Suppose you lift a box from the ground over your head. You will be using energy as there is work done. But when you keep the box lifted over your head, aren't you using any extra energy?
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1You need to differentiate between physical work & energy with Biological work & energy. The answer to your question can also be found out in the following example: Why is work done in both ascending the stairs and using a lift to cover the same height. Here work done does not refer to biological work done. – Sagnik Jan 18 '14 at 07:21
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Possible duplicates: http://physics.stackexchange.com/q/1984/2451 and links therein. – Qmechanic Jan 18 '14 at 08:25
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Yes, your body is utilizing stored chemical potential energy when you hold a box above your head.
Our muscles work in a funny way: it takes energy for them to flex. In other words, it takes energy for our muscles to exert a force, even if there isn't any large-scale macroscopic movement, such as in your box example. For us muscled beings, there is actually movement happening on the cellular and even molecular levels. Note that, in the usual cases you study in classical mechanics (think of a spring), most forces can be exerted indefinitely since no energy is "being spent."
(This whole muscle contraction business is quite fascinating and is worth researching, but is better elaborated on by biology and chemistry. See here for a useful Google search.)
BMS
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In short, No !
While you are lifting an objectupwards, you are moving it against the action of gravitational pull of the earth just like stretching a spring. While you are pushing it up you do work against gravity on the object which is stored in it as it's gravitational potential energy.
However when you have lifted the object to desired height and just keep it there, no work is done as no movement against any force is done. You may feel that you are doing work because of pain in your hand but pain is not characterised by work.
Rijul Gupta
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You are right in the case of springs or hydraulic arms or suchlike - but with human muscles you cannot hold the box above your head indefinitely. You will get tired and will have to put the box down, which suggests your muscles are expending energy just to hold the box in place. – Nathan Reed Jan 18 '14 at 08:52
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Our muscles expend energy even when we are not lifting somthing. As for getting tired, The longest time on a balance board holding a full glass of wine on the forehead is 1 hour and 8 minutes. Obviously I cant do it, so that does not mean that I do not have that much energy to keep my arms in that position it depends on person to person who can keep which posture for how long. – Rijul Gupta Jan 18 '14 at 09:06