Check my math (I'm a science dummy)

[bvz]

It appears that gravity is actually a pretty lousy mechanism for storing energy.

 

Could someone check/correct my logic here?

 

Assuming that an average house weighs about 110,000 kg (got this number from yahoo answers). This means that if you raised this house by 1 meter, you would be "storing" 110,000 Joules? (I get confused by the time element here).

 

If that's the case, and if you were to allow the house to settle the 1 meter over the course of about 8 hours, you would get back roughly 30 Watt Hours during that time - or enough to power a single 100W bulb for the duration of the sinking house. That seems incredible to me. Did I do the calculations right?

 

Just a thought experiment really. The idea of a house rising and sinking a full meter every day is absurd on the face of it anyway. Especially if the payback is so ludicrously small.

 

Bet I'm not the first to consider the idea and not the first to be disappointed at the results.

[Xittenn]

[math] U = m \cdot g \cdot h [/math]

 

[math] U = 110,000 \cdot 10 \cdot 1 = 1,100,000 \: Joules \: \neq \: 110,000 \: Joules [/math]

 

I didn't know the efficiency of forces was in question but irregardless gravity is considered a weak force when compared to other similar forces such as the strong nuclear force.

Edited January 13, 2011 by Xittenn

[bvz]

Thanks for the quick reply.

 

Could you elaborate a bit further on the math? I am mostly trying to figure this out with the bare minimum of experience with physics.

 

I looked up the definition of a Joule on wikipedia and the formula given was:

 

1J = 1Nm

N = kg x m / s²

 

so J = kg x m² / s²

 

 

(that was the time component that I didn't/don't get).

 

In your formula, I don't understand what the term g represents. Also, why is U the term you use for Joules? What does that stand for?

 

Sorry for all the basic questions but I have not had any physics since high school, and even that was a very long time ago.

 

 

 

P.S.

I would never think to disparage gravity. It's what keeps me grounded in my life

[John Cuthber]

Irregardless isn't a propper word.

http://en.wikipedia.org/wiki/Irregardless

 

Anyway, if your house was on the moon it would be easier to lift, so lifting it by one metre would store less energy.

Here on earth the strength of gravity (near the surface) can be considered as equivalent to an acceleration of about 9.8 m/s/s

 

So to calculate the stored energy you multiply the mass by the local value of the acceleration due to gravity

9.8*110000

That gives 1078000 Newtons of force

Work is force times distance, the distance is just 1 metre so the energy stored is 1078000 Joules

Power is work/time

There are 28800 seconds in 8 hours so the power is about 37 Watts

[bvz]

Aha. And that takes care of that pesky time component that I couldn't work out. Thanks.

 

My neighbor had his house lifted to add another floor beneath it and it got me thinking: How much energy did it take to lift that house, and how much of it could you get back if it were to be lowered to its original position again? And could something like this be used as a decent mechanical battery? The answers, it seems, are "not that much" and "not really".

[John Cuthber]

The packet of biscuits in front of me tells me that each biscuit gives 292000 Joules.

The energy stored is roughly the equivalent of 3 or 4 biscuits. (less if they are chocolate.)

[SMF]

In light of this question it interesting to note that a hydroelectric dam is using gravity to make electricity. This is quite efficient because the sun does the lifting of the water for free. However, water batteries that involve pumping water up hill to a reservoir for later power generation are already in use. They are about 75% efficient, which is not bad. What makes this strategy cost effective is the fact that many power sources, such as nuclear or coal, are difficult to modulate according to changes in the load, so there is a lot of wasted power during off peak times. The off peak power can be stored by pumping water up hill. SM

Edited January 14, 2011 by SMF