down to efficiency, how much effort is wasted? (A human can expend calories in muscle tension without doing any work, saying nothing of our core bio functions that run all the time).

Here is what you need to do: use a cardio machine that will measure your heart rate, and tell you the wattage of your effort.

Once you know that heart rate x leads to wattage y, just multiply the y of your average heartrate by the length of your workout (measured in seconds)
and by 1/4317 to get a vague idea of your expended calories.

If you want very detailed data of this kind specific to your own body, you would need to spend a few hour taking data and creating look-up tables. I wonder if I should start offering this as a proffesional service to people at my local gym and over the internet.
 
  • #6
Da-Force said:
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.
 
  • #7
Da-Force said:
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.
 

1. What is applied physics?

Applied physics is the branch of physics that focuses on using scientific principles and theories to solve real-world problems or create new technologies.

2. How does applied physics relate to weight-lifting?

Applied physics plays a crucial role in understanding the mechanics and forces involved in weight-lifting. It helps athletes and trainers optimize their techniques and equipment to maximize performance and prevent injuries.

3. What is work in the context of weight-lifting?

In physics, work is defined as the product of force and displacement, where force is applied in the direction of the displacement. In weight-lifting, work is done when a weight is lifted against the force of gravity, resulting in a change in displacement.

4. How does applied physics help in determining the amount of work done in weight-lifting?

By understanding the principles of applied physics, we can calculate the amount of work done in weight-lifting by measuring the force applied and the displacement of the weight. This allows us to track progress and make adjustments to training regimens.

5. Can applied physics help in preventing injuries in weight-lifting?

Yes, applied physics is crucial in preventing injuries in weight-lifting by understanding the mechanics and forces involved. This knowledge can help athletes and trainers identify potential risks and make necessary adjustments in techniques and equipment to minimize the risk of injuries.

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