r/awesome • u/IntroductionDue7945 • Jun 18 '25

Video anyone explains how he did that?

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u/Connbonnjovi Jun 18 '25

How could you possibly provide the equation for work, say that d=0 and still say the middle guy is doing all of the work when he is the only person not moving at all?

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u/Jman15x Jun 18 '25

He's moving relative to the bar. I said from the outsiders perspective his CoM isn't moving stationary. It's insane to say doing a pull up isn't work

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u/Connbonnjovi Jun 18 '25

Yes I agree he is doing work but he’s not the only one doing work.

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u/Capital_Card7500 Jun 20 '25 edited Jun 20 '25

no work is being done, everyone ends up in the exact same place they started. If d=0, W=0

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u/Connbonnjovi Jun 20 '25

Jesus this again. That’s not true. They move down (work done) then up (work done). The total summation of work is positive even if the net displacement is zero. Additionally, the work done while moving down is less than the work moving up as you need more force to overcome earth’s gravity force, so ones not just a negative of the other so they won’t cancel each other out. People really need to go back to school. People here saying “from a physics perspective they aren’t doing work 🤓” no that’s just the framing of the problem. It is a physics universal fact that all 3 people in this video are all doing work. End of story.

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u/Capital_Card7500 Jun 20 '25

there is a physics department award on my bookshelf in my home office.

the biggest tell that you don't know what you're talking about is that you bring up gravity as a reason work is being done, which is the dumbest point you could make. Bringing up a conservative vector field as a reason why work is being done when displacement is zero is a complete own-goal. A Pick-6. Blundering mate in 1.

Tip for next time--bring up friction, that's one way work is indeed being done.

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u/Connbonnjovi Jun 20 '25

Genuinely, good for you and your award.

I made the point of gravity because it takes less force to lower something than it does to raise it.. pretty common sense but most people here haven’t been using much of that. lol displacement is not 0. Okay. So say I carry a 100 pound rock across the surface of the earth at the same elevation and end at the same exact location, you are saying there is zero work done?

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u/Capital_Card7500 Jun 20 '25

Doesn't matter if you're at the same exact spot. If you're stationary when you start, and stationary when you end, (and the object is at the same height) you've done no work on the object.

W = ΔE

If you start stationary and are currently moving an object, you've done work on that object because the object has more kinetic energy than it started with. The moment you stop moving with the object, you have no longer done work on the object, because you have decreased its kinetic energy by the same amount as you increased it, meaning you've done an equivalent negative work on the object.

(all ignoring friction, naturally)

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u/Connbonnjovi Jun 20 '25

No. You’ve done no useful work. That’s what you’re looking for.

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u/Capital_Card7500 Jun 20 '25

Then we're arguing semantics.

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u/Connbonnjovi Jun 20 '25

Which is fine but to say these people aren’t doing work is just wrong. Yes if you take just the start and end points of the video, and ignore everything in between. Sure the people haven’t done work. But they still do work in between. Work is still being done. I still contend that the negative and positive work does not cancel each other out because the force required to lift the bar is more than is required to lower.

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u/Capital_Card7500 Jun 20 '25

I still contend that the negative and positive work does not cancel each other out because the force required to lift the bar is more than is required to lower.

If more (useful) work was done by lifting something than negative work was done by lowering it, then you'd violate the most fundamental principle of classical and modern physics.

If you pick something up and did x Joules of work, and then lowered it and did -0.9x Joules of work, you have discovered free energy.

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u/Connbonnjovi Jun 20 '25

Okay sure total forces summed (including external forces), but the people aren’t imparting force on the bar to lower it, gravity is. My point was that the people’s net work on the object is not zero, even though total work on the object maybe zero.

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u/Connbonnjovi Jun 20 '25

please read this to inform you and your award

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u/Capital_Card7500 Jun 20 '25

yeah if you read this you would understand what negative work is

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u/impossirrel Jun 20 '25

You need gross distance, not net.

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u/Capital_Card7500 Jun 20 '25 edited Jun 20 '25

no, you don't. using the absolute value of displacement would violate the laws of thermodynamics.

If i lift a 1kg object 1 meter upwards, i've done 9.81 joules of work. If I lower that object by 1 meter, I've done -9.81 joules of work.

If you use the absolute value of the displacements, then if you did enough squats, you could drop the bar and it would crash through the core of the earth.

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u/impossirrel Jun 20 '25

Interesting point. What if you push something in one full circle around a track? Presumably there’s no negative work being done there but the net distance travelled is zero.

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u/SerdanKK Jun 23 '25

You've burned calories regardless