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sleepsleep



Joined: 05 Oct 2006
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sleepsleep
i been thinking about this for some days,
maybe it is a very basic and simple question,

i know, my brain is not so tune to science channel,
so i hope whoever that got some free time willing to explain to me.

thank you.

the problem is like below.

in the left aquarium, we use our hand to push the ball into the bottom of aquarium, so, to keep the ball at the bottom we need to exert energy constantly, and we could feel it.

now, in the center aquarium, we tied the ball to the bottom of the aquarium, to prevent the ball float up, now, from where the ball gets energy to constantly stay in the bottom of the aquarium?

in the right picture, we use a hard steal to replace our hand to push the ball into the bottom of aquarium, the same question, where does the constant energy coming from to keep the ball in the bottom?

i assume, in the 3 situations, the same amount of energy is required to keep the ball in the bottom of aquarium. (or i am wrong)


now, if i am inside the ball (assume we got a big swimming pool) could we feel this energy, we might feel the pressure, but how about energy.


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Post 11 Nov 2012, 04:12
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revolution
When all else fails, read the source


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revolution
You have a bad assumption there. If the ball is not moving then there is no energy being imparted to it. The reason why you might think it takes energy to keep the ball from floating up when using your hand is just the nature of the way our muscles work. If, instead, you could sit on the ball and keep it from floating up then you feel no energy being used.
Post 11 Nov 2012, 05:09
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sleepsleep



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sleepsleep
revolution wrote:

If the ball is not moving then there is no energy being imparted to it.

but isn't it the ball constantly want to move upwards? (even if there is no movement in the ball)
if i let go my hands, or anything that tied the ball to the bottom, it would instantly floats up?

is that correct to say, the ball hold the energy i transmitted to it when i push it to the bottom?

so, if there is not more energy to maintain the ball position at the bottom, it would floats up.

so, if i exert constant energy to the ball, the ball remain same position,
if less, then the ball floats up a little bit
if more, the ball move deeper.

i don't really get if, because it seems, the ball will floats up if there is no "something, energy or etc" to prevent it going up.
Post 11 Nov 2012, 05:41
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revolution
When all else fails, read the source


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revolution
sleepsleep wrote:
so, if i exert constant energy to the ball, the ball remain same position,
No. If you keep supplying energy to the ball then it must move. If you only hold it in place then no energy is being transferred to the ball. The energy your muscles use to hold the ball in place goes to heat your arm. You are heating up the water and surrounding air, but not putting any more energy into the ball.
Post 11 Nov 2012, 05:51
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JohnFound



Joined: 16 Jun 2003
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JohnFound
Force is not equal to energy. Please, read the related articles in your physics text book (or Wikipedia). Smile
Post 11 Nov 2012, 06:14
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malpolud



Joined: 18 Jul 2011
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malpolud
sleepsleep wrote:
is that correct to say, the ball hold the energy i transmitted to it when i push it to the bottom?

Yes it is. The kind of energy is named potential energy. The same kind of energy has a book when you hold it over the ground, or a compressed spring or a compressed gas in a tank, or a charged capacitor.

sleepsleep wrote:
so, if there is not more energy to maintain the ball position at the bottom, it would floats up.

No. You do not need energy to prevent energy from being released.

Check this out: Force is [N] (Newton) [(kg * m) / s^2]

Work is how much force is used to e.g. push something on a distance. You push the wardrobe for two meters. To push it you use certain force. You multiple used force by the distance.

If you push e.g the wall and it does not move, the work equals 0 cause the distance was 0. So if you push something very strongly but it does not move no work is done.

Energy is just work being stored in one of many ways.

_________________
There's nothing special about it,
It's either there when you're born or not.
Post 11 Nov 2012, 10:03
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matefkr



Joined: 02 Sep 2007
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matefkr
energy is a "dumbed down" term for some calculations. it might work sometimes. It is related to mass and velocity. But the energy was always given to them, and obviously given off, if we are to measure it.
Post 11 Nov 2012, 11:00
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malpolud



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malpolud
How is energy dumbed down for calculations?
Post 11 Nov 2012, 11:26
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cod3b453



Joined: 25 Aug 2004
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cod3b453
malpolud wrote:
sleepsleep wrote:
is that correct to say, the ball hold the energy i transmitted to it when i push it to the bottom?

Yes it is. The kind of energy is named potential energy. The same kind of energy has a book when you hold it over the ground, or a compressed spring or a compressed gas in a tank, or a charged capacitor.
...
Actually, energy is transferred to the water - the ball in fact loses energy.

Initially, the ball has a higher gravitational potential energy (GPE). By pushing the ball down into the water and displacing the denser water, the ball is losing some of its GPE and (along with the energy you exert to push the ball down) increasing the water's GPE. Removing the fixing will see the water moving down, losing its additional GPE and displacing the ball back to the surface - increasing its GPE.

If instead the water was drained through a hole in the bottom, nothing significant happens to the ball even when the fixing is removed.
Post 11 Nov 2012, 12:34
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revolution
When all else fails, read the source


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revolution
malpolud wrote:
So if you push something very strongly but it does not move no work is done.
No work is done on the wall, yes. But if you are using your arms and legs to push the wall then work is done by your muscles. Chemical energy is converted to heat (and a few other chemical by-products).
Post 11 Nov 2012, 12:50
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sleepsleep



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sleepsleep
now let say, we are back to several millions years ago,

i am the only human on earth, and i want to calculate this "energy or what"
to solve the question, how much "x or any name" do i need to lift this 10 kg object to a height of 10 meter.

then how much "x" i need to constantly supply to maintain the 10 kg object at the exact position?

assume gravity is constant, the object is on the ground, how am i going to measure how much "x",

is this "x" something measurable?

and i come up with several name for x (based on memory from 2012),

energy, work, force ... but i still have no idea what is what,
what is x, how to determine x, how to measure x.
Post 11 Nov 2012, 13:03
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matefkr



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matefkr
energy is dumbed down, because you have interactions wich can be different based on different particles, while energy is not always exclusively influences these interactions, and you can only define energy based on interactions.
Post 11 Nov 2012, 13:35
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sleepsleep



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sleepsleep
ok, i found a story about energy,
http://www.energyquest.ca.gov/story/chapter01.html

Quote:

The term "joule" is named after an English scientist James Prescott Joule who lived from 1818 to 1889. He discovered that heat is a type of energy.

One joule is the amount of energy needed to lift something weighing one pound to a height of nine inches. So, if you lifted a five-pound sack of sugar from the floor to the top of a counter (27 inches), you would use about 15 joules of energy.


i could lift 1 pound to 9 inches, (one joule)

but what if i lift the 1 pound using 1 hour, slowly up to 9 inches, does it means, i still use one joule?
i don't think so.

or what if i lift the 1 pound to 9 inches using 12 hours? is that still 1 joule?
Post 11 Nov 2012, 13:41
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revolution
When all else fails, read the source


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revolution
sleepsleep wrote:
but what if i lift the 1 pound using 1 hour, slowly up to 9 inches, does it means, i still use one joule?
Yes.
sleepsleep wrote:
or what if i lift the 1 pound to 9 inches using 12 hours? is that still 1 joule?
Yes.

Edit: That is energy you put into the weight. If you are using your arms then remember to account for the heat generated by activating your muscles.
Post 11 Nov 2012, 13:44
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cod3b453



Joined: 25 Aug 2004
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cod3b453
sleepsleep wrote:
...how much "x or any name" do i need to lift this 10 kg object to a height of 10 meter.

then how much "x" i need to constantly supply to maintain the 10 kg object at the exact position?
...
The energy required to raise the object is (approx):
Code:
GPE = mgh
    = 10 kg x 9.8 kg m/s x 10m
    = 980 J    
The gravitational force is (approx):
Code:
F   = ma, a = g
    = 10kg x 9.8 kg m/s
    = 98 N    
Hence an opposing force of 98N is required to keep the ball in that position [or move it at constant speed - as it happens work done is W = Fd = 98N x 10m = 980J].

----

No matter how long this takes the same energy is transferred. As you can see above, there is no time parameter in these calculations.
Post 11 Nov 2012, 13:52
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sleepsleep



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sleepsleep
maybe some people could guess what i wanna write now,

yeah, remote control helicopter to an x pound object.

if i use a remote helicopter with full tank petrol,
stay float at a position 9 inches with 10 pound = 1 joule
from ground to 9 inches = 5 minutes

then in next experiment,
i use the same setup like previous,
but now for 1 hour from ground to 9 inches

now, both is 1 joule (energy)
but, obviously, experiment 1 uses less petrol than experiment 2.

so, in experiment 2, the chemical petrol converted to energy to lift the 10 pound up, but it uses 1 hour to do same work, which mean more petrol gets use, (is that more petrol converted to energy)

and why they didn't put time into energy calculation?
Post 11 Nov 2012, 14:13
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revolution
When all else fails, read the source


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revolution
sleepsleep: A helicopter does more than lift one pound of mass. It is also lifting itself by pushing air downwards. Just hovering requires pushing a lot of air around. It uses more fuel because it is doing more work. You have to account for all sources of energy usage. If you only measure the target mass then you will get a mistaken computation for energy used.
Post 11 Nov 2012, 15:04
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cod3b453



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cod3b453
As revolution hinted, they both transfer 1J to GPE, yes, but your helicopter has transfered the rest to a mixture of electromagnetic and kinetic energy (heat, sound, moving air).

If we eliminate these inefficiencies (and the atmosphere) and take time into account, the only useful value you will obtain is the power of the helicopter P = W/t:
Code:
P = W / t
  = 1J / 300s (5min)
  = 3.33mW
  
  = 1J / 3600s (1hr)
  = 0.28mW    
i.e. in both cases the same amount of energy (1J) is transferred (and no other transfers are occurring) but at a different rate. Once hovering, no change in GPE occurs: 0 = 0 / t - time has no effect.

Now if you add any of the other factors back in you will see a difference in the fuel consumption but this is because this extra energy is being converted to other, unrelated, forms that are a function of time. [Never done thermo-/fluid- dynamics to know what these calculations]
Post 11 Nov 2012, 15:12
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malpolud



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malpolud
sleepsleep no offence, but how come, you being a successful IT person have so small knowledge about basic physics? Wink
Post 11 Nov 2012, 15:38
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revolution
When all else fails, read the source


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revolution
cod3b453 wrote:
If we eliminate these inefficiencies (and the atmosphere) and take time into account, the only useful value you will obtain is the power of the helicopter ...
Erm, a helicopter cannot fly without air. It has nothing to push against. Wink
Post 11 Nov 2012, 15:42
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