標題:
～～Misuse of E = Fs～～
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發問:
Suppose there are 2 forces opposite in direction but same in magnitude adding on a resting object. As net force is zero, the object should remain at rest. As a result, its displacement should also be zero under the 2 forces.By E = Fs,E = F(0)=0 .But obviously it is wrong as energy has really been used.What is... 顯示更多 Suppose there are 2 forces opposite in direction but same in magnitude adding on a resting object. As net force is zero, the object should remain at rest. As a result, its displacement should also be zero under the 2 forces. By E = Fs,E = F(0)=0 . But obviously it is wrong as energy has really been used. What is the reason for this contradictory? Thanks for answering.
最佳解答:
energy has not been transfered to the object actually! what can be the energy gain by the two force? gravitational potential energy?no,it is not getting higher kinetic energy? no,it is not moving i guest you may want to ask if the two things is pushing by machines,the machines must use up energy,then what the energy use for? the machines(e.g motor) would be gettng hotter,as they cannot push an object to move. that's the energy transfer to~the reason of why the motor becoming hotter is another complex question.i won't answer it now. the object has not got any energy due to these type of pushing. if you place an object on an rough inclined plane, it would not move due to friction,there are forces acting oppositely on the object, however,it don't get any energy,no other energy is produced. don't think that force acting must transfer energy. i have ask the same question to myself before.therefore,i can realise your misconception.
其他解答:
I want to clarify what is energy in this equation about. Energy means the FORCE act on the OBJECT multipied the distance it move as you state in the question strictly. For example you move a object with constant velocity upward in certain distance upward, the object gain E=Ws (W=weight, s=distance moved), If you hold the object at the sky, there is no energy gain about the object, s=0 E=W(0), E=0. I think the energy that you mention here what really have been used is about two people act the same force in opposite direction, and they feel tired. The one feel tired, then must some energy used. But this is not right in the theorial physic world. The people feel tired because the muscle contract and cause extra energy needed, then you will feel tired. There may be no energy gain by the object, for example, hold the object in the sky. In the case you talking now. Since the distance is zero, So the energy is zero. Another scenario about your case. There is same forces applied on the object, the two forces are still equal in magnitute and opposite in direction. But it move with constant speed (According to newton first law, the object may rest or move with constant velocity if net force =0) Then the force which direction is same as the motion will give energy E=+F(+s)=Fs. The force which dirction is opposite to the motion will give energy E=-F(+s)= -Fs (-F means the direction opposite to the motion.) The sum of energy = +Fs+(-Fs)=0, the equation still give the same answer, zero Therefore, the equation is not wrong I hope this can help you to understand but not confuse you.
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