law of physics

[Michael]

If you take 2 identical square masses, one weighing 100lbs and the other weighing 1000lbs and drop them off a tall building, Will they both displace the same amount of air?
I was driving the motorhome this weekend, and noticed that when large semis would pass me on the left, some would push me to the right, and others wouldn't. the speed seemed the same, the trucks were nearly identical. this got me wondering. why would one displace more air than another? weight perhaps?


Michael

 

[EdFred]

If you take 2 identical square masses, one weighing 100lbs and the other weighing 1000lbs and drop them off a tall building, Will they both displace the same amount of air?
I was driving the motorhome this weekend, and noticed that when large semis would pass me on the left, some would push me to the right, and others wouldn't. the speed seemed the same, the trucks were nearly identical. this got me wondering. why would one displace more air than another? weight perhaps?


Michael

If the volume is the same the air displacement will be the same.

 

[Skip Miller]

the trucks were nearly identical. this got me wondering. why would one displace more air than another? weight perhaps?

I don't think weight has anything to do with it.

Without claiming to know much about the aerodynamics of a box, I think you have come close to answering your own question. Small changes can make a huge difference in aerodynamics. You say the boxes are nearly identical which leaves plenty of room for significant aerodynamic difference.

Also, the relative wind may have been different from truck to truck. Again small changes can be significant.

Finally, the distance in between your bedroom and the truck is also highly significant. For example, two ships passing in a narrow channel create a suction between them. This suction increases exponentially with decreasing lateral separation. I would assume the same forces are at work in your situation.

-Skip

 

[Richard]

Also, the loaded trucks will displace you moreso than the MT trucks. It's that whole laws of motion thing.

 

[Frank Browne]

Also, the loaded trucks will displace you moreso than the MT trucks. It's that whole laws of motion thing.

How is the loaded truck going to displace more air than an empry one? I would think it's more a matter of velocity than mass. If 2 trucks each w/same volume traveling at the same speed, wouldn't the displacement be the same? How would the outside air "know" one truck weighed more that the other? If the 2 trucks were traveling at different speeds, the air displaced
by the faster truck would be more energetic. Not a physicist or mathematician, just curious.

 

[Richard]

Frank, it is not displacement of air it is the increased resistance of the heavier truck. The increased inertia (resistance to change) of a loaded truck, relative to a lighter truck, will tend to reduce the movement of the truck as it passes Michael. But there is the same amount of air displacement involved so, because of the loaded truck's increased inertia, more of that force will act against the motor home.

There are two simple methods to reduce the effects of the truck's passage on the motor home. One is to increase the distance between the truck and the motor home. Second, reduce the amount of relative motion between the two vehicles.

Of course, in the second instance the motor home would be exposed to buffet for a longer duration but that buffeting results from aerodynamics which is separate from what I'm addressing.

I fear we will soon be talking momentum and acceleration. Actually, I'm looking forward to it.

 

[Richard]

But we're not talking about aerodynamics. We, okay, me, are talking about resistance. Michael was getting shoved around because he has less relative mass. Like when the same plane flying at the same speed through the same fluid medium reacts to the same turb differently when loaded to different weights.

 

[Frank Browne]

In the simplest example, two flat plates of equal surface area, rigidity, finish and planform perpendicular to a fluid stream will have the same drag profile at a given speed whether they are made of balsa or uranium. Weight has nothing to do with it. Both will "displace" (shove aside, actually) the same amount of fluid at any given moment.

Thanks Steve, that's exactly the mental picture I had.

The term "displace" is being used loosely here to describe the dynamic effect of an object moving through a fluid, not to be confused with "displacement" which is the measure of bouyancy an object has in a fluid while at rest, and is relative to the object's volume and the specific gravity of the fluid.

Again thanks. The clarification was in order.

 

[Frank Browne]

But we're not talking about aerodynamics. We, okay, me, are talking about resistance. Michael was getting shoved around because he has less relative mass. Like when the same plane flying at the same speed through the same fluid medium reacts to the same turb differently when loaded to different weights.

So...in the sense that 2 bodies orbit around a common center of gravity, the truck and motorhome are reacting to the same compressed air mass between them. Assuming the pressure is the same on the side of the truck and the side of the motorhome, a loaded and therefore heavier truck would be affected less than an empty one thereby forcing the motorhome to absorb more energy from the compressed air mass between them?

 

[kath]

Bernoulli's Equation:
p + 1/2 * rho * V^2 + m * g * h = constant
p = air pressure
rho = air density
V = air velocity
g = 9.8 m/s/s
h = height above the ground

In this equation, pressure, density, and velocity refer to the fluid (in this example, air).
In your highway example, the height of the air is not changing, so the "mgh" term is not relevant.

This is the equation that most of us know as "air travelling faster has lower pressure". Note first that this equation refers only to laminar air flow (and the trucks on the road produce anything but... lots of eddies and hydrodynamical messiness)... But if you want to use the equation to understand that "whoa!" swerving feeling as you're passed by a truck, you'll notice that the change in pressure (what makes you swerve) depends ONLY on the density of the air (which is constant) and the velocity of the air.

The velocity of the air is going to depend on the relative velocity between you and the truck, and the distance between you and the truck. The closer you are to the truck, the tighter the space in which the given amount of air will pass, and so the more it will speed up (V goes up) or compress (rho goes up).

Nothing in this equation has anything to do with the weight or either you or the truck. Is is true, however, that a pressure change (which exerts a force) will accelerate a lightweight vehicle much more easily than a heavy one.

Of course, this simple equation only applies to simple situations, and a REAL truck produces a lot of turbulence and non-laminar flow. But even then, it'll have more to do with the exact shape of the truck than its weight.

--Kath
Assistant Professor of Physics (starting next week!)

 

[flyingcheesehead]

More a case of aerodynamic effect, some trucks have a more streamlined outline. Truck makers are putting on cuffs and panels that smooth out the airflow over the chassis, saddle tanks, cab, trailer van, etc. Older trucks have sharper edges that disrupt the flow more, causing a more pronounced "bow wave" effect.

Yup. And the trucks aren't necessarily older, you can still buy a brand-new Peterbilt 379 or Kenworth W900 with a big, manly, squared-off hood that some truckers seem to like for some reason. Those will push you around a lot more than the newer round designs like my Freightliner Columbia.

One thing that will REALLY toss you around is double/triple trailer combinations. Those even bounce me around.

 

[flyingcheesehead]

Also, the loaded trucks will displace you moreso than the MT trucks. It's that whole laws of motion thing.

Richard,

I'm not sure I agree. If the truck had no power source, maybe... But I'll be hitting those air molecules at 65mph whether I'm empty (GVW ~32,700 lbs) or fully loaded (GVW 80,000 lbs). Without thinking about it too much, I would guess that a larger percentage of engine power is devoted to overcoming aero drag at light weights vs. pulling the load, but in terms of individual air molecules and the flow of air around the truck, I don't think it would be changed. I'll think about this some more while I'm driving tonight. I'm sure as soon as the proper equations pop into my brain it'll be obvious.

 

[Richard]

Kent, I take it you drive big rigs?

Here is where I am: the original question was all about the effects of the airflow coming off a given truck and the observation was made that not all trucks produce the same effects. In fact, the difference in effects of how that airflow acted upon the motor home was so significant that it was quite noticeable even though the size, shape or speed of the truck would not alone account for the difference in effects.

A vehicle of GVW such as a semi is moving quite a bit of air. Forget for the moment flat plate drag. That cushion reacts freely within the fluid medium until it comes in contact with a fixed or reltively fixed object. The cushion then begins to compress and exerts a force, measured in Hg, against the solid objects which inhibit the free airflow. Think of that force in terms of the moving air trying to return to an equalibrium. (The air was in relative equalibrium before the truck rammed into it but I'm concerned only about the change in pressure between the passing vehicles.)

The higher the GVW the more resistant that vehicle is to the force of the air acting upon it. Something has to give. Yes, the air will flow where it flows and the total force acting upon a solid surface is a function of surface area and speed of airflow, but the lighter GVW vehicle will show a greater effect to the pressure acting upon it, relative to the vehicle of greater GVW.

I suppose that difference in GVW 'tween the 2 vehicles could well be expressed as a percentage of total Hg but the larger the difference in GVW the higher that percentage will be for the smaller GVW.

 

[Richard]

Richard,

I'm not sure I agree. If the truck had no power source, maybe... But I'll be hitting those air molecules at 65mph whether I'm empty (GVW ~32,700 lbs) or fully loaded (GVW 80,000 lbs). Without thinking about it too much, I would guess that a larger percentage of engine power is devoted to overcoming aero drag at light weights vs. pulling the load, but in terms of individual air molecules and the flow of air around the truck, I don't think it would be changed. I'll think about this some more while I'm driving tonight. I'm sure as soon as the proper equations pop into my brain it'll be obvious.

I too don't think the airflow will change vis a vis the weight of the vehicle. But as the difference in GVW between the semi and the motor home increases so will the effects, expressed as percentage of total force, acting upon one or the other vehicles. For brevity, let's just say weight is a stabilizing force and the increased weight of the semi will reduce the effects of that force upon the truck. Since the truck is effected less, the effects upon the motor home will increase because the total force must remain constant.

 

[gismo]

Also, the loaded trucks will displace you moreso than the MT trucks. It's that whole laws of motion thing.

Only if they hit you!

 

[Skip Miller]

Since the truck is effected less, the effects upon the motor home will increase because the total force must remain constant.

Not strictly true as the system is not closed. The air in between the vehicles transmits the force between the vehicles, and some (much?) of the force may be used to accelerate the air out of the gap between the vehicles.

 

[Michael]

That cushion reacts freely within the fluid medium until it comes in contact with a fixed or relatively fixed object. The cushion then begins to compress and exerts a force, measured in Hg, against the solid objects which inhibit the free airflow.

I think Richard is onto something here. The air will react against the same object that is producing the displacement. A lighter object will react differently than a heavier object, thus creating different effect in the air flow.

maybe...