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Where is the "extra help" coming from when rolling the object on wheels?

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  • $\begingroup$ do the wheels take some of the weight? Have you checked if pulling is better than pushing with a wheelbarrow? $\endgroup$ – user207455 Jul 20 at 17:18
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Your example illustrates the fact that physical effort does not necessarily equal physics work. Even just holding a heavy object without even walking with it is taxing and yet no physics work is done

Hope this helps

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In principle, carrying a weight at a uniform speed on a level surface should not require any energy (if we ignore friction from the air and floor, which is fairly negligible at small speeds). Whether you are carrying the weight by hand or on a wheeled device, no work is being performed.

The reason why humans still feel like they are expending energy when carrying weights has to do not with physics but with biology, and specifically the way human muscles work. As Feynman explains in chapter 14 of the Feynman Lectures:

The word “work” in physics has a meaning so different from that of the word as it is used in ordinary circumstances that it must be observed carefully that there are some peculiar circumstances in which it appears not to be the same. For example, according to the physical definition of work, if one holds a hundred-pound weight off the ground for a while, he is doing no work. Nevertheless, everyone knows that he begins to sweat, shake, and breathe harder, as if he were running up a flight of stairs. Yet running upstairs is considered as doing work (in running downstairs, one gets work out of the world, according to physics), but in simply holding an object in a fixed position, no work is done. Clearly, the physical definition of work differs from the physiological definition, for reasons we shall briefly explore.

It is a fact that when one holds a weight he has to do “physiological” work. Why should he sweat? Why should he need to consume food to hold the weight up? Why is the machinery inside him operating at full throttle, just to hold the weight up? Actually, the weight could be held up with no effort by just placing it on a table; then the table, quietly and calmly, without any supply of energy, is able to maintain the same weight at the same height! The physiological situation is something like the following. There are two kinds of muscles in the human body and in other animals: one kind, called striated or skeletal muscle, is the type of muscle we have in our arms, for example, which is under voluntary control; the other kind, called smooth muscle, is like the muscle in the intestines or, in the clam, the greater adductor muscle that closes the shell. The smooth muscles work very slowly, but they can hold a “set”; that is to say, if the clam tries to close its shell in a certain position, it will hold that position, even if there is a very great force trying to change it. It will hold a position under load for hours and hours without getting tired because it is very much like a table holding up a weight, it “sets” into a certain position, and the molecules just lock there temporarily with no work being done, no effort being generated by the clam. The fact that we have to generate effort to hold up a weight is simply due to the design of striated muscle. What happens is that when a nerve impulse reaches a muscle fiber, the fiber gives a little twitch and then relaxes, so that when we hold something up, enormous volleys of nerve impulses are coming in to the muscle, large numbers of twitches are maintaining the weight, while the other fibers relax. We can see this, of course: when we hold a heavy weight and get tired, we begin to shake. The reason is that the volleys are coming irregularly, and the muscle is tired and not reacting fast enough.

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Carrying a heavy weight feels more strenuous and uses more energy because your muscles are resisting the force of gravity on the load, as well as supplying the forward momentum for it. When you put the load on wheels, as the Viet Cong did with their bicycles on the Hoe Chi Minh trail, then provided you have a smooth, flat surface to travel on, you only have to provide the forward momentum. The energy you would otherwise use in resisting the force of gravity on the load is no longer provided by your muscles. That accounts for the load, but of course you still have the force of gravity on your body to deal with.

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    $\begingroup$ "The energy you would otherwise use in resisting the force of gravity" ---- the question is all about why energy is required to do that. It is no use just saying "you would use energy" when the question is "why do I need to use energy?" $\endgroup$ – Andrew Steane Jul 20 at 18:14
  • $\begingroup$ @Andrew Steane : Perhaps you should read the question again, muscle verses gravity or wheels verses gravity would seem to be the proper answer $\endgroup$ – Adrian Howard Jul 20 at 18:42

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