Bonus Question

[UCLA Paratrooper]

Hey! In one of my classes my Prof made a bonus question he claims no one in the class will be able to figure out. I was wondering if you guys any solutions...

Here it is:
We all know that pressure decreases as we go up the atmosphere due to decreasing weight (of the atmosphere) pushing down on us and pressure is the greatest near the earth. Then why doesn't a 747 airplane crush us when it flies right above? Doesn't the 747 add additional weight thus adding additional pressure to us?

UCLA Paratrooper

---

Members don't see this ad.

 

[Hero]

if a Helicopter with the weight of a 747 airplane was flying CLOSELY and DIRECTLY over you, you will be crushed. the greater the distance the plane is from you, the lesser the pressure b/c the weight will be dispersed more. the 747 gains it's lift not from (insert forgotten physic phrase here) which it gets with it's speed. The helicopter pushes air down to get it's lift. I'm a bio major so some physics buff correct me if i am wrong.

I hope i get bonus credit

 

[P60001]

Sorry Hero, but I know a helicopter can hover over ones head and it have no noticable effect. I do not think a helicopter the size of a 747 would make any noticable difference. I would speculate that one reason may be due to the compressibiIity of air. I would also image if there was some way to force the air that is displaced by the volume of the plane directly on top of you, it might be a problem (again assuming the volume is compressed enough to change the pressure).
I am a biochem major, not physics (I hate physics), so I would be interested in knowing the correct answer.

 

[Hero]

i saw this experiement where they had this 15lb toy helicopter over a blance. when it lift off 1 inch from the balance, the balance still red 15lb and then started to drop from there as it went higher.

 

[ckent]

THe answer is because the weight of the airplane is diffusely spread over a large surface area (air). It wouldn't actually be the "weight" of the plane anyways, it would more likely be the force generated by the thrust of the plane keeping it airborne that you might "think" would generate weight below. But every time the airplane "pushes" against an air molecule, that molecule gets pushed down and to the side colliding with other air molecules spreading out the weight of the plane over pretty much the enire sky. It's just like pushing down on water actually. If you push down on the water in a jar with a piston covering the entire surface area of the water, then force will be felt on the bottom of the jar. But if you turn that jar into a tub, and push the same piston down into the water, a person at the bottom of the tub would not feel any force because the water would just be displaced to the side and upwards. Hope that makes sense.

 

[Jalby]

My Answer:
The 747 is weightless when flying. It is recieving lift from the wings that is exactly equal to the weight, so the net effect is that it doesn't add any net pressure to the air around it (not counting what the engines do to the air before and after it.

 

[Hero]

i think air is pushing the plane up rather than the plane pushing the air down.

the helicopter pushes the air down rather than the air pushing it up.

but wait, if I push a box w/ force N, the box pushes me back with force N as well argh i'm all confused now

 

[Mr. Z]

Ok, I'm gonna be brave and take a crack at it.

As air travels over the surface of a wing it is redirected in a downward direction. This redirection of the air molecules causes a downward momentum. According to basic physics every action has an opposite and equal reaction. Therefore, there is an upward force, equal in magnitude to the dowward momentum, this is known as lift. This can be described in terms of pressure... the downward momentum causes the pressure to be greater underneath the wing then it is above the wing. This is because air molecules that used to reside in area above the wing are being displaced (via the wing) to an area below the wing, leaving a low pressure zone above the wing. In effect there is no increase in atmospheric pessure underneath the plane because in order for it to fly it must decrease the pressure above the wing enough to offset its weight i.e.. it cancels out its own weight. The atmospheric pressure we feel is additive, so even though pressure increases below an airplane, it decreases by an equal amount above the airplane, we feel the combined effect of the two pressure differentials i.e.. nothing.

I have no idea if this explanation holds any water, but its all i got. The above poster was probably wrong about the helicopter analogy, but I think he may be onto something with the dispersive idea.

 

[Yogurt the Kid]

I hope this helps:

Think about Bernoulli's equation (look it up and solve for pressure) and the continuity equation (v1a1= v2a2). You'll notice that as velocity increases pressure decreases. Due to the shape of the wings of an airplane, the velocity of air passing over the top surface of the wings is greater than the velocity of air passing by the underside of the wings. According to Mr. B's equation, the greater the velocity of a traveling fluid, the lower the amount of pressure that it exerts against a surface. Hence, the slower-moving air on the underside of the wings of an airplane is exerting greater pressure (hence, force) on the wings--pushing the airplane upwards. The faster-moving air on the topside of the airplane is exerting less pressure on the wings; hence the plane is lifted upwards (this is the explanation of what we commonly refer to as "lift"). Ultimately, nobody is crushed because the net force applied is not in the downward direction; rather it is upward, against the underside of the plane?hence the plane flies J

But remember, I?m an English major, so please check with a bona fide Physics major.

 

[UCLA Paratrooper]

Any other ideas?

 

[Tobtolip]

Bleh, okay I'm gonna take a calc III approach (I'm probably terribly wrong cause this is just an out of the blue thought)

I think it has to do with force vectors. A vector, as I hope all of you know, is a unit of force with a direction. In the case of the Boeing 747, its force vector is not directed toward you, (if it was it would ram your head in=P). Instead, its vector is horizontal, thus you are not experiencing any of the force (its force isn't even directed towards you).

So what about Gas? Gas can be thought of as a force field (lotsa little vectors, an example of this is gravity!! Woop) Its vectors are pointed towards the center of the earth, so you feel their force on top of you (ie pressure). As you go higher in the atmosphere, you have less and less of these little vectors (which can be explained by vector addition, i think... havne't had calc III in awhile).

Okay.. that was my crack shot at it, its probably so wrong.

 

[Mr.D]

I think Jalbrektfast and Yogurt the Kid have it right. Yogurt the kid explains what causes an airplane to fly (due to an imbalance of forces above and below the wings thus creating a pressure gradient enabling a plan to move vertically), while Jalkbretfast further elaborates how a plane in pure horizontal flight (ie. not ascending or descending) does not have any weight associated with it since both vertical forces (the "weight" of the airplane pushing down and force of the plane pushing it up) balance each other out, only allowing for horizontal movement (force created by engines) to propel the plane longitudinally. Since the only force in question in your problem is presumably a net horizontal force causing the 747 to "fly" above a onlooker directly below, then we can safely assert that the person could not be crushed by a plane or any other large body flying above it since there is no net force to create a pressure (recall P=F/A) in the vertical direction to crush any object below it. Just a thought...I could be wrong. :wink:

 

[UCLA Paratrooper]

•••quote:•••Originally posted by UCLA Paratrooper:
•Hey! In one of my classes my Prof made a bonus question he claims no one in the class will be able to figure out. I was wondering if you guys had any solutions...

Here it is:
We all know that pressure decreases as we go up the atmosphere due to decreasing weight (of the atmosphere) pushing down on us and pressure is the greatest near the earth. Then why doesn't a 747 airplane crush us when it flies right above? Doesn't the 747 add additional weight thus adding additional pressure to us?

UCLA Paratrooper•••••

 

[UCLA Paratrooper]

Thanks for all your help guys!!!

UCLA Paratrooper

 

[Mr. Z]

Actually Bernoulli's equation is not an adequate explanation for why a plane can fly. Think about it... the equation explains lift as arising from a pressure differential above and below the wing caused by the faster velocity of air molecules over the top of the wing. This arises because the top of a wing is curved and longer than the bottom of a wing, therefore, an air molecule must travel faster over the top of the wing to have the same transit time as an air molecule going under the wing. Now here is the problem, how can a plane fly inverted?? they do it all the time, so something else is going on. Or what about wings that have a flat surface on both sides? they do exist. Flight is explainable based on air pressure differences above and below the wing, but not because of bernoulli's equation.

 

[Hero]

Hey UCLA Paratrooper: when your professor posts the answer, post it here as well. I'd like to see the 'correct' answer to this problem

 

[imtiaz]

take a garden variety peice of paper, 8.5 X 11 inches. orient this paper in a way as if you were going to write a letter to someone. now, take your two index fingers and thumbs and grab hold of the corners closest to you. lift up the paper, it should fall down and sag. now, gently blow over the top of the paper, you should see bernoulli's eqn. come to life. tada! the paper lifts.

•••quote:•••Originally posted by Mr. Z:
•Actually Bernoulli's equation is not an adequate explanation for why a plane can fly. Think about it... the equation explains lift as arising from a pressure differential above and below the wing caused by the faster velocity of air molecules over the top of the wing. This arises because the top of a wing is curved and longer than the bottom of a wing, therefore, an air molecule must travel faster over the top of the wing to have the same transit time as an air molecule going under the wing. Now here is the problem, how can a plane fly inverted?? they do it all the time, so something else is going on. Or what about wings that have a flat surface on both sides? they do exist. Flight is explainable based on air pressure differences above and below the wing, but not because of bernoulli's equation.•••••

 

[Mr. Z]

Imitaz, thanks for reinforcing my point.
No doubt in the experiment you describe, bernoulli's theory is fast at work. But it is inaccurate to say that the velocity of air molecules over the wing is what causes lift. The reasons for this i have stated above. I'm not making the sh#t up, its well known in the field of aerodynamics. Here is a website for you go check out, its pretty basic, though if you like you can consult a text on the aerodynamic engineering of wings.

<a href="http://www.geocities.com/galemcraig/" target="_blank">physics of flight</a>