Physics pros

[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)?

 

[PetunZ]

Assuming you're accelerating forward, the ball will tilt backwards until your acceleration stops. Once you're going at a regular velocity, the ball will be hanging straight down, and then tilt forward if you decelerate (brake). If you hang things from your rearview mirror, you'll notice that if you brake hard enough, it'll hit the windshield.

 

[enwar3]

Well.. apparently my roomie has a complicated explanation of why the ball will tilt forward (in a counterintuitive manner)...

 

[PetunZ]

Let's see if I can draw a quick picture in Paint with the forces involved...

 

[esun]

Just think of Newton's first law: things at rest tend to stay at rest. The ball wants to stay where it was, which causes the string to tilt backward.

 

[DangerAardvark]

You don't need a physics pro, you need a car.

 

[PetunZ]

Link

At rest, T=mg (gravity) so the ball is stationary. During acceleration to the right, the car puts a force on the top of the string causing a pendulum with the ball. Notice the ball is on the "back" of the tense string. Of course my drawing could be utter bs and, if so, I'll blame it on being 2:30am and you should be punished for making me think about physics at this time in the morning.

Maybe the physics people can double check it's validity...

 

[biggestmuff]

huh? This isn't AoaCB. Take an object, some string and tape out to your car and prove him wrong.

 

[enwar3]

We're at college, so neither of us own cars. I know I'm right, but he won't quit it. But it's no biggie. I just wanted to see if others thought the same way my roommate did (that the ball leans forward in the direction of the car)

 

[40sTheme]

Um.... inertia?
Basic property learned in like 4th grade.
Your roommate needs some junior high science maybe?
The string will tilt backwards; you need no explanation other than inertia.

 

[oynaz]

There is, of course, only one way to settle this:

1. Get a bunch of beers
2. Drink said beers, preferably while listening to Foo Fighters or Audioslave
3. Steal a car
4. Do the test

 

[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.

 

[Epic Fail]

I am disappointed, no treadmill?

 

[BlackTigers]

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.

 

[legoman666]

Originally posted by: enwar3
We're at college, so neither of us own cars. I know I'm right, but he won't quit it. But it's no biggie. I just wanted to see if others thought the same way my roommate did (that the ball leans forward in the direction of the car)

youre in college and still needed to ask this question? I seriously hope you're not a physics, engineering or math major.

 

[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.

 

[JohnCU]

wait isn't this the "imaginary force" thing?

 

[Anubis]

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.

but im lazy

BTW hows the weather out there, over here drive to work was not so fun

 

[DT4K]

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.

That was my first thought as well. So yeah, you're roommate is an idiot, but he probably saw the balloon experiment and that's what made him think he's right.
The part he missed is that the reason the helium balloons move forward is that they are less dense than air. So what happens is that when you hit the gas, the air goes backwards(just like a hanging ball), which causes higher pressure in the back and forces the (lighter than air) balloons forward.

Even though you don't have cars, I'm pretty sure he's been in a car. Ask him if he remembers what happened when someone slammed down the gas pedal. Did he find himself moving forward? Didn't think so.

 

[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.

 

[Beattie]

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.

Is this serious?

If the car is not moving (relative to the ball not the treadmill), the ball wont move either.

 

[Beattie]

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)?

I am fascinated to see what his logic on this is. He is of course, wrong though.

 

[DrPizza]

Originally posted by: Anubis

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.

but im lazy

BTW hows the weather out there, over here drive to work was not so fun

The list of school closings is a mile long. Normally it takes my wife 20 minutes to get to work. It took about 45 minutes today. Thick coat of ice on everything.

 

[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'

 

[Savij]

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.

Awww man, I was going to post about the helium balloon.