# I have a question about weight **URL:** \ **Category:** Factual Questions\ **Created:** [November 6, 2005, 6:34am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766 "2005-11-06T06:34:11Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![copperwindow](https://avatars.discourse-cdn.com/v4/letter/c/4af34b/32.png) [@copperwindow](https://boards.straightdope.com/u/copperwindow)\ **Post date:** [November 6, 2005, 6:34am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/1 "2005-11-06T06:34:11Z")
Let’s say I have a box that weighs 100 pounds. Now let’s say that I take a 5 pound hummingbird and out it in the box and close it. The box now weighs 105 pounds. If the humming bids starts flying and stays in the middle of the box, does it weigh 100 pounds or 105 ponuds? (I am pretty sure it will weigh 105 pounds, but I can’t figure out why…) ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [November 6, 2005, 6:37am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/2 "2005-11-06T06:37:46Z")
- 5 pound hummingbird\* I suggest that the hummingbird be tested for steroids. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [November 6, 2005, 6:42am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/3 "2005-11-06T06:42:11Z")
Let me ask you this: Does an airplane weigh nothing when in flight? Do the clouds have no weight? No! and No! The answer is 105 pounds because forces come in pairs. The force holding them in flight causes a reaction force upon the box. Even fish in an aquarium have weight… ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [November 6, 2005, 6:44am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/4 "2005-11-06T06:44:26Z")
[Here is a classic column you might enjoy.](http://www.straightdope.com/classics/a3_144.html) In this case, the box doesn’t weigh 105 pounds. The bird is supporting itself by flapping. Unless the box is air tight then the system isn’t closed and most of the weight is not transferred to your box. ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [November 6, 2005, 6:46am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/5 "2005-11-06T06:46:28Z")
I was assuming that the box was not air tight. I was thinking of something like a pet carrier. Closed systems have different answers. ---
**Author:** ![EllisDee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ellisdee/32/4531_2.png) [@EllisDee](https://boards.straightdope.com/u/EllisDee)\ **Post date:** [November 6, 2005, 6:48am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/6 "2005-11-06T06:48:03Z")
The OP is confusing mass and weight. The 105 is the total mass, whereas it is only 100 pounds of weight when considering the force it takes to lift it. Weight is a subjective measure that changes based on what planetary body (if any) you’re on. Mass is an objective measure and never changes. ---
**Author:** ![copperwindow](https://avatars.discourse-cdn.com/v4/letter/c/4af34b/32.png) [@copperwindow](https://boards.straightdope.com/u/copperwindow)\ **Post date:** [November 6, 2005, 7:12am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/7 "2005-11-06T07:12:39Z")
Okay, lets say that the bird isn’t flying in the box, but rather somehow magically floaing in mid air? ---
**Author:** ![uglybeech](https://avatars.discourse-cdn.com/v4/letter/u/82dd89/32.png) [@uglybeech](https://boards.straightdope.com/u/uglybeech)\ **Post date:** [November 6, 2005, 8:04am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/8 "2005-11-06T08:04:46Z")
> [@copperwindow](#): > > Okay, lets say that the bird isn’t flying in the box, but rather somehow magically floaing in mid air? Well, weight is a force, and if he’s magically floating - i.e. magically not exerting any force on his environment, then he doesn’t weigh anything. But basically all you’ve just asked is really what does he weigh if he doesn’t weigh anything? I actually have less of an objection to the introduction of magic than I do to a 5 lb hummingbird. By my calculations he would be the size of a large dinosaur. (5 lbs being 100 times the size of the largest hummingbird which is 8 inches long). ---
**Author:** ![treis](https://avatars.discourse-cdn.com/v4/letter/t/bc79bd/32.png) [@treis](https://boards.straightdope.com/u/treis)\ **Post date:** [November 6, 2005, 8:48am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/9 "2005-11-06T08:48:48Z")
> [@Ellis Dee](#): > > The OP is confusing mass and weight. The 105 is the total mass, whereas it is only 100 pounds of weight when considering the force it takes to lift it. > . This is wrong, assuming a closed system the box will weigh an average of 105 pounds. The bird by flapping its wings will exert a force upon the air and then that air will exert a downward force on the bottom of the box equalling the force required to keep the bird in the air. > [@copperwindow](#): > > ``` > Let's say I have a box that weighs 100 pounds. Now let's say that I take a 5 pound hummingbird and out it in the box and close it. The box now weighs 105 pounds. If the humming bids starts flying and stays in the middle of the box, does it weigh 100 pounds or 105 ponuds? (I am pretty sure it will weigh 105 pounds, but I can't figure out why...) > > ``` Force is commony written as F=ma but a different way to write it is F=change in momentum/change in time. (change in momentum=m_change in velocity and a=change in velocity/change in time.) The way a bird flies is that in pushes air with a certain mass and velocity downward. The force that it applies on the air is equal to change in velocity/time_mass. By the law of force reactions there is a force upwards on the bird and a force downwards on the air. Once the air gets to the bottom of the box its velocity is stopped by the box. Thus again force is equal to the change in velocity/change in time\*mass. The value of that force is the same as what the bird is exerting on the air. If the bird must exert 5 lbs of force on the air to accelerate the air downward and the bird upward the bottom of the box must exert 5 lbs of force to accelerate the air upwards (i.e. decellerate). ---
**Author:** ![EllisDee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ellisdee/32/4531_2.png) [@EllisDee](https://boards.straightdope.com/u/EllisDee)\ **Post date:** [November 6, 2005, 9:17am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/10 "2005-11-06T09:17:14Z")
Damn you physicists with your barbaric thought experiments! First you slice and dice that poor cat, and now you’re suffocating an innocent hummingbird! But seriously, good reply, **treis**. Makes sense to me. ---
**Author:** ![A.R.Cane](https://avatars.discourse-cdn.com/v4/letter/a/ee7513/32.png) [@A.R.Cane](https://boards.straightdope.com/u/A.R.Cane)\ **Post date:** [November 6, 2005, 9:29am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/11 "2005-11-06T09:29:12Z")
Please stop, you’re crackin’ me up w/ the 5lb. hummingbird thing. The visual is stuck in my head and my sides hurt from laughing so damn hard. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [November 6, 2005, 9:32am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/12 "2005-11-06T09:32:51Z")
> [@Ellis Dee](#): > > Damn you physicists with your barbaric thought experiments! First you slice and dice that poor cat, and now you’re suffocating an innocent hummingbird! Innocent, hell! The thing weighs FIVE POUNDS!! He could bite your head off! ---
**Author:** ![Nilor](https://avatars.discourse-cdn.com/v4/letter/n/a4c791/32.png) [@Nilor](https://boards.straightdope.com/u/Nilor)\ **Post date:** [November 6, 2005, 9:43am UTC](https://boards.straightdope.com/t/i-have-a-question-about-weight/329766/13 "2005-11-06T09:43:51Z")
> [@Smeghead](#): > > Innocent, hell! The thing weighs FIVE POUNDS!! He could bite your head off! I think he was fed a miniature black hole. A really really small one, mind you.