✔ 最佳答案
What you said is not generally true. The net work done equals zero for a closed path is only applicable for conservative force (e.g. gravitational force or elastic force on a spring).
For non-conservative force (e.g. friction), the net work done will not be zero.
2014-09-28 15:14:08 補充：
For conservative force, take gravitation as an example, when you raise an object from the floor with your hand, you are doing work.
But when the object falls back onto the floor (with the object still on your hand), work IS BEING DONE onto your hand....
2014-09-28 15:16:28 補充：
(cont'd)...That is, your hand absorbs energy from the falling object. Therefore, after a complete path (from the floor to a certain height and then back to the floor), there is no gain or loss of energy. The net work done is thus zero.
2014-09-30 16:34:18 補充：
Your suppl question:
Feeling "tired" is NOT a physical phenomenon. The use of "human hand" in the example is just for its easy understanding. Human activities indeed involve many physiological processes.
2014-09-30 16:39:06 補充：
(cont'd)...The main point that I want to bring out in the example is that raising an object needs the exertion of a force, whereas lowering the object down is being driven by a force (i.e. gravity). The work done in the two processes thus cancel each other out.
2014-10-02 11:08:18 補充：
Your further question:
I have said above that the use of "human hand" in the example is just for easy understanding of the physical phenomenon. In fact, the work-done equals zero after a closed path refers to a "physical system".
2014-10-02 11:11:59 補充：
(cont'd)...
As said, human activities involve many physiological processes. It is clearly NOT a pure "physical system". A person stands still for hours or even at rest still consumes energy.
2014-10-02 11:21:22 補充：
You may also think of an example that the object is raised by an electric motor. When the object rises, the motor exerts a force and does work. When the objects falls, a force acts on the motor and drives it,i.e. the motor absorbs energy from the falling object.
2014-10-02 11:28:22 補充：
continue on 意見 column...
2014-10-02 11:33:19 補充：
In terms of energy(work done), electricity is consumed by the motor when raising the object. But same amount of electricity is produced by the motor(which acts as a generator) when the object falls. There is no net gain or loss of energy in a complete cycle.
2014-10-02 11:36:54 補充：
Your further question:
Thermodynamics alone cannot explain human activity which involves many metabolic, physiological and chemical processes/reactions inside the human body. The human body comsumes energy even when in complete rest.
2014-10-03 23:30:41 補充：
I have already given the main points in my previous answer. The "work done" in your question is a term used in physics. But you seem to have mixed it with another non-physical intperpretation of "feeling tired".
2014-10-03 23:32:19 補充：
(cont'd)...The net work-done equals zero for a closed path applies only to conservative force in a physical system.
2014-10-05 14:38:01 補充：
I think you are still confused with human metabolism with mechanical work done in physics.
The human body comsumes energy at all times, even at complete rest. But this is not the "work-done" defined in physics. Hence, you cannot say a person is doing physical work when he sleeps.
2014-10-07 19:38:41 補充：
Be aware that human metabolism is not the type of work-done mentioned in your question, as it involves no conservative force.
