# The Naked Scientists Forum

### Author Topic: in the cab of a car...  (Read 6025 times)

#### gecko

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##### in the cab of a car...
« on: 12/10/2006 05:38:43 »
why is it that when objects are unsupported in the cab of a car, for example: a ball being dropped, they fall to the ground as if the car wasnt moving. if the car is moving at 70 mph, when something is unnattached in the cab shouldnt it stay stationary while the car moves forward? thereby flying to the back of the car?

instead it moves forward with the car and falls at the same time.

i just dont get it! but im sure some of you do.

#### Karen W.

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##### Re: in the cab of a car...
« Reply #1 on: 12/10/2006 06:25:04 »
The air inside of the car is contained and stable, at the same speed as the car, is it not? The car itself is moving fast through the air which is outside of the car, wouldn't the ball drop the same on the outside as the inside just down, and the only real movement would be the car speeding forward and hitting the ball! I don't understand how the ball would go to the back of car, because the air is enclosed inside the car, and traveling at the same speed without the wind factor hitting the ball.. Might be differnt if the windows were down! and there was that rush of air from the car breaking fast into the wind....Its inside no air movement so it should fall just as if it were standing still as the inside atmosphere has not changed.. Or am I way off too! Just my humble guess, I am no expert here!!

Karen
« Last Edit: 14/10/2006 12:28:07 by Karen W. »

#### lightarrow

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##### Re: in the cab of a car...
« Reply #2 on: 12/10/2006 08:15:47 »
I don't know what a cab is; however, if an object is inside a car and the car is moving at constant velocity, then the object moves with the same velocity. So the object velocity relative to the car is zero.

Example: we are on planet earth, which is spinning and moving around the sun, but every unattached object on earth doesn't remain stationary with respect to the sun!

#### Soul Surfer

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##### Re: in the cab of a car...
« Reply #3 on: 12/10/2006 09:28:19 »
This is a classic case of newtons laws of motion in action.  bodies at rest or in uniform motion continue to do so unless acted on by a force.  Assuming that you are travelling at a constant 70mph all the time the ball is doing so at the beginning continues to do so as it falls and when it sits on the floor of the car and so behaves perfectly normally.

You have to remember that while you are driving along the motorway at 70mph you are also going at a few thousand miles an hour rotating with the earth, about 18 miles per second going with the earth round the sun about 200 miles per second going with the solar system round the galaxy and about 600 miles per second going with the galaxy towards the great attractor  :-)

Learn, create, test and tell
evolution rules in all things
God says so!

#### eric l

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##### Re: in the cab of a car...
« Reply #4 on: 12/10/2006 16:13:46 »
It is actually very simple :  think of the car as standing still, and the universe moving around it.  The ball inside the car is part of the car, and will fall straight down if let loose.
Movement exists only in reference to something.  In reference to the car, your ball was not moving, in reference to the road the ball is moving with the same speed as the car (and continues to do so when dropped inside the car).
In the same way, a bomb dropped from an airplane does not fall perpendicularly to the ground, but would keep up with the airplane (if there was no friction of the air breaking down its speed in refernce to the ground below).  Without this friction, the bomb would still be perpendicularly below the airplane the moment it (the bomb) touches the ground (provided the airplane does not make a turn after dropping the bombs).

#### Zeig

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##### Re: in the cab of a car...
« Reply #5 on: 13/10/2006 00:24:19 »
yea, a Joe Shmo explaination is..the air in the cap is beign forced along at the same speed as the cab. when the ball is droped the air in the cab forces the ball along with it at the same speed. Also, the ball was already traveling at the cabs speed as you drop it bcuase your moving at the same speed as the cab.

#### lightarrow

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##### Re: in the cab of a car...
« Reply #6 on: 14/10/2006 11:23:45 »
quote:
Originally posted by Zeig

when the ball is droped the air in the cab forces the ball along with it at the same speed.
No. In the absence of air it would be the same (at constant velocity).

#### Karen W.

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##### Re: in the cab of a car...
« Reply #7 on: 14/10/2006 12:25:16 »
Hello Alberto, A cab is a car that's main purpose is for public hire, personal transportation when you don't have your own vehicle, or way to get from point A to Point B! Thanks for the information!

Karen

#### lightarrow

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##### Re: in the cab of a car...
« Reply #8 on: 14/10/2006 12:53:05 »
quote:
Originally posted by Karen W.

Hello Alberto, A cab is a car that's main purpose is for public hire, personal transportation when you don't have your own vehicle, or way to get from point A to Point B! Thanks for the information!

Karen

Hello Karen! you're welcome!

A related question just for you: if you have a ballon with helium inside (those for children, which fly if you let them go) and you fix it with a rope to a bus pavement, what happens when the bus brakes? The ballon goes ahead, backwards or remains in his previous position?

(Hint: in this case the air inside the bus is important).
« Last Edit: 14/10/2006 12:53:49 by lightarrow »

#### Karen W.

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##### Re: in the cab of a car...
« Reply #9 on: 14/10/2006 15:17:10 »
Alberto,WHO me...LOL You are asking the wrong girl that question!I am the least scientificly minded person here in the forum..., but here is my very un educated guess! As long as the windows were up, wouldn't the balloons remain in the previous position? I know "I" would move forward slightly from the stop, but because of the hellium, wouldn't it be the same staying in previous position....wait, Although I question that because once you have anchored it to the floor, then would it not have to move forward because of it's attachment on the rope or string when that went forward... I am not sure, I should experiment with it!

Karen
« Last Edit: 14/10/2006 15:26:25 by Karen W. »

#### lightarrow

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##### Re: in the cab of a car...
« Reply #10 on: 14/10/2006 18:46:33 »
Ok, so I'll tell you:

when the bus brakes, every object heavier than the air in it will go forward; the ballon (lighter than the air) goes backward.

Because: in this way air, which is more heavy, can go forward, leaving the ballon behind.

It's the same situation in which you have an air bubble at the bottom of a glass filled with water: the bubble goes up so that the water, more heavy, can go down.

Said in a more technical way: Archimede's principle applies in a gravitational field or, because of the equivalence principle, in any accelerated reference frame.

When the bus brakes, things inside of it become in an accelerated ref. frame, with acceleration directed forward. So, Archimede's principle makes the ballon move in the opposite direction, just as it moves the air bubble in the opposite direction with respect gravity.

#### Karen W.

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##### Re: in the cab of a car...
« Reply #11 on: 14/10/2006 20:16:22 »
Very interesting, but makes good sense... Cool.. Well I am greatful that I at least knew I would go forward...LOL I'M HEAVIER THEN AIR THATS FOR SURE!

Karen
« Last Edit: 14/10/2006 20:17:17 by Karen W. »

#### lightarrow

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##### Re: in the cab of a car...
« Reply #12 on: 15/10/2006 04:33:27 »
quote:
Originally posted by Karen W.
LOL I'M HEAVIER THEN AIR THATS FOR SURE!
I didn't know. I thought you were as light as a butterfly.

#### Karen W.

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##### Re: in the cab of a car...
« Reply #13 on: 15/10/2006 17:09:46 »
Well somedays are better then others my friend... Somedays I can fly! OH to be as beautiful and light as a butterfly would be a wonderous thing,....flittering around from one flower to the next..... what would that be like.... I think I'd see the world...LOL ! Goodmorning Alberto! How are you? I hope your day has been going well! Sorry for me its 9:15 am, what time is it there?

Karen

#### lightarrow

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##### Re: in the cab of a car...
« Reply #14 on: 15/10/2006 18:15:32 »
Probably I'm a little late for this answer, however it's 19:20 here now.
Good morning Karen!

#### Karen W.

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##### Re: in the cab of a car...
« Reply #15 on: 15/10/2006 23:55:37 »
Never too late for an answer...Wow so you are about 8 hrs difference also..  I hope you had a great Sunday and enjoyed your self today!! I seemed to have slept half of the day and half the night away.. so weird to get so much sleep.... was really tired.. Am used to 3 or so hours but haven't even got that this last week, so was totaly out of it... I got over 9 hours in two shifts last night and this morning after 9 am to about 12:30 pm... wow where did that come from...?? I'll take it though.. I needed it so bad... feel alot better now..Rest well friend!

Karen

#### gecko

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##### Re: in the cab of a car...
« Reply #16 on: 18/10/2006 05:01:41 »
the "cab" of a car refers to the interior space.

#### syhprum

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##### Re: in the cab of a car...
« Reply #17 on: 18/10/2006 12:42:48 »
Karen you seem to have a real wide time difference, are you in LA or even Alaska or Hawawi ?

syhprum

#### Karen W.

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##### Re: in the cab of a car...
« Reply #18 on: 18/10/2006 14:30:16 »
SYPHRUM,

Northern California, Friend!

Gecko,LOL Yes Your right and I did not see you said cab of car LOL...I had in mind you just said cab!! I should read the post better!!

Karen

#### The Naked Scientists Forum

##### Re: in the cab of a car...
« Reply #18 on: 18/10/2006 14:30:16 »