Physics pros

[gingerstewart55]

All you have to do is RIDE in a car (or any other vehicle for that matter).......the ball will do exactly what your body feels when you accelerate.....be pressed backward. Hit the brakes hard and you'll go forward.....into the windshield if braking hard enough and not restrained....and so will the ball.

 

[Number1]

Hum, when you fart in the space station, wich direction do you end up going?

 

[Gibsons]

See how much or what you can get him to bet on it.

 

[mugs]

The ball takes off.
/thread

 

[Sureshot324]

If you put a plane on a treadmill with a ball attached to it with a string, will the ball take off?

 

[imported_Baloo]

Since you can't test in a car, hang the ball from you hand, and move you hand. The ball will lag behind, then swing forward when you stop.

 

[Jeff7]

Originally posted by: Number1
Hum, when you fart in the space station, wich direction do you end up going?

If you do it enough, right out the airlock, when your fellow astronauts get sick of you constantly polluting their air supply.

 

[oiprocs]

Dude the plane took off. All other physics problems are now irrelevant. Get back your piehole back to that thread and join the masses in a never ending argument.

 

[sdifox]

Where is Smackdown when you need him?

 

[TuxDave]

Originally posted by: DrPizza
<-- physics teacher

It tilts backward. Also, I'm guessing that your roommate did see an experiment that's counter-intuitive: a helium balloon on a string (up from the floor) will tilt forward. That's probably what he's remembering.

edit: And, there are dozens of movie clips all over the internet showing both of these. I'll leave the searching up to you.

So I see how that makes sense but since there's opposing forces, is there a corner case? Like if you barely fill a helium balloon such that the gravity of the balloon and the lift from the helium perfectly counter-balance each other, will it still act funny?

It makes sense to me that the lower limit would be a case of zero bouyancy.

Edit Maybe my nerdiness is kicking in, but back to the regular case where the balloon is properly inflated, the moment you start to slow down, would the balloon ever move forward for a slight moment before being pushed to the back?

 

[trOver]

Originally posted by: PottedMeat
how hard is it for him to hold still a ball on a string then accelerate in some direction and see that the ball tilt backwards?

^^like above
i think you should mess with him, if you happen to be in the passenger seat while hes driving, when he accelerates forward, smash his head into the steering wheel. then say 'you were right man, im so sorry'

just make sure the airbag doesn't go off....

 

[mordantmonkey]

so what causes the helium balloon to lean forward?
increased air pressure toward the rear of the cabin? does the air in the cabin actually have that much mass?

 

[Tiamat]

it tilts backwards

/thread

 

[cubby1223]

Originally posted by: TwiceOver
Ask him if when he is in a car and the car accelerates if his head slams into the windshield. Then next time you are in a car with him, break really fast and have his head slam into the windshield.

But we're talking about something hanging upside-down from the ceiling, which mean the laws of physics behave in reverse.

If you were hanging from the ceiling you'd smash into the windshield when the car accelerates.

 

[silverpig]

Originally posted by: enwar3
Me and my roomie have an argument. So a car is at rest. A ball is hanging from a string tied to the ceiling, hanging straight down at first. So when the car accelerates, which way does the string tilt? Does it tilt backward (the ball moves behind the string's point of attachement, backward in relation to the car) or forward (the ball moves forward in relation to the car)?

Your post is ambiguous. You must specify the direction of acceleration. Maybe your roommate means the car is accelerating in reverse.

Or maybe it's falling down... or rocketing up.

*shrug*

 

[91TTZ]

Originally posted by: enwar3
Me and my roomie have an argument. So a car is at rest. A ball is hanging from a string tied to the ceiling, hanging straight down at first. So when the car accelerates, which way does the string tilt? Does it tilt backward (the ball moves behind the string's point of attachement, backward in relation to the car) or forward (the ball moves forward in relation to the car)?

Why do people ask for physics pro's, then ask ridiculously easy questions?


Hey, I have a question for people with PHD's in mathematics: what's 5+4?

 

[Mday]

ok, let's consider the density of the ball first, and assume the ball is able to move relative to the car.

If the density of the ball is higher than air in teh car, the ball will start to move in the opposite direction of the car, relative to the car. Newton's law is directly in effect here.

If the density is lower, then the ball will move in the direction of the car, relative to the car. Newton's law is indirectly in effect here. Why? because the air is denser, and moves accordingly (relative to the car). And basically pushes that ball forward since it's less dense than the air.

 

[IronWing]

Okay, now a classic physics question

The train is sitting in the railroad station and you've got a ticket for your destination, woo_oo wooo. You also have the world's best laser range finder with you, in fact, it is capable of making perfect measurements, absolutely precise, zero errors. So you point the laser straight up at the ceiling and turn it on. The photons leap forth from your range finder, smack the chromed ceiling, and bounce back down into the range finder's sensor. The range finder tells you that the ceiling is exactly 2 meters up.

The train pulls out of Kankakee, rolls along past houses, farms and fields.

You now repeat the previous action, pointing the laser range finder at the ceiling and firing away. Again, the range finder tells you that the ceiling is exactly two meters away. Satisfied that all is right in the world you start rockin' to the gentle beat and the rhythm of the rails is all you feel.

Then ZOWWWIE! It strikes you! The train was moving when you took the second measurement so the travel distance of the light beam between the range finder and the ceiling and back was longer than it was when you took the measurement while sitting in the station as the train moved forward a bit while the light was in flight. The range finder should have told you that the distance to the ceiling was farther for the second measurement but it didn't, they were exactly the same. Somewhere you learned that the speed of light is constant so what gives?

 

[Eeezee]

Originally posted by: Number1

Originally posted by: BlackTigers91
Well this provokes an interesting question.

If the car was on a treadmill, city size, would the tethered ball still have the ability to move? This is assuming that the speed of the treadmill matches the forward speed of the car.

I for one think that the ball would accelerate forward.

I think the ball would retain its ability to move however since the car is not moving, it would remain stationary I think.

I think (hope) he was just making a joke.

 

[Eeezee]

Originally posted by: ironwing
Okay, now a classic physics question

The train is sitting in the railroad station and you've got a ticket for your destination, woo_oo wooo. You also have the world's best laser range finder with you, in fact, it is capable of making perfect measurements, absolutely precise, zero errors. So you point the laser straight up at the ceiling and turn it on. The photons leap forth from your range finder, smack the chromed ceiling, and bounce back down into the range finder's sensor. The range finder tells you that the ceiling is exactly 2 meters up.

The train pulls out of Kankakee, rolls along past houses, farms and fields.

You now repeat the previous action, pointing the laser range finder at the ceiling and firing away. Again, the range finder tells you that the ceiling is exactly two meters away. Satisfied that all is right in the world you start rockin' to the gentle beat and the rhythm of the rails is all you feel.

Then ZOWWWIE! It strikes you! The train was moving when you took the second measurement so the travel distance of the light beam between the range finder and the ceiling and back was longer than it was when you took the measurement while sitting in the station as the train moved forward a bit while the light was in flight. The range finder should have told you that the distance to the ceiling was farther for the second measurement but it didn't, they were exactly the same. Somewhere you learned that the speed of light is constant so what gives?

The range finder is firing photons straight up in the rest frame of the train. Physics works the same in all rest frames. The travel time was not longer in the train's reference frame (moving relative to the Earth).

Let's interpret this problem in another way now. Let's say you dig a hole under the train tracks (without screwing up the tracks of course) and you point your range finder straight up from the bottom of the hole. You place a train there without a floor and measure the distance from the range finder to the ceiling. Now you repeat the test while the train is moving - obviously you will read the same thing regardless of whether the train is moving or not.

 

[Eeezee]

Originally posted by: 91TTZ

Originally posted by: enwar3
Me and my roomie have an argument. So a car is at rest. A ball is hanging from a string tied to the ceiling, hanging straight down at first. So when the car accelerates, which way does the string tilt? Does it tilt backward (the ball moves behind the string's point of attachement, backward in relation to the car) or forward (the ball moves forward in relation to the car)?

Why do people ask for physics pro's, then ask ridiculously easy questions?


Hey, I have a question for people with PHD's in mathematics: what's 5+4?

A lot of people freeze up when it comes to Physics. You see it in every introductory physics course. People get scared of the simplest concepts and they just go crazy and give up. That's why the averages in easy physics courses are generally low (I've never seen a midterm average greater than a 68 in any intro physics course I've ever taught or graded for).

People assume you need to be outrageously intelligent to solve simple physics problems. You don't. Anyone can do easy physics if they're willing to try. Most people aren't willing to try.

 

[IronWing]

Originally posted by: Eeezee

Originally posted by: ironwing
Okay, now a classic physics question

The train is sitting in the railroad station and you've got a ticket for your destination, woo_oo wooo. You also have the world's best laser range finder with you, in fact, it is capable of making perfect measurements, absolutely precise, zero errors. So you point the laser straight up at the ceiling and turn it on. The photons leap forth from your range finder, smack the chromed ceiling, and bounce back down into the range finder's sensor. The range finder tells you that the ceiling is exactly 2 meters up.

The train pulls out of Kankakee, rolls along past houses, farms and fields.

You now repeat the previous action, pointing the laser range finder at the ceiling and firing away. Again, the range finder tells you that the ceiling is exactly two meters away. Satisfied that all is right in the world you start rockin' to the gentle beat and the rhythm of the rails is all you feel.

Then ZOWWWIE! It strikes you! The train was moving when you took the second measurement so the travel distance of the light beam between the range finder and the ceiling and back was longer than it was when you took the measurement while sitting in the station as the train moved forward a bit while the light was in flight. The range finder should have told you that the distance to the ceiling was farther for the second measurement but it didn't, they were exactly the same. Somewhere you learned that the speed of light is constant so what gives?

The range finder is firing photons straight up in the rest frame of the train. Physics works the same in all rest frames. The travel time was not longer in the train's reference frame (moving relative to the Earth).

Let's interpret this problem in another way now. Let's say you dig a hole under the train tracks (without screwing up the tracks of course) and you point your range finder straight up from the bottom of the hole. You place a train there without a floor and measure the distance from the range finder to the ceiling. Now you repeat the test while the train is moving - obviously you will read the same thing regardless of whether the train is moving or not.

Except that light is special because its velocity is constant across reference frames. So something else has to give.

 

[Born2bwire]

Originally posted by: ironwing

Originally posted by: Eeezee

Originally posted by: ironwing
Okay, now a classic physics question

The train is sitting in the railroad station and you've got a ticket for your destination, woo_oo wooo. You also have the world's best laser range finder with you, in fact, it is capable of making perfect measurements, absolutely precise, zero errors. So you point the laser straight up at the ceiling and turn it on. The photons leap forth from your range finder, smack the chromed ceiling, and bounce back down into the range finder's sensor. The range finder tells you that the ceiling is exactly 2 meters up.

The train pulls out of Kankakee, rolls along past houses, farms and fields.

You now repeat the previous action, pointing the laser range finder at the ceiling and firing away. Again, the range finder tells you that the ceiling is exactly two meters away. Satisfied that all is right in the world you start rockin' to the gentle beat and the rhythm of the rails is all you feel.

Then ZOWWWIE! It strikes you! The train was moving when you took the second measurement so the travel distance of the light beam between the range finder and the ceiling and back was longer than it was when you took the measurement while sitting in the station as the train moved forward a bit while the light was in flight. The range finder should have told you that the distance to the ceiling was farther for the second measurement but it didn't, they were exactly the same. Somewhere you learned that the speed of light is constant so what gives?

The range finder is firing photons straight up in the rest frame of the train. Physics works the same in all rest frames. The travel time was not longer in the train's reference frame (moving relative to the Earth).

Let's interpret this problem in another way now. Let's say you dig a hole under the train tracks (without screwing up the tracks of course) and you point your range finder straight up from the bottom of the hole. You place a train there without a floor and measure the distance from the range finder to the ceiling. Now you repeat the test while the train is moving - obviously you will read the same thing regardless of whether the train is moving or not.

Except that light is special because its velocity is constant across reference frames. So something else has to give.

Nothing gives in this instance because you are measuring the height of the ceiling. The path of the light is perpendicular to the path of motion, so there isn't a Lorentz contraction along the direction of measurement and observation.

 

[AlienCraft]

Originally posted by: Sureshot324
If you put a plane on a treadmill with a ball attached to it with a string, will the ball take off?

there it is.... I knew someone would have it.
:thumbsup:

 

[Eeezee]

Originally posted by: ironwing

Originally posted by: Eeezee

Originally posted by: ironwing
Okay, now a classic physics question

The train is sitting in the railroad station and you've got a ticket for your destination, woo_oo wooo. You also have the world's best laser range finder with you, in fact, it is capable of making perfect measurements, absolutely precise, zero errors. So you point the laser straight up at the ceiling and turn it on. The photons leap forth from your range finder, smack the chromed ceiling, and bounce back down into the range finder's sensor. The range finder tells you that the ceiling is exactly 2 meters up.

The train pulls out of Kankakee, rolls along past houses, farms and fields.

You now repeat the previous action, pointing the laser range finder at the ceiling and firing away. Again, the range finder tells you that the ceiling is exactly two meters away. Satisfied that all is right in the world you start rockin' to the gentle beat and the rhythm of the rails is all you feel.

Then ZOWWWIE! It strikes you! The train was moving when you took the second measurement so the travel distance of the light beam between the range finder and the ceiling and back was longer than it was when you took the measurement while sitting in the station as the train moved forward a bit while the light was in flight. The range finder should have told you that the distance to the ceiling was farther for the second measurement but it didn't, they were exactly the same. Somewhere you learned that the speed of light is constant so what gives?

The range finder is firing photons straight up in the rest frame of the train. Physics works the same in all rest frames. The travel time was not longer in the train's reference frame (moving relative to the Earth).

Let's interpret this problem in another way now. Let's say you dig a hole under the train tracks (without screwing up the tracks of course) and you point your range finder straight up from the bottom of the hole. You place a train there without a floor and measure the distance from the range finder to the ceiling. Now you repeat the test while the train is moving - obviously you will read the same thing regardless of whether the train is moving or not.

Except that light is special because its velocity is constant across reference frames. So something else has to give.

You are in a rest frame. I need to stress that point. Yes, the speed of light is constant across reference frames; that does not change the answer to your question.

You specified that you measured the height once while the train was stationary relative to the Earth and once while the train was moving relative to the Earth. In both cases, you are measuring the height of the train car in the train car's rest frame. Thus, nothing has changed. You can't even claim to observe length contraction (which wouldn't apply here since you are measuring a length perpendicular to the direction of travel) or time dilation since you are in the same rest frame.

For a similar reason, the twin paradox isn't actually a paradox. One twin is in an inertial reference frame (on Earth). The other is not (in a spaceship that accelerates/decelerates relative to the inertial reference frame). One twin feels those forces, the other does not.