# The Naked Scientists Forum

### Author Topic: Moving vehicles.  (Read 5021 times)

#### Troll

• First timers
• Posts: 2
##### Moving vehicles.
« on: 19/10/2005 20:44:00 »
I was wondering if you can tell me why when you drop something within a moving car it will drop straight down and moving with the car but if you drop something outside a moving car it will drop straight down from the point you released it.

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#### rosy

• Neilep Level Member
• Posts: 1018
• Chemistry
##### Re: Moving vehicles.
« Reply #1 on: 19/10/2005 21:15:29 »
I don't think it does drop straight down. It will get slowed down by air resistance, but it will still be moving when it hits the ground.

#### Dr. Praetoria

• Guest
##### Re: Moving vehicles.
« Reply #2 on: 19/10/2005 22:51:47 »
Wouldn't that object that appears to drop straight down to your lap, be moving at the same velocity as your lap while if held outside the car, be moving at the same velocity as the car?  Isn't wind resisence a lot greater if you drive with your car windows open?  Would closing the car windows therefore reduce gas consumption?
Doc

#### rosy

• Neilep Level Member
• Posts: 1018
• Chemistry
##### Re: Moving vehicles.
« Reply #3 on: 19/10/2005 22:54:47 »
quote:
Isn't wind resisence a lot greater if you drive with your car windows open? Would closing the car windows therefore reduce gas consumption?

Yup

#### Solvay_1927

• Sr. Member
• Posts: 383
##### Re: Moving vehicles.
« Reply #4 on: 20/10/2005 00:25:23 »
Troll,
it's just Newton's 1st law ("inertia") - an object will continue to move in uniform motion unless acted upon by an external force (or some wording like that).

In the vertical direction, gravity obviously makes it fall.

In the horizontal direction, it's moving at the same uniform speed as the car, so when you let go of it inside the car, it just keeps pace with the car in the horizontal direction.  But drop it outside the car, and there IS an additional force which stops it from keeping pace with the car - wind resistance (aka friction from air molecules).

If you were travelling through a vacuum, however, then the object would keep up with the car until it hit the ground, even if dropped outside the car - because there would be no friction.

Incidentally, Newton's 1st law also means that if you were in a car without any windows, you wouldn't be able to tell if the car was stationary or moving at a constant velocity - the object would drop straight down in either case.  (This is just the principle of relativity.)
However, if the car was ACCELERATING (or decelerating), you WOULD be able to tell you were moving - the object would fall backwards (or forwards).  Try it.

Also, Rosy's right: outside the car an object (in air) probably won't drop straight down.  It's like when you throw a person out of a moving car, they tend to roll for a while before coming to a stop.
(I tried this experiment the last time my youngest kid asked me "Are we nearly there yet?".  And she did roll for quite a while before stopping.  Unfortunately, the policeman who saw me do this experiment wasn't very impressed with my explanation that I was just trying to teach my kids about Newton's laws of motion ... [:I])

"The early bird may get the worm, but it's the second mouse who gets the cheese."

#### Troll

• First timers
• Posts: 2
##### Re: Moving vehicles.
« Reply #5 on: 21/10/2005 20:10:21 »
Excellent.  I figured it was the wind resistance that caused it but wanted to check.  It just always sat at the back of my mind and since I've never really taken a science class past the beginning of High School I just wasnt sure what theory fit it.  Thanks everyone that really cleared the thoughts up on it.

Troll

#### johndiver

• Jr. Member
• Posts: 43
##### Re: Moving vehicles.
« Reply #6 on: 23/10/2005 22:42:40 »
Here's a variation on the moving car theme.
If you hold a string attached to a small helium balloon while sitting in a car, does the balloon move when the vehicle accelerates? Assume all windows are UP and heater fan is OFF.
In this case, the balloon will move toward the front of the auto while it accelerates! This is explained by remembering that helium is lighter than air, and that air has inertia. Thus, when the car accelerates, the air seems to rush rearward, causing the lighter-than-air balloon to "float" forwards.
And this is a much more socially acceptable demonstration than using one's annoying child :-).

#### Solvay_1927

• Sr. Member
• Posts: 383
##### Re: Moving vehicles.
« Reply #7 on: 27/10/2005 21:36:22 »
True, JohnD.  And for the same reason, a helium balloon moves INTO the bend when you're driving around a bend.

But helium balloons and kids don't mix - I remember driving my 4 kids back from the seaside once and they all had a helium balloon - which made looking out of the rear view mirror a tad awkward, to say the least.  I also discovered that the balloons kept drifting into the front of the car - even when I wasn't accelerating.
But was this a contravention of Newton's laws? No, it was just the *!£?&%* brats being annoying!

(Just kidding - I love my kids really.)

"Eagles may soar, but weasels don't get sucked into jet engines."

#### A Big Mug

• Jr. Member
• Posts: 21
##### Re: Moving vehicles.
« Reply #8 on: 22/11/2005 03:24:04 »
So if I follow this correctly, I should keep my kids in helium filled plastic bags to protect them injury in collisions.  In sudden stops they would be protected from flying out the front window.  By golly, I am going to get Dad of the year for this one.  Like I needed one more reason to frequest this wonderful forum.

#### neilep

• Withdrawnmist
• Naked Science Forum GOD!
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##### Re: Moving vehicles.
« Reply #9 on: 22/11/2005 03:36:27 »
Not only will your children be safely cocooned within the comfort of Helium bags, but their ensuing high pitch laughter and giggles will provide aid as an amusing executive toy for the journey ahead.

Men are the same as women.... just inside out !!

#### The Naked Scientists Forum

##### Re: Moving vehicles.
« Reply #9 on: 22/11/2005 03:36:27 »