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So there is this problem in my head, it may seem very simple, but I need an answer. The problem is, we have some object, and we apply a force by our hands with a constant speed from a position (1) to (2). And there is a theorem which says that the work done to displace an object from pos.(1) to (2) is the difference of kinetic energies of the second minus the first. But the thing is " we're moving it at a constant speed" which means by the equation the work becomes zero, but its not a logical answer since there has been a displacement..

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  • $\begingroup$ ...why does it matter that there's displacement when there is no force? $\endgroup$
    – ACuriousMind
    Commented Mar 3, 2016 at 18:59
  • $\begingroup$ I edit the question, I meant by moving it, applying a force... $\endgroup$
    – user65035
    Commented Mar 3, 2016 at 19:02
  • $\begingroup$ we move it with a constant speed from a position (1) to (2)....., so how it was moved ...i just wonder. $\endgroup$
    – drvrm
    Commented Mar 3, 2016 at 19:02
  • $\begingroup$ If you apply a force the object doesn't move at constant speed. Or, well, if you apply a force such that the object moves at constant speed, then yes, no work is done because the force then is orthogonal to the displacement. $\endgroup$
    – ACuriousMind
    Commented Mar 3, 2016 at 19:03
  • $\begingroup$ Oh okay now I understand, but what if the time of the process is 20 sec, and I began analysing the process at time =3 sec to time equal 18 sec, so there is no change in the velocity and there is the force done by my hand :) $\endgroup$
    – user65035
    Commented Mar 3, 2016 at 19:15

2 Answers 2

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Considering to your question and comments below it, I think that you have been confused about some assumptions. For example, when we assume that there is no friction, you should be able to imagine a situation that there is no friction in that. You shouldn’t imagine real life for that assumption, because in your common daily life, you cannot find a perfect friction-less experience.

We have some object, and we apply a force by our hands with a constant speed

If we apply a force, and if this force is the only force that is exerted to the object (try to imagine this situation); then the object won’t move with a constant speed. Because $a=\large{\frac Fm}\neq 0$.

If we apply a force and the object moves with a constant speed; then our force isn’t the only force that is exerted to the object. For example, it may that there is a gravitational force that makes the net force becomes zero.

We're moving it at a constant speed which means by the equation the work becomes zero

What is the work in work energy theorem? That work is work done by all external forces acting on the object. As I mentioned above, if you are exerting a force on an object and it is moving with a constant speed, your force isn’t the only force acting on the object. According to the work energy theorem work done by all external forces is equal to zero. I.e. work done by you + work done by other forces (for example weight force)=$0$. Your work isn’t zero. Net work done on the object is zero ($W_{\textrm{net}}=\Sigma W$)

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  • $\begingroup$ Oops thank you a lot for explaining , it was a misunderstanding in the principles.. :/ $\endgroup$
    – user65035
    Commented Jun 22, 2016 at 19:26
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actually you got it wrong work energy theorem states that the net work done is equal to change in k.e here you have to concentrate on the word net .. because net work done work done by you and work done by frictional force is zero......

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