A Silly Question

Where is your calculation?

I say it does move, but you have to point the air outlet to the other side!

Silly!

Hi az native,

Could it be, the low force of the vacuum can't overcome friction?

Just a thought, maybe it's a "silly answer" to a "silly question"?

Best regards,

John

There is no "reactive force" to draw the vacuum hose along.

Nothing's pushing the hose in the opposite direction.

Maybe it would move if you could remove all other forces acting on the hose, perhaps suspend it from a line then try.

Did you hook the vacuum hose to the exhaust to see if it move the hose that way. Have several shop vacs here and have hook the hose to the exhaust to blow somethings off. When drop on the floor it didn't move much at all. Most vacuum motors in a vacuum cleaner move air at about 100 cfm which pretty low velocity.

There's nothing wrong with your concept. The hose tries to move forward (the one on my vacuum cleaner does) but the hose has an internal spring which provides a force counteracting the 'force' of suction (the motion toward the low-pressure zone). If the hose were freely suspended, you could see the effect. Or take a shop vac on wheels, remove the hose, switch it on, then plug it in. It will move toward the direction the suction hole is pointed.

Dang...didn't see your post first

GA

The discharge port does not need to be opposite the intake port. Air pressure on the opposite side of the vacuum from intake port will push it toward the intake port. Having the exhaust port on the opposite side will help, but it's not required.

You might be right about the hose contracting; I hadn't thought about air friction internal to the hose.

Sorry, just not right, at all.

Air flow in the inside will put SOME force on the opposite side of the intake port, (against the direction of the hose end moving toward the vacuum). For this to work the hose will have to drag the vacuum in the opposite direction from the force exerted against the inner wall.

It ain't gonna go anywhere.

The hose contracts mostly due to a delta pressure created between outside the hose (higher pressure) and the inside of the hose (lower pressure).

Since the hose diameter can not shrink much due to its round corrugated structure, it contracts in length due to its accordion-like construction.

You can test this for yourself if you hold the hose end still or clamp it to something that does not move and then partially constrict the hose end. You will see the hose draw inward toward the fixed end as well as the end fastened to the vacuum cleaner. In other words, the hose will try to shrink in length.

As for the concept of creating a vacuum in front of a hose nozzle and asking if the hose nozzle would have a net force forward…

The answer would be yes, but there are some caveats.

First, the actual thrust component is not from sucking the air in, but thrusting it out. It is like standing on a frictionless cart with a pile of bricks. If you start throwing the bricks off the cart, the cart will move in the opposite direction.

The thrust component of a vacuum cleaner is in the body of the vacuum cleaner, not the hose. So the hose will not be propelled forward as it is not thrusting.

If we imagine a canister vacuum with an inlet port in the front and an exhaust port in the rear and they are along the same axis, there would be a net force in the forward direction when it is switched on.

Now, what happens if we were to fix a disk at the canister exhaust port so that the exhaust were diverted in all directions perpendicular to the long axis of the canister?

First, the sum of all thrust that is diverted in perpendicular direction would be zero. However, there is still a rearward thrust element , but since it is now striking a fixed plate attached to the canister the net force along the long axis of the canister is still zero.

But there is a pressure gradient between the front of the canister and the rear. That means there will be a slight force imparted on the tail end of the canister toward the front of the canister.

If we reattach the hose and hold the hose off to the side or face it rearward, what happens?

The pressure gradient between the front and rear of the canister is still there, but now it is diminished. If the hose is far enough away as to be outside the room, the net force would be 1/2 what it would be if the hose was removed from the canister because the low pressure no longer exists in front of the canister and there still is a high pressure at the rear.

What force is on the hose end outside of the room?

There is no thrust vector because the thrust is in another room and isolated from the hose end. All we have is pressure gradient between the inside of the hose and the outside of the hose. The hose will want to shrink just as I described in the beginning, but there will be no net force along the axis of the hose propelling the hose end forward or even backward for that matter.

The exception to that is the force of contraction of the hose as it contracts like an accordion to try to equalize the internal and external pressures. Again, that force has nothing to due with thrust, but an attempt to equalize pressures.

Anonymous Hero-

Thank you very much for your explanation. Does it hold true for upright vacuum cleaners too? Next time the wife uses hers (almost daily, one of those white glove freaks) with your permission I will quote you. With that she will probably delegate that task to me. She says she has a cute apron for me!

Good Luck, Old Salt

Wow, AH. That's a lot of text to say that vacuum flow does not = thrust and the forces generated by the air flow into the vacuum are not enough to overcome the external forces (inertia, ambient air pressure, etc.) acting on the vacuum.

Good effort though.

The Hoover Constellation was a vacuum cleaner without wheels, blowing some of the air downwards to create a hovercraft effect. The inlet port and outlet port were opposite each other, and indeed you could put the hose on the outlet port to make a weak pressure source for a paint sprayer. If you took the hose off altogether it did try to move.

Planes have devices hanging from their wings (usually in pairs) that combine both your vacuum cleaner AND rocket theory.

Internal resistance to air flow acts on the hose.

+ the turbulent flow

yah...I think that a vacuum cleaner ranks right up there with incandescent bulbs as far as efficiency goes.

look at this link:

http://www.daviddarling.info/encyclopedia/J/jet_engine.html

It shows jet/rocket engine theory using a "balloon".

forces are at BOTH ends of the jet/rocket (Newton's 3rd Law of Motion).

Every action has an equal and opposite reaction.

Not even close.

A vacuum cleaner does not expel air the same way a jet, or even a balloon, does.

The air entering a vacuum hose will IMMEDIATELY begin to lose velocity, due to friction.

As I said earlier, unless the air outlet is exactly opposite to the air intake (the hose end) the forces are not the same.

Look at the inside of a vacuum. The air is deflected numerous times as it enters the hose and travels through the vacuum to the outlet. Each with a reduction in velocity.

It seems that this "silly question bothered" me, so I woke up at 0300, laid and begun thinking.

as sleep didn't come, I rose up and took a piece of cardboard long and wide enough to cover the out let port of the fan in my bath room.

This fan is mounted on a small window, with vertical hinges that allow it to move 90 degrees horizontal.

When the fan is operated, it has power enough to move the [ unlocked]window, for about 40 degrees.

But now I attached this cardbord, about 3 c.m away from the outlet, and when the fan was on, it stood, didn't move! The air was flown in all the directions 90 degrees of the fan!

This explained me that without straight flow, there will be no motion!

Shouldn´t we ask Dyson about this?

Amazing design!

in a weightless environment it would move......otherwise not enough thrust

Dear Mr.aznative,

I think the explanation below answers your question.

Air craft jet engine developes thrust when the hot gases are jetting out after combustion, in straight line with out any obstruction at very high velocity to the extent of 450 to 460 Metres/Sec. where the pressure at the exit will be very high. By applying Bernoulli's theorem we can calculate the pressure.

The thrust developed will be M x V where M is the mass of gas escaping/Sec. and V is the Velocity of gas in M/Sec. and this thrust will push the air craft forward i.e opposite to gas flow, asper Newton's law.

But in the Vaccuum Cleaner the vacuum will be much less than 760 MM Hg. and air is entering from the pheriphery of the mouth entry and it is connected to hose where the air turns, and the mouth has sufficient weight.

Hence Newtons 3^rd law is not applicable here.

DHAYANANDHAN.S

There is no such thing as a silly question; only a silly answer!

You may have heard that jet boats were invented in New Zealand but what you may not have heard is that before he got his idea the inventor was at a site where someone else was testing a boat that had a water intake in the bow. This persons idea was to 'suck' the boat through the water and so his exhaust was just through the bottom of the boat. Like your vacuum cleaner there was almost no forward motion. The jet boat inventor simply used the idea and put the inlet and outlet in the proper places and made a lot of money. I have also seen an experimental naval design where the idea is for the ship to be similar to a girder trolley with a 'beam' of ocean water pumped through the ship. FAIL!

My simple explanation is that the entry for the air lets the air come from all around the mouth, even from behind. If you could see it, it would be a sphere stuck on the end of your hose, whereas the exhaust can be made to go through a particular shaped nozzle giving thrust. In short: volume x velocity of low pressure sphere (inlet) equals volume x velocity of high pressure jet. And when it comes to movement high pressure trumps low pressure.

I like silly questions. I can give silly answers.

Is it switched on?

Why is a chicken? - because one of it's eggs are both the same!

I believe your instincts are correct. The hose will contract, but the force is low because both the mass and the velocity are low. Velocity can be increased to a certain extent by use of a nozzle.

This is not a silly question. It was asked by Richard Feynman a long time back in the guise of a lawn sprinkler sitting underwater with water being sucked in instead of sprayed out. Would it rotate forward, backward, or not at all? The way I heard it was that he had to test it using a large jar and pushing the water back out by pressurizing the jar. When it didn't move, he increased the pressure until the jar exploded, making him unwelcome in the lab. Wonderful story.

http://www.youtube.com/watch?v=xQrkiH0U1Yg

It seems that this "silly question bothered" me, so I woke up at 0300, laid and begun thinking.

as sleep didn't come, I rose up and took a piece of cardboard long and wide enough to cover the out let port of the fan in my bath room.

This fan is mounted on a small window, with vertical hinges that allow it to move 90 degrees horizontal.

When the fan is operated, it has power enough to move the [ unlocked]window, for about 40 degrees.

But now I attached this cardbord, about 3 c.m away from the outlet, and when the fan was on, it stood, didn't move! The air was flown in all the directions 90 degrees of the fan!

It seems that this "silly question bothered" me, so I woke up at 0300, laid and begun thinking.

as sleep didn't come, I rose up and took a piece of cardboard long and wide enough to cover the out let port of the fan in my bath room.

This fan is mounted on a small window, with vertical hinges that allow it to move 90 degrees horizontal.

When the fan is operated, it has power enough to move the [ unlocked]window, for about 40 degrees.

But now I attached this cardbord, about 3 c.m away from the outlet, and when the fan was on, it stood, didn't move! The air was flown in all the directions 90 degrees of the fan!

It seems that this "silly question bothered" me, so I woke up at 0300, laid and begun thinking.

as sleep didn't come, I rose up and took a piece of cardboard long and wide enough to cover the out let port of the fan in my bath room.

This fan is mounted on a small window, with vertical hinges that allow it to move 90 degrees horizontal.

When the fan is operated, it has power enough to move the [ unlocked]window, for about 40 degrees.

But now I attached this cardbord, about 3 c.m away from the outlet, and when the fan was on, it stood, didn't move! The air was flown in all the directions 90 degrees of the fan!

This explained me that without direct flow will be no motion!

Here is a better link. I think I was too hasty in putting up a youtube link before looking at it. Youtube seems to be a playground for video tricksters.

http://blog.warningsciencecontent.com/2011/12/31/feynman-sprinkler/

However, I think the final proof should be building one. You could use a vacuum cleaner with rubber hose and fittings to make a nozzle, or you could adapt a lawn sprinkler to the vacuum port in a swimming pool.

The hose not, but the vacum cleaner would move if on a low friction base.

Gas exiting a rocket is all directed in generally the same direction. Due to conservation of momentum, the rocket is forced in the opposite direction.

Air that is sucked into the vacuum cleaner approaches the inlet from all directions. The air direction is not the opposite of the "rocket" case. There is no thrust generated.

Not necessarily correct, there maybe thrust depending on the configuration of the vacuum cleaner - if it is a barrel configuration like the old electrolux or hoover with inlet at one end and outlet in line at the other end there will be thrust but it is small due to the low airflow - not dissimilar to a ducted fan running at low speed.

If the outlet is filtered and diffused as it is on many modern vacuum cleaners then there will be minimal thrust ...... it is not the inlet which provides thrust but the outlet!

Most vacuum cleaners filter the intake air/dust in order to prevent the motor/fan from being exposed to the dust. That would accelerate wear if the dust goes into the motor. Shop vacs filter the input air since either the motor is enclosed in a removable filter or on some a filter bag is connected to the inside end of the inlet fitting.

The uprights with the bag strung from the handle, which was usually behind the handle, most often had the motor separated from the air fan by housing. Erosion of the fan still occurred but not as serious as if the motor was exposed to dust. This arrangement would not provide any useable thrust.

Diffusing of the exhaust air causes it to be slower than needed for any useable thrust. If the old Hoover canister types with the bottom exhaust and the ring around the bottom were modified it could move with its own exhaust thrust. If the exhaust air, functioning similar to a hovercraft, was partially diverted to a side port this could make it move from that thrust. The friction between the vacuum and the floor pre-empts most vacuums from moving from their self provided thrust.

Good Luck, Old Salt

Agreed

I'm thinking the original question concerned the suction side only. The exhaust side is a different matter.

Nothing wrong with your "calculations"!

-To understand how the jet engine works (a good example for your question!)

-See the design restriction required of any jet engine,especially the outlet contours !!

Ofcourse based that the inlet tube doesn't contract under vacuum conditions...