My friend asked me-

Imagine that we pull up a pen which is initially at the ground slowly upwards.At a certain point we see that its total energy is due to its P.E.So,how can we say total energy is constant?

My thinking is that it does gain some P.E. at that height but as gravity is a conservative force it will regains its previous energy once we put it down to the previous level.Hence the total summation of energy is 0 when we return to the starting position.

Is this explanation correct?

Thanks for your opinion.

  • 1
    $\begingroup$ "In physics, the law of conservation of energy states that the total energy of an isolated system remains constant" - Wikipedia. When you pick up a pen, the pen gains energy but you lose it. The pen is not isolated from the person lifting it. $\endgroup$ – RedGrittyBrick Dec 4 '15 at 15:50

The potential energy of the pen increases as you raise it. This increase in energy is equal to the amount of work you do to raise it. The total energy of the Pen + Raising Apparatus is constant because the energy of just the pen increases and the energy of the raising apparatus decreases.


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