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Energy used in doing work


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Hello everyone,

lets say we have a object of mass 1 Kg kept on a table it will have energy at rest & potential energy. If I lift it to 1m up and kept it to same location back, object will have same energy.

Can we say all energy is used in motion of object. Where that energy go? is it wasted/used? or converted to other form?

Hope I have made myself clear.

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When you are lifting the object you are doing work (W=f*d=m*g*h). By lifting the object you have done POSITIVE work. When it falls back to the ground the EARTH does positive work and you have done NEGATIVE work. This sums to having done ZERO work. Work is just an easy way of saying change in energy.


Hope I've helped.

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First you spend energy to get the object up. The object gains potential energy.


When it comes back down again, all that energy has become heat. Normally, the potential energy would be converted to kinetic energy first. It would accelerate under the influence of gravity. There is no reason to stop, except friction. This can be a break, or a parachute, or just the table or ground that stop the object. In all those cases, the energy was converted to heat. (But not very much, so it's hard to measure/notice the heat when it falls from just 1 meter).

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Also worth noting is that your muscles consume energy both when lifting the object up, and lowering it down again gently.. This is not a necessity. A machine could be built which re-absorbed some or most of the energy it used to lift the object, it's just the way muscles work (resisting motion is close to the same process as moving in the first place). They also consume some energy when exerting a constant force (ie. just holding something heavy). A table (or even electric motor if it has some kind of lock/ratchet/etc) does not have to do this.

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