brainteaser:the helicopter and the airplane
02-16-2012, 11:18 AM
#22
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It goes to the back of the plane. It is fairly frictionless flying in the air of the cabin, theres not enough friction to keep it with the air as the airplane accelerates. Just like if you put a soda can on the floor in the aisle during takeoff. Its going to roll to the back of the plane.
02-16-2012, 01:52 PM
#24
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The helicopter will move forward.
Because as the aircraft accelerates it pushes the cabin air forward.
In a recent WSJ article they posted a bunch of GOOGLE interview questions. They used an example of a helium balloon in an accelerating car. The balloon moves forward because the air is pushed forward. It has nothing to do with any of Newton's laws.
KIRK, OUT.
Because as the aircraft accelerates it pushes the cabin air forward.
In a recent WSJ article they posted a bunch of GOOGLE interview questions. They used an example of a helium balloon in an accelerating car. The balloon moves forward because the air is pushed forward. It has nothing to do with any of Newton's laws.
KIRK, OUT.
I think the answer to the helicopter Q is it will initially move back until there is no more acceleration. It will slow down in reference to the airframe due to air friction against it, and will stop moving when either the plane stops accelerating, or it stops due to the friction of the air.
02-16-2012, 04:11 PM
#25
With The Resistance
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The helicopter will move forward.
Because as the aircraft accelerates it pushes the cabin air forward.
In a recent WSJ article they posted a bunch of GOOGLE interview questions. They used an example of a helium balloon in an accelerating car. The balloon moves forward because the air is pushed forward. It has nothing to do with any of Newton's laws.
KIRK, OUT.
Because as the aircraft accelerates it pushes the cabin air forward.
In a recent WSJ article they posted a bunch of GOOGLE interview questions. They used an example of a helium balloon in an accelerating car. The balloon moves forward because the air is pushed forward. It has nothing to do with any of Newton's laws.
KIRK, OUT.
The reason a helium balloon does that is because it is lighter than the surrounding air. Air has mass, and when an object (airplane) accelerates, the air inside it is in fact slightly compressed toward the back of the plane (really it is the back of the plane that compresses the air slightly as it's pushing the air forward and the mass of the air is initially resisting). So the helium balloon simply does what it is designed to do, and moves to the area of (very slightly) less pressure. When a car is in a turn, (remember the definition of acceleration includes change of direction, as well as speed), the air is pushed against the outside of the turn, and the balloon moves to the area of less pressure, which is the inside.
I think the answer to the helicopter Q is it will initially move back until there is no more acceleration. It will slow down in reference to the airframe due to air friction against it, and will stop moving when either the plane stops accelerating, or it stops due to the friction of the air.
I think the answer to the helicopter Q is it will initially move back until there is no more acceleration. It will slow down in reference to the airframe due to air friction against it, and will stop moving when either the plane stops accelerating, or it stops due to the friction of the air.
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Let us suppose we have a killer whale loaded in a tank of salt water(roughly 600 times denser than air) onboard an MD-11 and the whale is in a state of neutral bouyancy, what will happen as the aircraft accelerates.
Inertia does not change with weightlessness.
02-16-2012, 05:15 PM
#26
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It will go where the air around it goes. If the parcel of air in which it hovers remains still, then so shall the 'copter.
Newton's first law of motion: The velocity of a body remains constant unless the body is acted upon by an external force.
The external force, in this case, must be the parcel of air. However, I wonder about the effect of compressibility on the air. The parcel of air wishes to remain stationary but the cabin moves forward causing the air to remain still relative to it's position whilst the airplane was stationary, therefore as the rear of the cabin pushes forward on the parcel of air, I wonder if the air compresses upon the air in front of itself, relatively, therefore making anything suspended in the parcel of air seem to move rearward.
Hmm... I don't think so though. Trying to recall the times I've watched dust particles float around in the sunlight within a cabin, they don't seem to be affected by anything other than the airflow from the various vents and movements of people in the area.
Newton's first law of motion: The velocity of a body remains constant unless the body is acted upon by an external force.
The external force, in this case, must be the parcel of air. However, I wonder about the effect of compressibility on the air. The parcel of air wishes to remain stationary but the cabin moves forward causing the air to remain still relative to it's position whilst the airplane was stationary, therefore as the rear of the cabin pushes forward on the parcel of air, I wonder if the air compresses upon the air in front of itself, relatively, therefore making anything suspended in the parcel of air seem to move rearward.
Hmm... I don't think so though. Trying to recall the times I've watched dust particles float around in the sunlight within a cabin, they don't seem to be affected by anything other than the airflow from the various vents and movements of people in the area.
The correct answer...
It "goes backward". But this requires an important assumption: that no new control inputs are made to the helo.
Conservation of momentum means it wants to stay put (while the airplane leaves). In order for it to leave with the airplane, a force must be imparted to generate an acceleration (F=MA).
The air around the helo moves, and imparts a SMALL aerodynamic force on the helo, but that force is not going to be enough to immediately overcome the momentum of the helo. The helo is affected by the air mass, but it is not rigidly fixed in the airmass (unlike passengers in seats). So the helo will move forward, but not as fast as the airplane...at least until the aft bulkhead catches up to it.
02-16-2012, 05:15 PM
#27
With The Resistance
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Not sure but it did remind me of this video: [ISS] Station Re-boost From Inside Station - YouTube
02-16-2012, 05:16 PM
#28
With The Resistance
Joined APC: Jan 2006
Position: Burning the Agitprop of the Apparat
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The correct answer...
It "goes backward". But this requires an important assumption: that no new control inputs are made to the helo.
Conservation of momentum means it wants to stay put (while the airplane leaves). In order for it to leave with the airplane, a force must be imparted to generate an acceleration (F=MA).
The air around the helo moves, and imparts a SMALL aerodynamic force on the helo, but that force is not going to be enough to immediately overcome the momentum of the helo. The helo is affected by the air mass, but it is not rigidly fixed in the airmass (unlike passengers in seats). So the helo will move forward, but not as fast as the airplane...at least until the aft bulkhead catches up to it.
It "goes backward". But this requires an important assumption: that no new control inputs are made to the helo.
Conservation of momentum means it wants to stay put (while the airplane leaves). In order for it to leave with the airplane, a force must be imparted to generate an acceleration (F=MA).
The air around the helo moves, and imparts a SMALL aerodynamic force on the helo, but that force is not going to be enough to immediately overcome the momentum of the helo. The helo is affected by the air mass, but it is not rigidly fixed in the airmass (unlike passengers in seats). So the helo will move forward, but not as fast as the airplane...at least until the aft bulkhead catches up to it.
02-17-2012, 03:27 AM
#29
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It goes to the back of the plane. It is fairly frictionless flying in the air of the cabin, theres not enough friction to keep it with the air as the airplane accelerates. Just like if you put a soda can on the floor in the aisle during takeoff. Its going to roll to the back of the plane.
Last edited by JamesNoBrakes; 02-17-2012 at 05:45 AM.
02-17-2012, 10:29 AM
#30
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Not so good. It has everything to do with Newton.
Pretty good.
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Let us suppose we have a killer whale loaded in a tank of salt water(roughly 600 times denser than air) onboard an MD-11 and the whale is in a state of neutral bouyancy, what will happen as the aircraft accelerates.
Inertia does not change with weightlessness.
Pretty good.
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Let us suppose we have a killer whale loaded in a tank of salt water(roughly 600 times denser than air) onboard an MD-11 and the whale is in a state of neutral bouyancy, what will happen as the aircraft accelerates.
Inertia does not change with weightlessness.
This. If it was in a cockpit filled with 50 or 70 weight gear oil, then it might move forward with the aircraft, but there's not enough of a relationship between a particle of air and the helicopter to make this happen, so for all intents and purposes, it's frictionless. It's also compressible too, which further allows the helicopter to move. I'm having a hard time believing people have never accelerated in flight and had stuff move in the cockpit, and in those cases, was the force of the "air" on objects enough to keep them moving? Now just think about an oject with mass (balloon has very little mass) floating in the middle with almost no friction. If you had a runway long enough, then maybe you could get that minimal friction and resistance to compression to keep it "in place", unlikely though with any kind of significant acceleration.
Also if the airplane was long enough, and I agree. Very minimal air friction, but it is there.
