Another Physics Brain Teaser

And why does a vacuum cleaner's motor go faster when it's got a submarine stuck up it's pipe?

It's because the water pressure has made the submarine smaller and less bouyant so it has to either increase it's volume as a scuba diver does or decrease it's weight either way it is now small enough to get jambed upa vacuum cleaner pipe and the vacuum cleaner motor now goaes faster because the impellors aren't sucking anything just rotating the air round and round in the impellor casing.

This is until the rotating air creates enough friction to generate enough heat to melt, warp or seize the vacuum cleaner and goes to prove that vacuum cleaners and submarines don't mix.

 

Yeah I had a vacuum cleaner like that once, Cheapos are no good.

If you get a good one you would not have had that problem.

Can you claim on warranty. I wouldnt mention what you were using it for though.

 

Mikey - Yes but mines with corners make em hard to throw (?) I don't know either

PI remember that targets is the modern term for 'skimmer' BUT how come when two properly operated skimmers of the frigate type come up against a sludgemarine the skimmers ALWAYS win! Also please remember the old motto "we come unclean" 'casue it don't happen on nukes which makes me think that if the crews actually wash they ain't real sludgemariners - what is the world coming to -Sorry people in jokes abound on this subject and I rather fancy PI and I will be trading them and abuses for some time - still it leaves Jack alone (but not I hasten to add on the pirate forum - we has a duty to others!)

 

Goes down for sure. A ton of feathers will float on water; lead will not.

 

Vaccum cleaner can be use by housewife illegally for other purposes, still for Hmmm... sucking things... I seen it somewhere.

My IQ have just drop 10 need to meditate to recover it

 

Do you want a job? You qualify! You can either be the submarine designer or the cleaner I need one of each. Pay rate is better for the cleaner!

 

Ok you build an aquaduct accross a railway line say,( doesnt matter) Oh by the way an aquaduct is a bridge with water on it.

This simple arch bridge has a weighing scale under each of the two legs of the arch, lets say the bridge wieghs 2000 tons so each scale reads 1000tons.

Ok so far?-- Now fill the canal --and aquaduct ( bridge ) with water. This puts up the wieght some more --say 2000 tons more.

We now have a aquaduct of 2000 tons and 2000 tons of water on it. The scales read 2000 tons each.

Heres the question. You sail over the aquaduct with a 1000 ton barge-- what is the reading of wieght as the barge is passing over the aquaduct.

 

This one is tricky

1. As the ship chugs along it will be using fuel and some of the lubrication oil will also evaporate so there is no real constant. For arguments sake lets say it takes the ship 5 minutes to cross the aqueduct and it burns 1litters per minute with a weight of…
2. The material of the aqueduct is of importance as concrete will soak up more water from the waves created than a steel one but one could agree on maybe 0.5 tons for the water stuck to the sides of the aqueduct witch can’t be taken into the displacement calculation if one ever needs to be made.
3. The prismatic and other coefficients play a role in the amount of water the ship pusses in front of it as due to resistance of the aqueduct the water will build up in front of the ship, also the put-put boat effect of once displaced water on the rear side (scale 1) re entering at a higher velocity will bring in more water into the aqueduct than the ship displaces.


After giving this some careful thought and 2 beers and using the Hamilton-Jacobi-Bellman equation


I came up with this

Scale 1 Scale 2
Aqueduct 1000 1000
Water 1000 1000
Result 2000 2000

Enter ship 500 500
Displaced water * 397 410
Difference 103 90
Total Wight 2103 2090

4193

* the displaced water takes into account the whole voulume of the aqueduct, not just the physical area of the boat

 

I don't think the weight will change much
if you lifted the barge through the air and dropped it in the middle of the aqueduct, it will be a matter of seconds(?) before the displaced water has distributed itself ofer a long distance.

 

And there, I think, is the answer. The barge was in the canal long before it reached the aquaduct, so the whole system has seen no increase in displaced volume, or mass.
In any event, as Raggi says, even if the barge was ballooned into position, a weight of water equal to the displacement of the barge will flow off the aquaduct into the rest of the canal.

 

It would depend on the speed of the barge. As the barge moves forward it would force water under the front and lift the barge upwards. This would have a greater effect in an aquaduct as the bottom of the duct is so close to the bottom of the boat. So the scales at the end where the barge entered would increase as the bow wave entered the aquaduct and then further increase as the water was forced down under the boat.

The weight would slowly decrease as the barge moved along the aquaduct until it made the same effect for the same reasons at the other end.

However, if the barged was slowly pulled on the aquaduct by a rope and not by self propulsion you'd probably get the answer you want.

Poida

 

I'm not sure.
You say that the pressure will be higher under the moving barge.
Bernoulli and experience say that the barge will float lower in the water because the pressure drops when the water accelerate under it's bottom.
It can even be sucked to the bottom if the gap is too small.

It's an interesting mind exercise.

 

level of water goes down obviously

here's why:

density of iron is 7.9 that of water

you remove an iron ingot from the boat. what made this float was the displacement of a mass of water equal to the mass of the ingot. Since water is 7.9 times less dense than iron, the volume of water is 7.9 times larger. So the boat goes up and "frees" that volume of water, so the level of water goes down.

then you drop the ingot in the enclosure, which raises the level of the water a bit.

So overall the water volume has gone down by 6.9x the volume of one ingot.

as you repeat this and the boat goes higher and higher, the level of the water in the enclosure goes down.

 

Oh, Antoine,

We're already a bit further than that.

But thanks for the splendid numbers!