Ootheca

The thickies are welcome to click it all they want... they might find they get more than they bargained for.


It's actually nothing to do with energy or inertia. The easiest way to see it is to cut the problem in two straight down the middle. Imagine you cut the rope in the middle of the horizontal section and are left holding each end. On the right you have one pulley and a 250kg weight, on the left you have 2 pulleys and a 250 kg weight.

Obviously your right hand would be experiencing the full 250kg whereas your left hand would be experiencing only 125kg. If you then tied the ropes together again you would have 125kg difference acting to the right and the right hand weight would fall to the ground, raising the left hand weight by half the distance.

fast bloke

It doesn't all necessarily follow the path of least resistance though. You know when the wife tells you you are going shopping. You can either agree (POLR*) or look less than overly excited (PTID**). Now if you changed the non stated background information to say that you had left her at the altar, you would have an option for stay at home drinking beer and watching ****, which would in this case be the new POTR*


*POLR - Path of least resistance

** PTID - Path terminating in death

dpb

Jezus Fb even i got that one



gods truth i used to know how to add up stuff in parallel and series <shakes head>

fast bloke

1. - I am going to the garage to find three pulleys

2. - I am nearly tempted to click it and see

Ootheca

There is of course in reality a small consideration that must be made... for the question we assume the mass of the rope and the pulleys is insignificant. Obviously if you use very heavy pulleys and/or chain instead of rope then you must take this into account!

fast bloke

Are you saying I am stupider than you

[Wheres that infarction button]

speedking

Happy with 96% But Q16 and 17, 50kg is the force required for equilibrium, not to lift the weights, that would require 51kg.

dsmith

There's nothing quite like competitive pedantry between engineers

AndyC_772

iTrader: (2)

50kg is not a force The force is, of course, 50g N.

The answer is 50, not 51; if you applied a force of 51g N, not only would you be lifting the weight, you'd be accelerating it upwards at a rate of 1/50g, which is more than needed to meet the definition of 'lift'.

AndyC_772

iTrader: (2)

It's nothing to do with 'coping' with the load, though. That's a concept used by electricians worrying about heat, fire and long runs of cable - it has no real meaning here unless you're specifically told that the current is high or the wire is very thin.

Normally, at this most basic level of electronics, wires would be considered ideal, ie. zero resistance. Therefore, there can be zero potential across the bulb and, therefore, zero current through it.

Consider the wire to have finite resistance, and it's a 3 step process to understand what's going on:

- work out the combined parallel resistance of the wire and the bulb (Rp = 1/ (1/Rw + 1/Rb))

- from that, work out the potential across the wire//bulb combination (Vp = Vo (Rp / (2* Rb + Rp))

- then you can work out the current through the shorted bulb (Ib = Vp / Rb)

There are mathematical simplifications, of course, but this method describes the behaviour of the system without having to resort to misconceptions like current 'choosing' to take a particular route, or a wire 'coping' with a load.

You could also work it out by considering current:

I = Vo / (2* Rb + Rp)

Vp = I * Rp

Therefore, Ib = Vp / Rb

*sigh* it's too early in the morning for this... I do this for a living and my coffee hasn't even soaked in yet

[Davey]

The pulleys totally messed me up lol! I've never really paid much attention to the physics of pulleys

dpb

thats what you meant to say wasnt it fb

bonerp

74 % and I know which ones I answered wrong before I even pressed continue! D'oh!

I think its sucked in too!! haha. I remember my old cortina didnt have induction like these new fangled cars today, and it made a sucking noise in the carb!!!!!! Ever stuck your hand over the top of a carb????

RB5_245

From Wikipedia:

"Suction is the creation of a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Physicists consider the notion of "suction" to be specious, since vacuums do not innately attract matter. Dust being "sucked" into a vacuum cleaner is actually being pushed in by the higher pressure air on the outside of the cleaner.

Higher pressure of surrounding air can push matter into a vacuum but a vacuum cannot attract matter."

dpb

Ok so what it boils down to really is whether we can can reasonably label your average prospective ford deisel mech. a scientist or glorified grease monkey

Luminous

iTrader: (3)

I still don't get the water going through pipe with two tubes coming off A and B. I just cannot work out why the fluid goes higher in A, when the restriction is in B.

Anyone have a nice scientific explanation, not a gut feeling one

speedking

The restriction causes an increase in backpressure, so the fluid rises higher at A. (I got that one wrong at the time though )

speedking

You are of course correct that 50kg is not a force, I was answering in the style of the question. However, the definition of lift is "raise to a higher level", a force of 50g N would be insufficient to achieve this effect HTH

AndyC_772

iTrader: (2)

I wasn't 100% sure on this one, but I think the explanation is:

- the rate of flow in terms of mass per second is the same all along the pipe (obviously),
- in the restricted section it's moving faster
- therefore, the pressure is less
- therefore, it can't support such a high column of liquid.

Why is the pressure less? It has to be, in order to accelerate the water from the low speed in the wide section of pipe, up to the high speed needed to maintain the mass flow rate in the restricted section. Just imagine the forces acting on a water molecule at the start of the restriction - why should it accelerate? A: there's a pressure difference acting on it from left to right.

AndyC_772

iTrader: (2)

Just competitive pedantry, pay no attention

Why let the mathematicians have all the fun?

Luminous

iTrader: (3)

I never did do engineering. I seem to remember that pressure is equal throughout a fluid, but I guess that is only in a static system then.

I was thinking the pressure inside the constricted pipe increased due to its narrow width. The pressure will be lower after the constriction right? As the pipe after the constriction is bigger and therefore you don't need as much pressure to cope with the flow rate being supplied.

I applied that principle to the pressure in the pipe where tube A is.

AndyC_772

iTrader: (2)

It acts equally in all directions. It's not equal everywhere within the fluid, though - you don't need a deep-sea diving bell to go paddling.

The pressure in the constricted pipe is lower, that's what causes the water molecules to accelerate as they enter the constricted section of pipe, and to slow back down again once they exit it.

Luminous

iTrader: (3)

Hmmm, more food for thought. Just had this pipe thing explained by my dad. He took one look at it, before I had even finished explaining, and said

1) If its a static system then A and B are the same height
2) If there is flow, which there is, then the constriction is a red herring. The pressure at the start of the pipe must be the highest in order to get the water to flow. The pressure falls as you travel along the pipe. The rate of fall of pressure is greatest over the restriction, that is all. Therefore as the pressure is highest at the start of the pipe, A is higher.

Seems damn simple if he is right

dpb

is it the path of least resistance again

RB5_245

It's the venturi effect.

I got that one wrong, as in a high flow system as the question says, you'd expect it to draw a vacuum?

billythekid

70% here, but actually quietly quite happy with that considering my education.

RB5_245

Well given it probably takes 50% to pass, as per your average exam... well... maybe the grease monkey doesn't even need to be glorified..



Dave (a grease monkey)

dpb

50 %

ive missed my vocation