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How much energy is saved when using luggage with wheels as opposed to carrying the luggage? Thanks.

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closed as too broad by user10851, Kyle Kanos, John Rennie, Kyle Oman, Qmechanic Jun 20 '15 at 7:03

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ I think there are too many variables, at the moment, to give a decent answer to this question. Could you try narrowing it down some? $\endgroup$ – Kyle Kanos Jun 20 '15 at 2:02
  • $\begingroup$ Transporting an 8 pound piece of small luggage across a flat surface, is more energy used just carrying it or pulling it (if it had a wheel)? $\endgroup$ – Pepe Jun 20 '15 at 4:16
  • $\begingroup$ Energy used when lifting and walking the object = gravitational potential energy of the object after lifting it to height h + energy used to transport the object from point A to point B. Energy conserved when wheeling the object = gravitational potential energy which was used to lift the object to height h - the amount of friction the wheels face. However notice that the amount of energy you spend to lift the bag is RETURNED at the end when you place the bag down again, with the only disappointment being that you cannot possibly utilize that energy for anything useful (mostly). $\endgroup$ – Youstay Igo Jun 20 '15 at 11:28
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There is no fundamental physics reason why carrying a bag should use more energy than wheeling it. Whether you be carrying a bag or wheeling it, you are essentially sliding it along a line of almost constant gravitational potential (aside from a little jiggling up and down with your stride), so the bag's total energy isn't changing and in theory does not need to be topped up.

The reason why animals like us expend more energy carrying than wheeling their bags is wholly to do with how animal muscles work. If you could clamp your hand shut around the handle, you would expend no effort carrying the bag. A muscle, even though it does no work on the bag, must be constantly supplied with energy in various chemical forms by the body to stay in a contracted state. This work is dissipated as heat in your forearm muscles as they holds the hand clamped shut. The process is explained in more detail in Malabarba's Answer to the Physics SE Question, "Why does holding something up cost energy while no work is being done?".

A further two factors are that:

  1. In the contracted state, muscles are metabolising ATP and other energy stores and outputting waste, which swiftly "poisons" the tissue if it builds up faster than the circulatory system can get rid of it, leading to a lowered ability of the muscle to work and "tiring";

  2. The contracted state of a muscle, and the accompanying stretched state of connective tissue actually hinders blood circulation to both.

Your muscles and connective tissue can brook neither of these things for long without needing a rest: this is why your arm feels like it's going to drop off if you need to walk without a wheeled truck for you bags for long.

AS for the exact amount of energy used by a muscle to stay in the contracted state, i.e. a quantitative answer to your question, you may need to put this to the biology stack exchange.

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  • $\begingroup$ Then why is it so much easier to pull along luggage with wheels than to carry it? Why are most people using wheeled luggage, why not pick it up and carry it? It sure seems easier to pull. Never mind the hand grasping it, assume it's latched onto your belt and it's just being pulled. Thanks. $\endgroup$ – Pepe Jun 20 '15 at 14:31
  • $\begingroup$ You do indeed need to apply less force to a bag to pull it on wheels than lift it up. This is different from the energy required. If it's latched onto your belt, then you are describing a backpack, which is a great deal easier than holding onto it with your hand. Clipped to your belt, it would almost certainly interfere with your gait. $\endgroup$ – WetSavannaAnimal Jun 21 '15 at 0:08

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