Now I'm trying to give an answer of the question. Satellite rotates about earth by the gravitational force of earth. Now if we place the satellite in such an orbit so that it rotate in opposite to earth rotation and having a greater angular rotational speed.   Now there is an existence of earth magnetic field. Though it is very poor but strength of magnetic field is not the major factor of energy conversion. So as in electrical machine rotor rotates and stator gives magnetic field and electricity generates is it not possible to extract gravitational energy by using satellite as rotor and geomagnetic field as stator? how much it is effective?   Edited: OK eventually this question becomes duplicate. Now I am asking how can we store this energy. Can we store this energy? or not.
Since the Earth has a magnetic field, can it, in theory, be run through a conductive metal coil to create electricity?
Why are date-based hats awarded too early? I just got the  hat which states "post or vote on December 21".  Does SE have a time machine now? How to query MongoDB with "like"? How can momentum but not energy be conserved in an inelastic collision? Bug with Excavator Badge I edited this post today  on stack overflow. I was approved of the edit, so I waited a few minutes to get my  badge because this answer was posted on Jan 28 '10 at 1:06, but it never came. I have read multiple articles on meta that there have been bugs with this badge, but they have been about winning it multiple times (), or earning it then losing it (). Am I missing something here? Just don't show UI for actions that can't be actioned Recently I just deleted one of my answer in stackoverflow where there was one comment. After deleting the answer it also show for sometime that we all know but in the comment it still allow us to up to make it useful but as I clicked it it just showed me the message:  If we are not allowed to do then what was the meaning? Why don't it just be removed or make it disable if the answer is deleted. Just don't show UI for actions that can't be actioned I have:  &lt;?php  $file=fopen(date("Y-m-d").".txt","r+") or exit("Unable to open file!");  if ($_POST["lastname"] &lt;&gt; "") {    fwrite($file,$_POST["lastname"]."\n"); }  fclose($file);  ?&gt;   but it overwrites the beginning of the file.  How do I make it insert? Getting "Not found" message when running a 32-bit binary on a 64-bit system Rubik's Cube Combination Could anyone explain why the number of legal or reachable combinations of a $3\times 3\times 3$ Rubik's Cube is $1/12\mbox{th}$ of the total. I understood the logic behind the total number of combinations: $8! \cdot 2^{12} + 3^{12} \cdot 12!$. What I am not able to understand is why do we divide this quantity by $2 \cdot 3\cdot 2$. I don't have any idea about parity or group/set theory. Is there another possible explanation for it? How is a Kelvin Pod different than a standard Escape pod? Why shouldn't we allow NULLs? I cannot boot to the desktop because I filled the file system. How do I reclaim some space? My memory is completely full when I tried following the data recovery and copied the wrong file using command line tool Test Disk. As it was taking so long to copy, I turned it off and I realized I copied the wrong file. Now my laptop memory is now full and lead me to more problems.  I have had similar problems to :       Now my screen keeps blinking:   Can anybody help? Many thanks. Wish you all the best. Currently, a user does not get any notification when a trivial edit (less than 6 characters, by a user who has the ) is made on one of his/her posts. As in .  Even though the user is trust-worthy (owing to the rep of the user), wouldn't it be nice for the OP to get a notification whenever an edit is made, even trivial? Can truesight see through a druid's Wild Shape? The rules for truesight state:     A creature with truesight can (..) perceive the original form of a shapechanger or a creature that is transformed by magic   A druid player argued that since the druid is still a "humanoid" (not a "shapechanger"), and since , a creature with truesight should not see through it.  As the DM, I ruled against it because I believed the intent of truesight is to see through all kinds of shapechanging effects but I was still left wondering if I missed anything. I couldn't find anything about this in the errata either.  So, going by strict RAW, should a creature with truesight be able to see the druid's true form while a druid is using wild shape? Suppose that $f:\Bbb R\to \Bbb R$ is continuous and that $f(x)\to 0$ as $x\to \pm\infty.$Prove that $f$ is uniformly continuous. Suppose that $f:\Bbb R\to \Bbb R$ is continuous and that $f(x)\to 0$ as $x\to \pm\infty$. Prove that $f$ is uniformly continuous.   Since $f(x)\to 0$ as $x\to \pm\infty$ we have $|f(x)|&lt;\epsilon $ whenever $x&lt;-K$ and $x&gt;K$ where $K&gt;0$.  Also $f$ is uniformly continuous on $[-K,K]$ as the domain is compact.  Hence $f$ is uniformly continuous in $(-\infty,-K)\cup [-K,K]\cup (K,\infty)$.  But how to conclude that $f$ is uniformly continuous on $\Bbb R$ from above as I may chose $x\in (-\infty,-K);y\in [-K,K]$ ,how to conclude that $|f(x)-f(y)|&lt;\epsilon $ from above for a given $\epsilon$?  Please suggest the edits required. Changing Default Apache version on MacOS Sierra A DHS sweep of the network showed that one of the researchers Macs is running a non-safe version of Apache (2.4.25) and they would like him to update to at least 2.4.27.  He is having difficulty getting MacOS to use the new version of Apache rather than the one installed by default with Sierra. His issues are:  Mac OS X 10.12.6 Sierra   I have installed httpd 2.4.27 . . .  /usr/local/apache2/bin/ but the default httpd running is located at /usr/sbin/ I cannot install a new httpd into /usr/sbin/  - no write persmissions for sudo I made a root user active - but "root" does not have write permissions for /usr/sbin/ either   I cannot find any source on the web that describes how to change the httpd path from the default system version (/usr/sbin/httpd) to a new USER installed version, like /usr/local/apache2/bin/httpd  What steps am I missing? How many irreducible monic quadratic polynomials are there in $\mathbb{F}_p[X]$? Can some of you help me with my homework?     I had to count the irreducible, quadratic, monic polynomials in $\mathbb{F}_p[X]$ for arbitrary $p$.    I will show you what I tried myself.  Research effort  First of all the set $\{X^ 2 + bX +c : b, c \in \mathbb{F}_p \}$ contains $p^2$ elemenst. We need to substract the amount of elements that is reducible. These elements are exactly the set $A = \{(X+c)(X+d) : c, d \in \mathbb{F}_p \}$. There are $p(p-1)$ pairs of $(a,b) \in \mathbb{F}_p \times \mathbb{F}_p$ such that $a \neq b$, and there are $p$ pairs such that $a=b$.     When $a$ and $b$ are unequal, whe know that $a$ and $b$ can be swapped without changing anything to the polynomial $(X+a)(X+b)$, so I obtained that $|A| \leq p +(p-1)- \frac{1}{2}(p-1)=p-\frac{1}{2}(p-1)$.  In other to find out these are the elements, or that there are still less, I thought I had to assume that $(X+c)(X+d) = (X+\gamma)(X+\delta)$, to deduce some information about the realation between $(c,d)$ and $(\gamma, \delta)$, but this led to taking square roots in $\mathbb{F}_p$, and I am not familiar with that.  Could you please provide me some more information? Getting percent sign into an URL in a footnote I'm using the url package. Now, I have some some url with a percent sign in it and want to place it in a footnote. When I let the % where it is, I get a comment, and the whole thing fails to compile because of some unmatched brackets. When it's at the end of a line, it doesn't show up at all in the result. When I write \% instead, this gets literally into the output—but I want the url to be displayed correctly.  What could I do? Where can I find Ghost/Poison/Psychic type Pokémon? Are there fields and ordered fields of every infinite cardinality? On the top of my head, I cannot think of any fields of cardinality more than that of the reals. (It is known that the process of algebraic closure does not increase the cardinality of an infinite field.)  What is the simplest way to give an example of a field (and an ordered field) of a specific cardinality $\alpha$?  I see there is the "Field" of surreal numbers, but it is a proper class rather than a set (and hence do not have a cardinality as such). However, there seems to be some modified construction which gives proper fields with the cardinality of some strongly inaccessible cardinal. Questions about right ideals