Help with science questions.

[dullard]

Suppose you have a black box. The definition of a black box is that it is something that you don't know what it is, or how it works - it could be anything. But lets limit it a bit - the black box does not perform any form of nuclear reaction.

Suppose that black box has a mass of 70 kg (154 pounds of weight on Earth). This 70 kg is the sum of the box mass plus any unknown contents of the box. You add 10 kg of mass with lots of chemical potential energy to the inside of the black box. Finally you remove 8 kg of mass that has low chemical potential energy from the black box. Nothing else escapes (no reaction products, nothing) - so please don't try to alter this!

My questions:
1) How much mass does the black box now have?
2) Why does this answer change if the box is human?

 

[BooneRebel]

If it's an inert mass, then adding 10kg and taking away 8kg gives you a net gain of 2kg to the original 70kg. However, in a chemical reaction (i.e., human), you have to take into account any possible release of energy, conversion from one form to another, etc.

 

[BooneRebel]

Here's a simple example for you:

I have a black box weighing approx 2 tons. I add 20 gallons of gasoline to this mass & drive it around 400-500 miles. The gasoline has disappeared and the box now weighs it's original mass. What happened?

 

[dullard]

<< I have a black box weighing approx 2 tons. I add 20 gallons of gasoline to this mass & drive it around 400-500 miles. The gasoline has disappeared and the box now weighs it's original mass. What happened? >>



You broke a simple laws of nature - Your gasoline combustion products disappeared. Where did the CO, CO2, H20, etc go? Mass cannot be created or distroyed as long as there isn't some form of nuclear reaction. So your example doesn't fit my question...

 

[BooneRebel]

I didn't say the combustion products disappeared, I said the gasoline disappeared. That was the point of my example, a chemical reaction released energy and by-products of the reaction, the exhaust gases.

Your 'human' also processes fuel to release energy and by-products.

 

[dullard]

<< I didn't say the combustion products disappeared, I said the gasoline disappeared. That was the point of my example, a chemical reaction released energy and by-products of the reaction, the exhaust gases. >>


Please, you aren't understanding me. Let me try the question in a different mannor:

Suppose you have a 70 kg black box. You add 10 kg of high energy mass to the black box. ABSOLUTELY NOTHING LEAVES the black box. What would a scale show as the mass of the black box? Why would the answer be different if you are human?



<< The gasoline has disappeared and the box now weighs it's original mass >>


As long as the gasoline products are still inside the box, then it doesn't have its original mass - the mass is still the same as the box+gasoline mass.

 

[BooneRebel]

<< Suppose you have a 70 kg black box. You add 10 kg of high energy mass to the black box. ABSOLUTELY NOTHING LEAVES the black box. What would a scale show as the mass of the black box? Why would the answer be different if you are human >>

By your definition, if you add 10 kg to 70 kg and no matter is destroyed and no matter leaves, then the sum would be 80kg.

And as Capn pointed out, your human would now be decaying rapidly...

 

[Capn]

the box would weigh 72kgf as long as there are no nuclear reactions.

If the box is human, you have a dead human that weighs 72kgf cause you didn't let anything enter or exit him.

 

[BooneRebel]

So, is this your homework? What's the next question?

 

[dullard]

<< the box would weigh 72kgf as long as there are no nuclear reactions.

If the box is human, you have a dead human that weighs 72kgf cause you didn't let anything enter or exit him. >>



Now why did you think I didn't let anything enter or exit the human? Suppose that 10 kg entering included food, water, and air in a given amount of time. Also suppose that 8 kg leaving included feces, urine, lost cells (skin and hair), exhaled air, lost blood, etc in the same given amount of time. Then wouldn't the human still be alive and have a mass of 72 kg?