Ask a physicist anything. (4)

[Cabal]

95%!

I dispute the two I got wrong

"6) If there were no energy losses due to friction or unwanted heat transfer, an internal combustion engine could be built for an automobile which would be capable of converting 100% of the combustion energy in gasoline into energy used for moving the car. (Assume 100% of the fuel is burned during combustion.) "

With no friction or unwanted heat transfer, it's quite possible... yet the answer is false! Boo.

I can't remember why I got that one right! Not sure what was going through my head at the time. I think I suspected they were going to throw some entropy-based thing at you from another source other than the two they'd specifically ruled out.

"8) Energy is the ability to do work. In other words, if Ben needed 1000 joules of energy to move a box 5 meters across the floor it would make no difference whether he had 1000 joules of thermal energy or the same amount of mechanical energy. He could still do the work required to get the job done."

Energy is energy. If he 'had' 1000 joules of thermal energy, it's his to use as he wishes. Build a turbine and off you go. Boo.

Well....depends on the entropy. I agree though, a lot of these questions are based on somewhat weasely wording.

 

[shinbits]

Please take this very simple, basic test as fast as possible without cheating.....
Intuitor Basic Physics Savvy Quiz

Then go ahead and post your scores.

And no, I'm not telling anyone my final score was 40% so forget about even asking me.

I have a question about one that I got wrong:

14) When a person on Earth stands on a floor, of the two external forces acting on her, her perception or sensation of weight directly depends on the upward force the floor exerts on her feet, not the downward force of gravity.


I answered "false", and the answer is "true".

I read the explanation which says this:

When a person stands on the floor she has both a weight force acting on her in a downward direction and a normal force which is the force the floor exerts on her in the upward direction

This doesn't make sense to me. How is the floor "pushing up"? This implies that if there gravity on earth was weak enough, the floor would propel you upward toward the sky. Or is "pushing" merely a term for anything that stops you from falling?

 

[shinbits]

95%!

I dispute the two I got wrong

"6) If there were no energy losses due to friction or unwanted heat transfer, an internal combustion engine could be built for an automobile which would be capable of converting 100% of the combustion energy in gasoline into energy used for moving the car. (Assume 100% of the fuel is burned during combustion.) "

With no friction or unwanted heat transfer, it's quite possible... yet the answer is false! Boo.

"8) Energy is the ability to do work. In other words, if Ben needed 1000 joules of energy to move a box 5 meters across the floor it would make no difference whether he had 1000 joules of thermal energy or the same amount of mechanical energy. He could still do the work required to get the job done."

Energy is energy. If he 'had' 1000 joules of thermal energy, it's his to use as he wishes. Build a turbine and off you go. Boo.

I got those two "wrong" as well. Knowing you dispute them makes me feel better about my pathetic 55%.

 

[Wiccan_Child]

I have a question about one that I got wrong:

14) When a person on Earth stands on a floor, of the two external forces acting on her, her perception or sensation of weight directly depends on the upward force the floor exerts on her feet, not the downward force of gravity.


I answered "false", and the answer is "true".

I read the explanation which says this:


This doesn't make sense to me. How is the floor "pushing up"? This implies that if there gravity on earth was weak enough, the floor would propel you upward toward the sky. Or is "pushing" merely a term for anything that stops you from falling?

It really is pushing you. The reason you don't go soaring into the sky (what an image) is because it only pushes you up with the same force with which you push down on it - thus, the forces cancel out, and you remain perfectly still.

The force comes from the electromagnetic repulsion between the electrons in your feet and the electrons in the surface of the floor, bolstered by all the other atoms that push up as well.

Gravity is always pulling you down, and it's only the electrons that stop you from accelerating through the floor. If it weren't for them, there'd be nothing to stop us walking through walls

 

[Wiccan_Child]

I know, that's why my question said "compressed to the size of the moon" not "turned into the moon".

I think I got my question answered anyway, but just to clarify - density has an effect on the gravitational pull, right?

Yup. There is a sphere, defined by a radius called the Schwarzchild radius, that a mass must be squeezed into before its gravity will overcome all other the other forces, and thus become a black hole. The amount of mass isn't that important, it's how squished it gets.

 

[shinbits]

It really is pushing you. The reason you don't go soaring into the sky (what an image) is because it only pushes you up with the same force with which you push down on it - thus, the forces cancel out, and you remain perfectly still.

The force comes from the electromagnetic repulsion between the electrons in your feet and the electrons in the surface of the floor, bolstered by all the other atoms that push up as well.

Gravity is always pulling you down, and it's only the electrons that stop you from accelerating through the floor. If it weren't for them, there'd be nothing to stop us walking through walls

Wow.

Okay. So this means, if I stack encyclopedias on a chair, there's more force pushing down, right?

So, what happens to electrons when force is applied to it, that causes it to repel things?

Thanx for your input.

 

[SithDoughnut]

Yup. There is a sphere, defined by a radius called the Schwarzchild radius, that a mass must be squeezed into before its gravity will overcome all other the other forces, and thus become a black hole. The amount of mass isn't that important, it's how squished it gets.

Ah yes, I remember the Schwarzchild radius. Thanks for letting me know that my memory of GSCE physics hasn't gone completely.

 

[shinbits]

P.S.: Wiccan, do u ever think you could use your four "Ask a physicist" threads as sort of an online resume for some sort of teaching position or consultant?

But then again, anyone could pretend to be an expert on the internet.

 

[Supreme]

I'd say if it wasn't for chemistry, modern physics wouldn't exist. Certainly my field (quantum optics/atomic physics) all grew out of thermodynamics and stoichiometry.

I remember a harsh lesson during my second year of my undergrad, where we were all complaining about having to solve the Schrodinger equation for the hydrogen atom, and then the dual major students (physics and chem) just rolled their eyes and showed us the stuff they were having to learn in their chem classes about resonance hybrid structures, and we were struggling with a lone 1s orbital.

Chemistry is definitely a subject I regret not being able to study more of.

That said, chemists are total electron-obsessives and nowhere near as cool as physicists

Lies! Physics fails in comparison to chemistry! Or biology for that matter...

Thanks for your answer and Wiccan Child's answer- helpful and interesting as ever

 

[Wiccan_Child]

Wow.

Okay. So this means, if I stack encyclopedias on a chair, there's more force pushing down, right?

Right. And the electrons in the floor may not be able to push back with enough force - thus breaking the larger molecular bonds, breaking the floor on a macroscopic scale.

So, what happens to electrons when force is applied to it, that causes it to repel things?

Thanx for your input.

When you push against a wall, what you're doing is, among other things, moving the electrons in your hand against the electrons in the wall. Since they're both negatively charged, they repel each other.

But because you also have positively charged protons in your hand, from a large distance, your hand is basically electrically neutral. It's only when your hand is so close to the wall the distance is measured in nanometres, that the slight negative charge becomes noticeable.

 

[Wiccan_Child]

P.S.: Wiccan, do u ever think you could use your four "Ask a physicist" threads as sort of an online resume for some sort of teaching position or consultant?

But then again, anyone could pretend to be an expert on the internet.

Something like that: this thread (these threads?) have given me the confidence boost to start private tutoring. I've got a few kids that live locally that I teach physics and mathematics to. It always pays to keep abreast of basic trigonometry

 

[Tinker Grey]

Please take this very simple, basic test as fast as possible without cheating.....
Intuitor Basic Physics Savvy Quiz

Then go ahead and post your scores.

And no, I'm not telling anyone my final score was 40% so forget about even asking me.

77.5%

 

[Nostromo]

87.5% and quite frankly I'm embarrassed.

 

[grace24]

Wiccan Child, is "nothing" the same as "non-existence"?

Hypothetically speaking, "non-existence" is abstract idea describing a state from which existence can not arise from. Since existence is real, there is something. Which means that "non-existence" is and was never an option. We cannot trace back to "non-existence" because there is no such thing. Does that make sense? Correct me where I go wrong.

 

[Wiccan_Child]

Wiccan Child, is "nothing" the same as "non-existence"?

Meh, semantics are a funny thing. 'Nothing' is the name given to the absence of things, 'non-existence' is the name given to the linguistic opposite of 'existence'. We say something is non-existence, which doesn't make literal, but there you go.

Hypothetically speaking, "non-existence" is abstract idea describing a state from which existence can not arise from.

It can't arise 'from' non-existence, because non-existence isn't a thing whence come others. But this quirk of language doesn't mean everything must come 'from' something else.

Since existence is real, there is something.

Agreed.

Which means that "non-existence" is and was never an option.

I disagree. There's no guarantee that, before there was something, there 'was' nothing.

We cannot trace back to "non-existence" because there is no such thing. Does that make sense? Correct me where I go wrong.

Your mistake, I think, is that you assume that existence precedes essence.

 

[grace24]

I will try to answers your response later. Continue on....

Imagine that space-time is expanding at accelerate rate. Just like a balloon.
For the balloon to expand, there must be something inside of it. If you can contract the balloon until it pops and disappear, what are you left with? Do you still have something inside the balloon? Then again, contract whatever that "something" until it disappear, what do you have left? Do you still have something? Or do you have "nothing" (minimalistic condition (state) of existence)?

 

[Wiccan_Child]

I will try to answers your response later. Continue on....

Imagine that space-time is expanding at accelerate rate. Just like a balloon.
For the balloon to expand, there must be something inside of it. If you can contract the balloon until it pops and disappear, what are you left with? Do you still have something inside the balloon? Then again, contract whatever that "something" until it disappear, what do you have left? Do you still have something? Or do you have "nothing" (minimalistic condition (state) of existence)?

You run the risk of over-extending the analogy - just because a balloon must be filled with 'stuff' to expand, the universe doesn't necessarily have to operate in the same way. It may be the case that the universe can expand infinitely (or very largely) while it can only contract to a certain size.

 

[shernren]

It really is pushing you. The reason you don't go soaring into the sky (what an image) is because it only pushes you up with the same force with which you push down on it - thus, the forces cancel out, and you remain perfectly still.

You're a geocentrist!

Hehe. You don't actually remain perfectly still. The normal force from the floor doesn't exactly cancel out the gravitational force of the Earth. The residual force is exactly the centrifugal force required to make you follow the Earth's 24-hour-rotation about its center.

 

[Steffenfield]

Is it possible to build a deadly air gun?

Imagine this...

Have a container build pressure to something like 300 psi.

Expell that down the long nozzle of a gun, the end of it would be covered so the air wouldn't escape just yet.

Have some type of mechanical seal draw behind that pocket of air, compressing the chamber greatly down to the size of a bullet.

Open up the end of the nozzle to discharge the round.

I've seen some of the stuff they have in Mythbusters which just sprays air. It only becomes deadly when it's loaded witth an object.

Just wondering if air alone could be pressurized and condensed enough to make it deadly say at least a distance of fifty yards.

 

[Assyrian]

Dry ice pellet?