petyr 20181030 06:41:46  dp=v*dm=v*rho*dVolume=v*rho*A*dx=v*rho*A*v*dt\r\n\r\ntherefore dp/dt=F=rho*(v^2)*A.\r\nshowbox  
Baharmajorana 20140919 11:25:44  I have a sulotion with Bernoulli's equation. I assum that the initial and the final situation that P1+1/2.ru.v1^2=P2+ 1/2.ru.v2^2 , then we have that the initial speed is Zeroo then we have: deltaP = 1/2 ru v^2 and F= PA , so we have F= 1/2 ru v^2 A. I think this is partially true? Hmm?  
beforebeing 20130911 00:14:50  Even easier, no calculus:
(from continuity equation)
The change in momentum is since there is no horizontal component left after the stream hits the wall.
So:
 
ticklecricket 20101107 12:37:47  Quick dimensional checks.
Should have a mass term, which is * A, so D and E are out.
C has a height dependence, and that doesn't make *any* sense for this problem.
Which leaves A and B, one has units of momentum and one has units of force (v vs. v^2)  
vtakhist 20091021 06:29:59  You can think about it as force of Drag which is proportional to the quantities in the right answer choice (A).  
ramparts 20091008 11:09:01  I always try to take limits first, then use dimensional analysis  looking at the units is great, but it can be timeconsuming (you'd have to calculate two or three separate combinations of units here) and it's really easy to make a mistake, especially for folks like me who are really prone to making stupid mistakes. Limits are much easier.
So the force most certainly will not blow up as goes to 0. Much easier way of eliminating D and E than units. As Andresito said, choice C has the right units but don't bother checking, there's no h given in this problem. Finally, check the units to distinguish between A and B. You've saved two possibly mistaken units calculations :)
BerkeleyEric 20100718 19:18:38 
I think in this case you can just check the limits of (A). Once you see that they are right, (B) can't work because there needs to be another v, (D) and (E) can't work because the rho would need to be in the numerator.
And (C) is just odd since there is no h in the problem.

 
anmuhich 20090315 12:33:54  Dimensional analysis definitely doesn't hurt in this case. Thinking about this in terms of Newtons third law with equal and opposite forces, you know that you have an amount of mass per unit time striking a wall. It's easy to see that A*v is a volume per time element. So if you multiply this times a density you get a mass per unit time. So you know that rho*v*A has to be in the answer. This is a mass per time thing. Then by looking at dimensions you know that you need another meters per time to make this mass*meters per time squared (aka Newtons). Answer A is the only answer left that makes sense because C talks about h and g which intuitively shouldn't matter at all.  
linford86 20090107 20:39:02  Use Bernoulli's law to get an approximation. Unfortunately, if you do this, you'll notice that you're off by a factor of a half. But none of the other choices depend quadratically on v (this comes from the kinetic energy term in Bernoulli's law; the kinetic energy is proportional to the square of the velocity.) Multiply the pressure by A to get the force from the pressure. Also, note that the fluid velocity must be zero at the wall since this is a stagnation point.  
michealmas 20061230 13:00:19  nice technique senvas  
Healeyx76 20061102 19:50:13  scottopoly, i serioulsy hope thats not true :)  
scottopoly 20061029 23:03:11  I heard they no longer have questions with answers that can be reduced by dimensional analysis. Am I right?
kevglynn 20061103 09:05:25 
Could you maybe tell us where you heard that? Because, well, "...frankly, it sounds made up." Cosmo Kramer

jesford 20080405 10:17:41 
I have also heard something like this. Not that there are no more problems that can be solved by dimensional analysis, but that ETS has tried to cut down on the number of problems that can be solved this way.

 
senvas 20060912 12:20:32  alternatively:
dp=v*dm=v*rho*dVolume=v*rho*A*dx=v*rho*A*v*dt
therefore dp/dt=F=rho*(v^2)*A
as in choice A
Anastomosis 20080410 20:05:21 
Me love LaTeX.
so, dividing both sides by dt:
because

walczyk 20110407 11:40:55 
this is how i did it originally, then when i looked at it again i couldn't figure it out without dimensional reasoning :x. props to you good sir

jdbro 20141024 22:32:07 
nice

jdbro 20141024 22:32:28 
nice

 
Andresito 20060312 16:27:15  Actually choice (C) has the correct dimension but "h" is a variable which is irrelevant. We are assuming uniform flux of water.
thanks Yosun  