# Thought Experiment: A dynamically variable weight set **URL:** \ **Category:** Factual Questions\ **Created:** [April 6, 2011, 6:59pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437 "2011-04-06T18:59:01Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![The\_Controvert](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@The\_Controvert](https://boards.straightdope.com/u/The_Controvert)\ **Post date:** [April 6, 2011, 6:59pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/1 "2011-04-06T18:59:01Z")
You know how sometimes there’s a neat science fiction technology that the writer uses to advance the story line, but upon further thought, you realize there are much better uses for that sort of technology? Example, time machines. So how about this? A character in the story works out with a newfangled weight set. But it’s just a single bar with some buttons that adjusts the weight. So the character doesn’t need to fumble with heavy weight discs and doesn’t need a spotter. But I’m sure a material that can control its apparent weight (say by a factor of 10) at the push of a button has much better practical applications. Would it result in perpetual motion and free energy? Can conservation of energy and conservation of mass be preserved? ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [April 6, 2011, 7:03pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/2 "2011-04-06T19:03:59Z")
If you could change the apparent weight, you could have free energy. Put weights on both ends of a see saw, change them alternately from 100lbs to 1000lbs and you could attach a generator to it. ---
**Author:** ![The\_Controvert](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@The\_Controvert](https://boards.straightdope.com/u/The_Controvert)\ **Post date:** [April 6, 2011, 7:11pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/3 "2011-04-06T19:11:38Z")
Unless the weight bar contained an energy pack that uses more energy to adjust the weight than what you get from the generator. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [April 6, 2011, 7:26pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/4 "2011-04-06T19:26:49Z")
> [@The\_Controvert](#): > > So how about this? A character in the story works out with a newfangled weight set. But it’s just a single bar with some buttons that adjusts the weight. So the character doesn’t need to fumble with heavy weight discs and doesn’t need a spotter. Wouldn’t it be easier to place the weight bench and bar over a variable electro-magnet of some sort and just increase the field as you “raised” the weight? Granted, that doesn’t address the heart of your question. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [April 6, 2011, 7:40pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/5 "2011-04-06T19:40:07Z")
> [@The\_Controvert](#): > > Unless the weight bar contained an energy pack that uses more energy to adjust the weight than what you get from the generator. Exactly. We already have devices that can change their apparent weight. A hot-air balloon, for example, uses heat to reduce its apparent weight. An air compressor with a high-pressure tank can increase its weight by pumping more air into the tank. They don’t violate the conservation of energy because they require energy to operate. ---
**Author:** ![The\_Controvert](https://avatars.discourse-cdn.com/v4/letter/t/dec6dc/32.png) [@The\_Controvert](https://boards.straightdope.com/u/The_Controvert)\ **Post date:** [April 6, 2011, 7:41pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/6 "2011-04-06T19:41:16Z")
If you use an electro-magnet, how do you keep the apparent weight constant as the bar is lifted to various distances from your magnet? ---
**Author:** ![Shodan](https://avatars.discourse-cdn.com/v4/letter/s/9f8e36/32.png) [@Shodan](https://boards.straightdope.com/u/Shodan)\ **Post date:** [April 6, 2011, 7:51pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/7 "2011-04-06T19:51:09Z")
[Nautilus machines](http://www.livestrong.com/article/237503-about-nautilus-exercise-equipment/) are built around the concept of variable resistance. Regards, Shodan ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [April 6, 2011, 7:53pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/8 "2011-04-06T19:53:44Z")
> [@The\_Controvert](#): > > If you use an electro-magnet, how do you keep the apparent weight constant as the bar is lifted to various distances from your magnet? You have a physically large electro magnet. It needs to be big enough for the distance of the weight above the magnet is much smaller than the width of the surface of the magnet. This will approximate an infinite plane of magnets and the force will remain relatively constant over the range of motion. A more feasible idea is to vary the strength of the magnet based on the position of the weight. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [April 6, 2011, 8:23pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/9 "2011-04-06T20:23:54Z")
> [@gazpacho](#): > > A more feasible idea is to vary the strength of the magnet based on the position of the weight. Seems like an even more feasible method would be to use some kind of a hydraulic fluid system to adjust the apparent of the bar, based on an easily adjustable control lever. We used hydraulic loaders on our farm tractors nearly 50 years ago; that’s a pretty well-understood technology. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [April 6, 2011, 8:26pm UTC](https://boards.straightdope.com/t/thought-experiment-a-dynamically-variable-weight-set/577437/10 "2011-04-06T20:26:12Z")
> [@The\_Controvert](#): > > If you use an electro-magnet, how do you keep the apparent weight constant as the bar is lifted to various distances from your magnet? I dunno. It’s what they always use in comics. 🙂