# Collision of liquid with floor

Hi friends,
I have an issue in solving a Collision of liquid with floor.

The problem is as:

https://fbcdn-sphotos-d-a.akamaihd.net/hphotos-ak-ash4/q71/s720x720/1379505_1432382913655481_500299018_n.jpg

Attempt:

https://fbcdn-sphotos-a-a.akamaihd.net/hphotos-ak-prn2/q77/s720x720/1383432_1432380890322350_1121703847_n.jpg

Friends according to question Option (A) is correct. Please try to help me in this.
I will appreciate the help.

Related Introductory Physics Homework Help News on Phys.org
Simon Bridge
Homework Helper
When the tap is first turned on, there would be some reaction but while the water is gushing, there is no change in momentum.

haruspex
Homework Helper
Gold Member
To answer the question, you have to presume that the hose reduces in diameter greatly in the section of pipe the gardener controls. Thus, it effectively has no momentum before reaching that point. All the momentum the water leaving the pipe has therefore comes from a force supplied by the gardener.
(A dropped firehose thrashes around in a dangerous manner.)

$$F=\frac{d(mv)}{dt}$$
$$F=v.\frac{d(m)}{dt}$$
$$F=v.\frac{d(A.v.ρ)}{dt}$$
$$F=v.A.ρ.\frac{d(v)}{dt}$$
But once the water is constantly flowing ##\frac{d(v)}{dt}=0##.

Like Simon Bridge said,

When the tap is first turned on, there would be some reaction but while the water is gushing, there is no change in momentum.

haruspex
Homework Helper
Gold Member
Looking at the diagram, there may be another interpretation. If we take that right-angled turn in the hose as accurate, the gardener must supply the force to turn the flow of water through that angle.

Looking at the diagram, there may be another interpretation. If we take that right-angled turn in the hose as accurate, the gardener must supply the force to turn the flow of water through that angle.
But looking at the scale of diagram, no living man could support the tank!!!!:tongue:
The tank must be supported by some other structure. So the force you are talking about must be balance by the rigid tank itself, not the man.
The diagram shows, I think, that the tank is rigid till the (white-black) pipe junction and black portion which could be assumed as flexible is straight, so no force should be acting on the man.

haruspex
Homework Helper
Gold Member
Ha! Just realised I misread the OP as saying zero was the wrong answer, so I've been scratching around for other interpretations unnecessarily. Sorry for the noise.

Ha! Just realised I misread the OP as saying zero was the wrong answer, so I've been scratching around for other interpretations unnecessarily. Sorry for the noise.
:tongue:

Simon Bridge
Homework Helper
The temptation is probably to think of the hose as being like a rocket... it's throwing mass out one way so the hose must move back the other way.

The temptation is probably to think of the hose as being like a rocket... it's throwing mass out one way so the hose must move back the other way.
Yes, but its just a temptation!!!

pbuk
Gold Member
The temptation is probably to think of the hose as being like a rocket... it's throwing mass out one way so the hose must move back the other way.
Anyone who has handled a fire hose knows that this is in fact the case.

Or just think about it - if you mount the whole system in the diagram on wheels would you not expect it to accelerate to the left? Where do you think the force that causes this acceleration acts?

pbuk
Gold Member
Oh I've just noticed the answer is supposed to be zero too - in that case the question is wrong. I notice that the English is not correct (waters the plants by a pipe), perhaps there is something else that has been lost in translation. Or perhaps the hose is in fact rigid so absorbs all the nozzle reaction itself.

1 person
Anyone who has handled a fire hose knows that this is in fact the case.

Or just think about it - if you mount the whole system in the diagram on wheels would you not expect it to accelerate to the left? Where do you think the force that causes this acceleration acts?
Oh I've just noticed the answer is supposed to be zero too - in that case the question is wrong. I notice that the English is not correct (waters the plants by a pipe), perhaps there is something else that has been lost in translation. Or perhaps the hose is in fact rigid so absorbs all the nozzle reaction itself.
the tank should be connected to a rigid support other than the man, and seeing the scale of the diagram it clearly is the case. In reality, many other complications arise thats why force is non-zero. only the black portion is the hose, which is straight so there is no momentum change and hence no force is exerted.

P.S. : The assignment carries a logo of MIET, located in India. English isn't their primary language. Nor is mine.:tongue:

Simon Bridge
Homework Helper
... and they tend to be a little glib in their descriptions.
See the other threads from this member off the same test/assignment :)

... and they tend to be a little glib in their descriptions.
See the other threads from this member off the same test/assignment :)
yeah, they haven't cared to mentioned crucial information. Its up to the reader, to make assumptions!!!

Simon Bridge
Homework Helper
OTOH: we don't know the preparation - sometimes the school does "example questions" in class and the whole thing becomes a memory exercise... or the missing info is effectively provided in class.

pbuk
Gold Member
... and they tend to be a little glib in their descriptions.
See the other threads from this member off the same test/assignment :)
Well yes, the only way the force at the nozzle can be zero is if there is no change in velocity at the nozzle so the whole pipe must be 1mm internal diameter (to be fair this is what it says, but against that is (i) the loose grammar means that you can't rely on any statement; (ii) whoever heard of watering a garden with a 1 mm bore pipe; (iii) I can't be bothered to calculate the pressure drop in 1 m of 1 mm pipe at 12+ m/s flow, but I bet the header tank would have to be a LOT taller than shown in the picture to sustain this).

Trick questions like this are not IME a good way to reinforce learning, but if you are going to set a trick question you'd better make damn sure it is right, otherwise the joke is on the person setting the question.

1 person
OTOH: we don't know the preparation - sometimes the school does "example questions" in class and the whole thing becomes a memory exercise... or the missing info is effectively provided in class.
It could be but nonetheless question is loosely constructed as MrAnchovy already pointed out!!

Trick questions like this are not IME a good way to reinforce learning, but if you are going to set a trick question you'd better make damn sure it is right, otherwise the joke is on the person setting the question.
This is regular here, even most of the regular course books include these kind of questions!!!

Simon Bridge
Homework Helper
It could be but nonetheless question is loosely constructed as MrAnchovy already pointed out!!
Of course. I have said nothing to contradict that - I'm just not arrogant enough to armchair quarterback what happens in a classroom on the other side of the World.

It is quite something to wrestle with when you get course material like this - however it was constructed.

1 person
It is quite something to wrestle with when you get course material like this - however it was constructed.
Yes, it is. But it is so common with Physics that "most of us" associate it with difficulty of Physics. Its just that the variety of questions that can be create related to a particular topic is so vast, that the one with even a little doubt in basic concepts will face huge problem solving these kind of questions.

Simon Bridge
Homework Helper
In a way that is part of the value of the training - learning how to handle badly phrased problems.
IRL: those are mostly what you get.

In a way that is part of the value of the training - learning how to handle badly phrased problems.
IRL: those are mostly what you get.
Yeah, I guess!

the tank should be connected to a rigid support other than the man, and seeing the scale of the diagram it clearly is the case. In reality, many other complications arise thats why force is non-zero. only the black portion is the hose, which is straight so there is no momentum change and hence no force is exerted.

P.S. : The assignment carries a logo of MIET, located in India. English isn't their primary language. Nor is mine.:tongue:
There is no relation of of the logo with the assignment. On the logo paper the assignment is done by me. But the assignment is from a foreign writer.

But apart from this Thanks for a good discussion. Many doubts clear.

Last edited:
... and they tend to be a little glib in their descriptions.
See the other threads from this member off the same test/assignment :)
Sorry for the inconvenience Simon Bridge. please don't mind. You are the guys are like the candles who enlighten me in the light of physics.

Thanks for the help guys.

There is no relation for of the logo with the assignment. On the logo paper the assignment is done by me. But the assignment is from a foreign writer.

But apart from this Thanks for a good discussion. Many doubts clear.