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Old Jan 12th 2015, 09:13 AM   #1
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potential energy

Suppose, I have an object of mass 'm' kg. By applying a force of F=mg, if I withdraw this object with constant velocity and place it at 'h' metre from ground level, then potential energy stored in it will be "mgh". If I apply the force more than 'mg', what will be the amount of potential energy stored in it? If in this case (when I will apply force more than 'mg' to place it at 'h' meter from ground level), the potential energy equals to 'mgh', how it is possible?
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Old Jan 12th 2015, 09:56 AM   #2
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If you apply an upward force F greater than mg, then you will be lifting the object and it will be accelerating upward. The sum of forces acting on it is F minus its weight mg, and since F=ma you have:

a = (F-mg)/m = F/m - g

Its potential energy is mgh, but now h is a variable with respect to time:

h(t) = h_0 + (1/2)at^2 = h_0 + (1/2)( F/m - g)t^2

So its PE is:

PE = mgh(t) = mgh_0 + (1/2)(Fg-mg^2)t^2

And because the object has velocity v(t) = at, it also has kinetic energy:

KE = (1/2)mv^2 = (1/2)m(at)^2 = (1/2)m(F/m-g)^2t^2
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Old Feb 8th 2015, 09:17 PM   #3
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Hi ChipB - I see you're at it again. You know very well that you're not supposed solutions to problems like that and you keep on doing it thus violating the forum rules. Why do you keep doing that? See

Must read before posting a problem : please show your attempt!
Must read before posting a problem : please show your attempt!
The Helpers and the staff of the forum have agreed that they won't give a full answer to a problem posted with no attempt (except maybe in some cases). The Helpers are here to help you to understand your weaknesses by going through the asked questions with you and not here to solve your physics problems. It is imperative to show an attempt to a problem. If you don't know how to start, say it. Helpers will give you a push in the right direction.
I recall mentioning this to you in the past and you've always ignored it. That's why I left this forum.
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Old Feb 9th 2015, 04:38 AM   #4
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Pmb - sorry if I caused you to leave this forum. When I answered this question I went into some detail based on my judgment that this was not a homework problem, but rather a question from someone trying to understand how work and energy inter-relate. In rereading it I think now I probably went into too much detail, providing a detailed answer to a question not asked. In retrospect I should have simply noted that the excess work done by the force increment above mg goes into KE.
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Old Feb 9th 2015, 08:42 AM   #5
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With Energy of Del(PE) is also Energy of Del(KE)

Back to the student's interesting statement...

Isn't it fact (a Block as system) that del(PE) from stationary at z(1) to stationary at z(2) cannot happen without meaningful del(KE)'s along the way? Even idealized, increments of del(KE) must happen.

Stated elsewise, when a block is lifted there is always energy (work)
associated with the KE activity. Even with initial/final speeds zero. Where does that energy go?

TSH for PHF
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Old Feb 9th 2015, 09:34 AM   #6
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Originally Posted by ChipB View Post
Pmb - sorry if I caused you to leave this forum. When I answered this question I went into some detail based on my judgment that this was not a homework problem, but rather a question from someone trying to understand how work and energy inter-relate. In rereading it I think now I probably went into too much detail, providing a detailed answer to a question not asked. In retrospect I should have simply noted that the excess work done by the force increment above mg goes into KE.
Thanks Chip. I very much appreciate the apology. It was hard for me to leave but harder to stay at a forum whose members seemed to be doing the homework for anybody who'd ask a question. That's very unethical.

That was the main reason I left. I guess I'll stick around now. Thanks.
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Old Feb 9th 2015, 03:35 PM   #7
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Originally Posted by THERMO Spoken Here View Post
Stated elsewise, when a block is lifted there is always energy (work)
associated with the KE activity. Even with initial/final speeds zero. Where does that energy go?
Yes - I agree - the block must be accelerated from v=0 to some value, then decelerated at the end. It takes energy to do both, and that energy comes from the mechanism that initially accelerates the object and whatever the braking mechanism is that slows the object at the end. Consider an elevator ascending from floor 1 to floor 10 of a building. I think the answer to your question is that work is done by the lift mechanism to increase the car's PE and also to accelerate it from v=0 to some positive value, and then more work is done to decelerate it to v=0 again. That slowing down phase could be accomplished either by (1) removing the upward lift force at just the right moment so that the elevator car glides to a halt at precisely the correct height, in which case it's a simple exchange of KE for PE, or (2) the car gets to floor 10 and brakes are applied to slow it, in which case KE is exchanged for heat. In this second case the work done by the lift mechanism + brakes went into delta PE of the elevator car plus the initial delta KE as it began its lift plus the delta KE as it slowed at the end.
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Old Apr 24th 2015, 08:53 PM   #8
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Magnetism refers to physical phenomena arising from the force between magnets, objects that produce fields that attract or repel other objects. To know more about the magnetic science fun please visits.
………………………………………………………………………
MAGNETIC SCIENCE FUN
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Old Apr 25th 2015, 03:36 AM   #9
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Originally Posted by jhonmax12 View Post
Magnetism refers to physical phenomena arising from the force between magnets, objects that produce fields that attract or repel other objects. To know more about the magnetic science fun please visits.
Welcome to the forum!

Note: It's improper to advertise in this forum as you did by posting a link to that website.

What you said is not quite correct. Magnetism refers to the physical phenomena that are mediated by magnetic fields. Those fields can only exert forces on charged particles. They also exert forces on currents too but a current is just a charge in motion so it really only affects charged particles. The general expression for the force on a charged particle is called the Lorentz Force and is given by the expression

F = q(E + vxB)

Neither a magnetic field nor a magnet can exert a force on anything by charged particles and a magnetic force can't do work either. When you observe one magnet apparently exerting a force on another magnet what is actually taking place is that the magnetic field produced by the magnet is only exerting a force on the charged particles of which the magnet is made up of. And that includes the intrinsic magnetic moment of elementary charged particles. For details on what I've said about please see the following articles

Magnetic Forces Doing Work by Eugene P. Mosca, Am. J. Phys., 42(4), Apr. (1974). Abstract: http://scitation.aip.org/content/aap...1119/1.1987675
Abstract -Consider a conducting circuit moving with velocity v[/v]moving through a constant magnetic field B. The induced emf is given by (integral vxB*ds where the integral is taken once around the circuit. Some texts refer to b]vxB as the force the magnetic field exerts on a unit charge moving around the circuit. This is incorrect as magnetic forces can never do work. The force the conductor exerts on an electron is shown to do this work.
Work Done on Charged Particles in Magnetic Fields by Charles A. Coombes, Am. J. Phys., 47(10), Oct. (1979). No abstract in article.

What is spin? by Hans C. Ohanian, Am. J. Phys. 54, June 1986
Abstract - According to the prevailing belief, the spin of the electron or of some other particle is a mysterious internal angular momentum for which no concrete physical picture is available, and for which there is no classical analog. However, on the basis of an old calculation by Belinfante [Physica 6, 887 (1939)], it can be shown that the spin may be regarded as an angular momentum generated by a circulating flow of energy in the wave field of the electron. Likewise, the magnetic moment may be regarded as generated by a circulating flow of charge in the wave field. This provides an intuitively appealing picture and establishes that neither the spin nor the magnetic moment are ‘‘internal’’—they are not associated with the internal structure of the electron, but rather with the structure of its wave field. Furthermore, a comparison between calculations of angular momentum in the Dirac and electromagnetic fields shows that the spin of the electron is entirely analogous to the angular momentum carried by a classical circularly polarized wave.
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