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the_bullet
2010-Feb-23, 05:43 PM
Hi all. I didn't know if this belongs here or the science forum, but as it relates to gravity I have put it here. After recently watching a Mythbusters episode with a falling car, I was reminded of a question I have had for a while. Why would a falling object fall heaviest end first. There is probably something simple I am missing, but shouldn't all parts of the car fall at the same rate, as acceleration due to gravity is not dependant on mass ?. Thanks...

astromark
2010-Feb-23, 06:21 PM
Earths atmosphere offers some resistance. A 1 Kilogram pillow of feathers is not going as fast as a 1 Kilogram block of Iron. Air resistance friction effects the lighter end of the car so it goes nose down...

hhEb09'1
2010-Feb-23, 06:33 PM
The force of air resistance will be about the same across the entire body, but the body will rotate around the center of mass--so if there is a "heavy end" that end will tend to be rotated into the force.

BigDon
2010-Feb-23, 10:06 PM
AND cars are designed with a fair bit of aerodynamics in mind.

ShinAce
2010-Feb-23, 10:29 PM
Hi all. I didn't know if this belongs here or the science forum, but as it relates to gravity I have put it here. After recently watching a Mythbusters episode with a falling car, I was reminded of a question I have had for a while. Why would a falling object fall heaviest end first. There is probably something simple I am missing, but shouldn't all parts of the car fall at the same rate, as acceleration due to gravity is not dependant on mass ?. Thanks...

I just wanted to mention the sheer irony of someone with the name bullet asking this question. Think of an arrow. Because of the heavy streamlined head and feathered tail, the head always points in the direction of travel.

I tend to think of it as the lighter part getting pulled back instead of the heavier part pushing forward. Semantics, but my personal view.

the_bullet
2010-Feb-24, 12:38 AM
Thanks for the replies. I still can't see what the forces involved are that would cause an object to fall heaviest end first. In the given example of an arrow, it is the drag of the tail through the air that keeps the arrow straight. If you take the flight off a dart and through it the wrong way round it will not straighten. But with an object such as a car, will drag have an effect so that the lighter end is accelerated less? Things obviously do fall heaviest side first, as in the aforementioned car, but I still don't understand why :confused:..

spode
2010-Feb-24, 01:22 AM
The force of air resistance will be about the same across the entire body, but the body will rotate around the center of mass--so if there is a "heavy end" that end will tend to be rotated into the force.

If that's true, then does it not follow that two spheres, identical except for their mass, would fall at the same rate if they fell seperately but at different rates if they were joined by a weightless rigid pole? Doesn't seem right to me somehow.

Would a block that is heavier at one end but is otherwise uniform fall heavy end first? I kind of think so (but I don't know). Maybe it would have to so with bouyancy and upthrust in air?

excaza
2010-Feb-24, 02:14 AM
hhEb09'1 has already answered the question, but probably not as clearly as some may like.

For the most basic explanation, take two masses connected by a rigid pole. For this problem it doesn't really matter if the pole weightless or not as long as the density is constant.

(m1) - - - - (m2)

In a vacuum, where m1 > m2, and both are the same size. Only thing we need to consider here is gravity.

If you drop the stick perfectly vertically, either orientation, doesn't matter:

(m1) (m2)

(m2) (m1)

In an ideal world, the orientation won't change because the forces are applied through the center of mass, giving you zero torque (trying to open a door by pushing it toward the hinges instead of around its axis). Remember for later that Torque = Force * moment arm.

Now if you angle the construction slightly (had to use underscores as leading spaces get deleted):

(m1)

___(m2)

You've now introduced some non-zero torques into the mix. Though both masses produce a torque, the magnitude of the torque produced by m1 will always be greater than that of m2, so the system will want to rotate in the direction of the gravitational force of m2 (in this case, counter-clockwise).

In a perfect vacuum, perfect world, etc. the system would actually oscillate (a pendulum), but here in this universe we've got plenty of things to resist the oscillation, slowing it down until it becomes negligible. For your car example air resistance plays a large role, as the car will want to fall vertically because that offers less resistance than falling horizontally. Differing densities can play a part, but for these cases it's negligible.

Hope this helps.

Jeff Root
2010-Feb-24, 02:24 AM
I didn't see the Mythbusters episode, but until the car is falling fast enough
for drag to be significant, one end of the car will NOT fall faster. I'll guess
that for a typical car that might be 30 miles per hour, which is reached in a
little under two seconds, over a distance of about 50 feet. At that point the
car would begin to rotate due to the lesser gravitational force on the light
end of the car, allowing that end to be pushed back by the airstream.

-- Jeff, in Minneapolis

Jeff Root
2010-Feb-24, 02:31 AM
spode,

Two spheres of identical size and different mass will accelerate toward the
Earth at the same rate in vacuum. In air, the sphere with the lesser mass
will reach its terminal speed first. It will be slowed more by drag than the
more massive sphere is slowed, even before it reaches its terminal speed.

-- Jeff, in Minneapolis

BigDon
2010-Feb-24, 02:33 AM
The pinto used in the the "Blues Brothers movie" had its engine removed prior to being dropped, as do a lot of other cars used for that effect in movies.

BigDon
2010-Feb-24, 02:34 AM
The pinto used in the the "Blues Brothers movie" had its engine removed prior to being dropped, as do a lot of other cars used for that effect in movies.

Sorry, I hit enter too soon. Which was why it spun flat the whole way down.

spode
2010-Feb-24, 02:42 AM
I didn't see the Mythbusters episode, but until the car is falling fast enough
for drag to be significant, one end of the car will NOT fall faster. I'll guess
that for a typical car that might be 30 miles per hour, which is reached in a
little under two seconds, over a distance of about 50 feet. At that point the
car would begin to rotate due to the lesser gravitational force on the light
end of the car, allowing that end to be pushed back by the airstream.

-- Jeff, in Minneapolis

Thanks, Jeff, that was what I thought. I just didn't agree with the idea that the heavier end would rotate about the centre of mass, as had been suggested.

violentquaker
2010-Feb-24, 02:54 AM
Imagine balancing a brick with more weight at one end (similar to a front-heavy car) falling flatwise on your finger. You would need to place your finger under the center of mass (not the mid point) of the brick in order for it to balance. Air velocity acts as a force on the mid point (evenly distributed), so it causes the car to rotate just like the brick falling off of your finger.

I didn't see the Mythbusters episode, but until the car is falling fast enough
for drag to be significant, one end of the car will NOT fall faster. I'll guess
that for a typical car that might be 30 miles per hour, which is reached in a
little under two seconds, over a distance of about 50 feet. At that point the
car would begin to rotate due to the lesser gravitational force on the light
end of the car, allowing that end to be pushed back by the airstream.

-- Jeff, in Minneapolis

FYI, the fall in Myth Busters was several hundred feet. IIRC sportier cars with a center of mass nearer to the middle did a better job of staying flat.

astromark
2010-Feb-24, 03:57 AM
To Bullet; Air flow offers resistance. Gravity force draws the central mass down to Earth. The lesser weighted end will become effected by airflow more than the greater mass end... The dart and the arrow will turn around if sufficant hight is allowed. The standard car with engine in, will tend to stay upright and front down as thats the way airflow and gravity effects a car... I hope you are not suggesting Jamie was wrong....:)

Delvo
2010-Feb-24, 04:28 AM
Even ignoring air resistance and any difference in weight between the front and back, there's another reason why a falling car would fall with the front end down. Most cars usually drive forward, so if one falls off of some kind of edge, the front end will come off the edge first while the back end is still rolling on the ground and hasn't gotten to the edge yet. So the front starts falling sooner than the back does. There's a short time during which the front end is falling and the back end isn't.

NorthernBoy
2010-Feb-24, 01:30 PM
The centre of mass will lead the centre of pressure. If they are both at the same point then any orientation is equally likely, but if not then the object will turn until the centre of pressure is directly above the centre of mass.

It is (hopefully) obvious if you think about it, the earth pulls every piece of the falling object towards it, but you can think of it as pulling only through the centre of mass and get the same result. Similarly the air is giving an upthrust all over the surface, and if you integrate it, you get a point in the car that is the effective centre-point for this force. One point is being pulled down, one is being pulled up, so one will end up above the other.

djellison
2010-Feb-24, 01:46 PM
Why would a falling object fall heaviest end first.

For the same reason an arrow flies with the fletchings at the back.

If the centre of mass is toward the front of the car, but the aerodynamic centre of pressure is more central, then the car acts like a giant fletching, pitching the car around the centre of mass so it's going nose first.

hhEb09'1
2010-Feb-24, 02:50 PM
If that's true, then does it not follow that two spheres, identical except for their mass, would fall at the same rate if they fell seperately but at different rates if they were joined by a weightless rigid pole? Doesn't seem right to me somehow.It is true, but that does not follow.

In your example, the heavier sphere would experience the same air resistance as the lighter sphere. Since the heavier sphere has more mass, it is decelerated less by the air resistance--which means it would tend to fall faster. They'd still fall at the same rate, if they were joined; if they weren't joined, the heavier sphere would have a higher terminal velocity.

sabianq
2010-Feb-24, 04:44 PM
hi there!
everybody is correct.. the reason things fall a paticular way is only due to the atmosphere and how it effects the falling body..

an arrow will fall point down because of the air restistance on the feathers..

if there were no air resistance at all like in the vacuum of space, the object will fall in the orientation it started with, if there is any other force applied, the object will "tumble"

you can see a direct example:
think of the space station in orbit. it is falling around the earth. it has many parts that are heavy and many parts that are light, the mass (weight) is not even close to being evenly distrubited around the structure.
if heavy things fell first (or faster) then the space station would be a tubbling wreak and would scatter it self all around the earth in orbit.

excaza
2010-Feb-24, 06:04 PM
think of the space station in orbit. it is falling around the earth. it has many parts that are heavy and many parts that are light, the mass (weight) is not even close to being evenly distrubited around the structure.
if heavy things fell first (or faster) then the space station would be a tubbling wreak and would scatter it self all around the earth in orbit.

Not correct, the layout of the components of a space station or satellite are very carefully planned out in order to minimize the moments of inertia of the system. This makes the system much easier to stabilize and/or make any attitude changes.

JohnD
2010-Feb-24, 06:52 PM
bullet,

Suggest you try an experiment.
Take a brick, a house brick, and make a cardboard copy of it, just an empty box the same size.
Tape the brick and the copy together, narrow end to narrow end, so you have a double brick, with one end very much heavier than the other.
Consider this as a model of a car, with the asymetric position of the Centre of Mass exaggerated.
Now hold the brick from above its Centre of Mass, which will be about the real brick's CoM, and drop it. Note how often it will hit the ground real-brick first. Do this from various heights (you make have to make a few extra cardboard 'bricks'!)

I predict that except from a very low height (less than a meter?) it will ALWAYS land real-brick first. In other words, given enough velocity and time, drag will turn it in flight (the flight of the brick!) to point heavy end down.

You should note that aerodynamic drag is proportional to the square of velocity, so while drag is small at low speeds, it rapidly increases as the object accelerates. Most objects in an atmosphere reach a terminal velocity, in real flight or free fall, because drag exceeds the force accelerating them.

John

Jeff Root
2010-Feb-25, 12:36 AM
John,

Unless his vision is much sharper than mine, or he uses a camera to record
the falling brick, he will need to drop it from much higher than one metre to
detect any rotation before it hits the ground. I simultaneously dropped a
ping pong ball and a steel ball bearing from as high as I could reach (~2m),
and could not detect any difference in the time they hit the floor.

I also dropped the ping pong ball simultaneously with a dodecahedron
made of paper, having the same mass (2 grams) but 7.5 times the volume.
The ping pong ball obviously landed first.

-- Jeff, in Minneapolis

JohnD
2010-Feb-25, 08:51 PM
Well, it was thought experiment, but I suspect that the cardboard end would show which end impacted.

Bullet, please report back. I don't want the glory of being right, you tell us.

mugaliens
2010-Mar-01, 08:32 AM
If you take the flight off a dart and through it the wrong way round it will not straighten.

It won't straighten in time to hit a dart board 8' away, but drop it from 100 ft and I guarantee the pointy end will contact the dirt first.

Jason Thompson
2010-Mar-01, 12:36 PM
Even ignoring air resistance and any difference in weight between the front and back, there's another reason why a falling car would fall with the front end down. Most cars usually drive forward, so if one falls off of some kind of edge, the front end will come off the edge first while the back end is still rolling on the ground and hasn't gotten to the edge yet. So the front starts falling sooner than the back does. There's a short time during which the front end is falling and the back end isn't.

While this is true, it is not applicable to the example in the Mythbusters episode, as the cars dropped there were winched up (two by crane to about 400 feet and one by helicopter to 4000 feet) and dropped in a horizontal orientation, not driven off a cliff.

The episode also further illustrated that air has an effect on the falling car, as the one dropped from 4000 feet missed the target drop point by a couple of hundred feet despite hovering directly over it at the moment of release.

Dragonchild
2010-Mar-01, 04:02 PM
BigDon's point almost got entirely missed. When considering the effects of mass, gravity and drag, bear in mind many cars are specifically designed to be aerodynamic. They're also mostly hollow; it's an ad hoc number, but in the name of perspective, assume 99% of a car's volume is air. Most also have the engine (and thus most of the weight) in the front. The combination of these characteristics will result in a falling car to act much like a shuttlecock, with excessive weight in the front and excessive drag in the back, making the chassis dive nose first. Depending on the vehicle, it wouldn't take long for this to take effect.

As violentquaker mentioned, sports cars are more likely to fall flat because they are designed to move the center of gravity as close to the center of the car as possible. This is to avoid under/oversteering problems in races.