I suppose in the first scenario, lifting the bar off my shoulders by an inch, my core is providing an equal and opposite force downwards, to offset my arms pushing up.
I guess I'm good, then. Makes sense. Thanks, you guys!
I think it must be that you can't make it harder on your legs by pulling down - the top half of your body is a closed system with fixed weight. That your core muscles are just doing more work to counter your arms.
I think this is right in this case. But generally, it shouldn't be a problem that you can make a task harder but not easier by doing it differently, if you're starting from the most efficient way of doing it.
As long as the bar does not move around you won't change the force at your legs. If you move the bar around you will change the force at your legs.
3 both makes sense and doesn't make sense to me.
Makes sense version:
If I think about how muscles move, it makes sense that any deviation from most-efficient will be harder.
Doesn't make sense version:
If I think about how a closed system (ie, from the waist up) has a fixed amount of weight pushing down on your legs, then no amount of futzing around with the closed system should alter the weight pushing down on your legs. So it seems like the difficulty of the task on your legs should not change.
4 - that's what I meant by the "jumping in an elevator" stipulation. Bar doesn't move relative to upper torso during the back squat.
So, I'm taking 4 as affirmation of my thinking.
To expand on 5: I guess both versions make sense.
Since your core and arms are now working, your body is less efficient and the task is harder.
For your legs alone, nothing has changed.
Oh! LB! Have I asked you my rowing question? Or anyone, really.
I think 5 is right -- that is, if nothing changes at the level of your pelvis, then nothing changes below that level (which is what you said). I'm mostly thinking that you've got intuitions about how using your body differently seems as if it should make it harder in practice, and it's plausible that the intuition is derived from imagining situations that would change what you were doing in ways that did impact your legs.
For me, there's a natural place to rest the bar where much of the load is supported by my skeletal structure. I could stand like that for a bit. If I move the bar to a different point, I'm stressing my arms and I'm needing more effort from my stabilizers to keep the bar movement in its natural track. I chalk up any extra fatigue on the quads to extra work in stabilizing the weight.
When we have to use the rowing machine, this is what I think about:
A rowing machine can't possibly be replicating the motion of real rowing, because your butt moves on the machine. Your butt doesn't slide dramatically in a boat, right? And your feet are in a locked position? So the fact that rowing machines have your legs extending fully and then bending fully is just totally made up wholesale - it does not resemble what you do on a boat?
I find rowing machines incredibly awkward and I think they'd make more sense if your butt were stationary and you were actually mimicking rowing motions.
Your butt doesn't slide dramatically in a boat, right?
Yes it does, to let you use the force of your legs to drive the oar. There's a sliding seat just like on the rowing machine.
It is an awkward motion until you get the feel for it, and it sounds as if you haven't gotten good coaching. Did they start you out with breaking up the stroke into three parts, legs then back then arms?
If F=ma and W=Fd and you hold the bar really really still in theory you should be able to hold it forever because you're not doing any work.
11- I thought crew shells have seats that slide, no?
14: No, not the Vikings, it's an early 20th (late 19th?) century innovation. I can't remember the guy's name, but this one competitive rower wore leather pants and greased his seat with oatmeal for slipperyness, and won everything for a while. (I may be misremembering details, but I swear I'm not making it up.)
I keep my back very very straight, and that seems to be enough for the coaches - they sometimes use me as an example. I think I have heard the three parts thing, but haven't registered it where I actually think about that while rowing. This conversation might do the trick.
(Mostly we barely ever use the rowers, though. There's only 2 of them there, mostly for people who need an alternative to biking or running.)
Straight is good, and firm is even more important. You're probably doing this right already if they're using you as an example, but a standard error is to start leaning a little forward (which is right), and then as you extend your legs lean forward more (which is wrong), so that you don't end up moving the handle much when you straighten your legs.
Try breaking the stroke up to get a feel for it -- do a couple of minutes just using your legs: leave your arms straight, and your back motionless in the initial leaning forward position. And then just your back -- straight legs, straight arms, and just lean forward and back (you will get very very little power this way, but you're not expecting much). And then just your arms -- straight legs, leaning back some, and extend and pull in your arms.
Once you've got all the parts separated, try a full stroke by doing the parts in order, not overlapping: legs then back then arms. That's how you should be doing it for real (I mean, you don't pause inbetween, so there's a tiny bit of overlap, but the motions really aren't simultaneous.)
(More than you wanted to know, right? I should fix my rowing machine. I like rowing a lot, and the fact that it annoyed Tim is no longer a problem.)
Oh, slip, but doesn't actually need fixing. It's not as if it's an uncommon name. And wanting to be called Buck was always idiotic.
Googling is not backing me up on the leather-pants/oatmeal thing. Possibly that was rowing coach folklore. What I'm finding is that a sliding seat on tracks was a technical innovation in the 1850s.
Yeah, I started calling him Mr. Breath way, way, way back when I first mentioned him, and he requested 'Buck' as an alternative. I, being nothing if not accommodating, went along with it.
To the physics question in the OP: are you doing squats in a magnetic field? That might affect the answer. Also, what's your acceleration?
You're not meaningfully changing the distribution of mass. But squeezing changes some angles and tensions between bones in your body, it is effectively a changed stance. Different stances, different leg muscles.
Re rowing: the leading manufacturer of rowing machines has pretty good technique videos on its website: http://www.concept2.com/indoor-rowers/training/technique-videos
After a brief and unsuccessful rowing career in college I've recently starting taking sculling lessons. It's fun! Trickier in the boat than on the machine, though.
Straight is good, and firm is even more important.
Where's a 19-year-old when you need one?
I definitely do pull the bar down onto the shoulders when I squat heavy. If nothing else it's reassurance against losing the weight over the back, which is No Fun.
are you doing squats in a magnetic field?
Gravitational, primarily.
I finally got around to buying a kayak. Been having a lot of fun exploring the waterways here.
I think your answer is in the "for the sake of argument" exclusion. If you're using your core to stabilize the bar an inch off of your shoulders, that's work that's not being done when it's resting on your back. Plus what lw said.
I had no idea that this is what physicists do.
I am not a physicist*, but it seems to me that the answer is obviously neither pushing down nor pulling up on the bar makes a difference in the level of difficulty for your legs. Why would it? (Setting aside the "maybe more difficult form" points raised above, which are valid.)
*(but I am a white man, so)
Well, I've come to that conclusion, too. But on "why would it?", well, it feels like more weight is pushing down on you.
Oh, so this was an experimental result? That's more interesting. I thought it was just theoretical.
That's the weight of expectations. It's heavier for women, because patriarchy.
If you want to replicate Viking rowing (or just rowboat rowing) on a rowing machine, you just don't move your legs. Easy?
39: It was an experimental thought experiment.
Ragnar Lodbrok got his breeches for the dragon, but kept them for the advantage the lanolin gave him while rowing.
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Since this is the physics thread: an ad for Sherwin-Williams paint shows off some fancy fluid dynamics.
Impressively, everything in the video is a practical effect, even the segment that flies past multicolored turbulent plumes. You can see how they filmed everything in their behind-the-scenes featurette below.
(ad is 30 seconds, "making-of" is 2 minutes. Both feel like commercials but also like work that they're justifiably proud of)
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I don't know if this is relevant, but . . .
Part of lifting something heavy is locking an inhaled breath in your chest -- exhaling while you lift makes it harder. Some of the muscles that contribute to breathing attach to your rib cage and also to your arms; futzing around with them might put pressure on the shape of your chest and affect your ability to hold your breath simply.
Since this is the lifestyle thread, more on the Juicero front:
Ivanka Trump @IvankaTrump
Daily cold-pressed juice we can make at home? Yes, please: bit.ly/juicero #Healthy @juicero #Juicero
One reply around time it was posted (March 31), and then an avalanche starting about 40 minutes ago when someone "discovered" it.