Hands off other than during the multiple times compressing it. Spring ends in same position. Net work done?
I’m not sure why “net work” would be a meaningful number here. But if you repeatedly do work on a spring, and the spring doesn’t do any work on you, then obviously “net work” is positive (non-zero).
Either the system isn’t frictionless, or it’s not settling to rest. If it settles to rest, then ultimately, work is being done against whatever the force is that’s making it settle. If it’s frictionless, then work is done on the mass to get it oscillating, and the energy remains in the oscillating mass-spring system.
By this I was referring to net work done by you, on the spring.
If you mean net of all work done on/by the spring, then it’ll be zero for an ideal spring (i.e. no internal friction) if it ends up stationary at the same length as in the beginning. As Chronos notes, this will not happen if the spring itself and its environment are frictionless.
The comment was made that motor units are doing no “net work” since they end up in the same position and I am trying to clarify my understanding of what is meant by that.
I don’t think so.
Spring is compressed/stretched/deformed and that deformation of the material cause some energy loss as heat - again, second law. Same as a vertical spring with a weight pulled down and released even in a vacuum - it won’t be a perpetual motion machine.
On edit I see the comment about “internal friction” - not so idealized. There is some heat loss. I am trying to make a rough analogy to muscle function here.
When you compress the spring, you are doing work on the spring.
When you let the spring uncompress while still applying a force, the spring is doing work on you.
If the spring ends up where it started, then the net work is zero - you’ve got back all the energy you put into the spring.
And again, letting go of the spring without still applying force, “hands off” when not compressing it. I’ve received no energy back. The energy the spring lost was to heat loss due to material deformation in this case.
Again, I am only focusing on “net work” because it was claimed that such is “what is usually meant” when discussing work.
Hence we have established that do 100 reps of bench presses of 150 pounds does no physics work, in that usual sense, if the barbell is returned to its original position. (Gravity is conservative.) Individual motor units inside complete muscles function a bit differently, a slight bit closer to what I am attempting to set up as my hypothetical with the spring (albeit still a very imperfect analogy) and I am trying to get closer to understanding if by these terms there actually is no work being done by muscle fibers within the frame of reference of a body really ever, even though the body as a whole can perform work on objects.
Yes, but the OP asks about expending calories. And under Earth’s gravity, he most certainly is.
I feel I’m repeating myself. If you push a spring to compress it, then let go, yes, you have done a non-zero amount of net work.
I really don’t see the analogy here.
If you are looking at a real-world situation (e.g. not ignoring friction), and consider holding a weight and moving it up and down, the individual muscle fibers do non-zero net work, because they are working against various frictional losses.
But I don’t think that’s why your muscles get tired when holding a weight in the air. They get tired because muscles burn energy just to keep contracted, like a helicopter that uses fuel just to stay in one spot. Or a car on an uphill slope using the clutch to stay still.
And talk about repeating one’s self. No that is not how muscles work when engaged in an isometric contraction as in holding a weight overhead. They do not simply statically “keep contracted.” Each muscle consists of multiple motor units each made up of variable numbers of muscle fibers Different motor units also have different sorts of muscle fibers with different contraction patterns. A muscle, even at apparent rest, is constantly firing some number of these motor units repetitively. A muscle engaged in significant isometric activity is firing these individual motor units more often and more of them at a time and also rotating through them as some fatigue. All of the fibers within any given motor unit fire in response to one motor neuron’s command, contracting (by sliding fibers across each other) and producing force across a distance, and then with the addition of energy in the form of ATP elongating again to the same original length. That’s where the increased caloric need comes in. And in energy lost as heat. We are far from perfectly efficient machines.
They do return to the same length after each contraction. The process of holding a weight steady overhead for ten minutes requires many thousands upon thousands of muscle fibers taking turns contracting, producing force across distance, and then returning to the same length, many many many thousands of times. But they do end at the same lengths on average. By your take on the spring analogy this seems to nevertheless count as net work.
Anyway a stationary aloft helicopter is also doing work, real physics work. It is moving a mass of air some distance which is what produces the force that keeps its weight aloft. I cannot comment too intelligently about the clutch example but I suspect highly that it is not like either of the other two.
I still don’t see what analogy you were trying to set up by talking about friction-less springs. Muscles are far from being friction-less.
Chronos was in response to my question focusing on the work as the work needed to overcome the friction, which was immaterial to the pertinent points of the crude and imperfect analogy. The major issue in muscle fiber function is not overcoming friction. Loss as heat, yes.