# A physics question regarding water. **URL:** \ **Category:** Factual Questions\ **Created:** [April 23, 2002, 7:38pm UTC](https://boards.straightdope.com/t/a-physics-question-regarding-water/105195 "2002-04-23T19:38:36Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![rookie523](https://avatars.discourse-cdn.com/v4/letter/r/71c47a/32.png) [@rookie523](https://boards.straightdope.com/u/rookie523)\ **Post date:** [April 23, 2002, 7:38pm UTC](https://boards.straightdope.com/t/a-physics-question-regarding-water/105195/1 "2002-04-23T19:38:36Z")
An ordinary sponge is soaked with water so that this wet sponge is 1 kg. The mass of the water is 99% that of the wet sponge. The sponge is left outside for a certain period of time. When you come back, you find that the mass of the water is now 98% of the sponge. How much water is left in the sponge? ---
**Author:** ![Kamandi](https://avatars.discourse-cdn.com/v4/letter/k/5daacb/32.png) [@Kamandi](https://boards.straightdope.com/u/Kamandi)\ **Post date:** [April 23, 2002, 7:47pm UTC](https://boards.straightdope.com/t/a-physics-question-regarding-water/105195/2 "2002-04-23T19:47:28Z")
The system (sponge and water) weighs 1 Kg or 1000g. The water weighs 99% of that, or 990g. Therefore, the sponge weighs 1000 - 990 = 10g. If in the new system the water is 98% of the system, then the weight of the sponge is 2% of the system. If 10g = 2%, then the whole system weighs 10/0.02 = 500g and the new weight of the water is 500 - 10 = **490g**. ---
**Author:** ![rookie523](https://avatars.discourse-cdn.com/v4/letter/r/71c47a/32.png) [@rookie523](https://boards.straightdope.com/u/rookie523)\ **Post date:** [April 23, 2002, 8:06pm UTC](https://boards.straightdope.com/t/a-physics-question-regarding-water/105195/3 "2002-04-23T20:06:48Z")
i see almost everything … but where did the 10g /0.02 come from? i haven’t looked at a physics question in 6 years… ---
**Author:** ![UncleBeer](https://avatars.discourse-cdn.com/v4/letter/u/977dab/32.png) [@UncleBeer](https://boards.straightdope.com/u/UncleBeer)\ **Post date:** [April 23, 2002, 8:11pm UTC](https://boards.straightdope.com/t/a-physics-question-regarding-water/105195/4 "2002-04-23T20:11:42Z")
The weight of the sponge remains constant. Therefore, in the inital system, the sponge is 100% - 99% of the mass of the entire system. After evaporation, the sponge is 100% - 98%, or 2% of the entire system. 2% obviously equals .02 And this in an algebra question; it’s purely mathematical. Has nothing to do with physics. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [April 23, 2002, 8:13pm UTC](https://boards.straightdope.com/t/a-physics-question-regarding-water/105195/5 "2002-04-23T20:13:41Z")
This is not a physics question at all, it’s algebra. > [@](#): > > If 10g = 2%, then the whole system weighs 10/0.02 = 500g The water weighs 98% of the sponge and the water together, therefore the sponge weighs 2% of the two. We already established that the sponge weighs 10g. So if 10g is 2% of the total weight, that is expressed as the formula 10g = 2% \* total g =\> 10g = 2/100 \* total g =\> 10g = 0.02 \* total g =\> 10g/0.02 = total g ---
**Author:** ![rookie523](https://avatars.discourse-cdn.com/v4/letter/r/71c47a/32.png) [@rookie523](https://boards.straightdope.com/u/rookie523)\ **Post date:** [April 23, 2002, 8:28pm UTC](https://boards.straightdope.com/t/a-physics-question-regarding-water/105195/6 "2002-04-23T20:28:11Z")
ahhh i see the light… of course, i haven’t taken algeo in 6 years either… studying psychology tends to dull the math senses… i should really brush up on that…