Re: Homework Thread

by **Big Boy Bulley** on Wed Oct 22, 2008 5:03 pm

Right ive had to resurect this thread cus i dont have a clue whats going on with my physiscs homework!
Any help (answers even) would be greatly appriciated!

1. When liquid flows under pressure P through tube of length l, the volume flow rate (i.e. the volume per unit time), V, depends on the radius r, of the tube, the pressure gradient, P/l, and the viscosity, n, of the liquid. Use the method of dimensions to derive the form of an equation for V in terms of the other quantities.

2. A sphere moving through a fluid of density p experiences a drag force that depends on its speed, v, and its radius, r.

(a) Use the method of dimensions to derive the form of an equation for the force, F, in terms of p, r and v.

(b) sketching graphs, so i figure that aint gunna work, but if u can help with those bits great!

Sorry if i put anyone off taking A level physics! lol

by mr. brodie on Wed Oct 22, 2008 5:19 pm

I mean, who does h/w these days?

by **elbanaan** on Wed Oct 22, 2008 5:38 pm

well the first is Delta P = 8*n*Q*L/(Pi*r^4) , ths in laminar flow
for the speed: what speed? average or max?

I don't know what the "method of dimensions" is though.

2nd: F = 1/2 p v² * A * C

A being r*r*Pi
C being a drag coefficient (depends on velocity and viscosity)

by **Big Boy Bulley** on Wed Oct 22, 2008 8:16 pm

Right thanks, but i still dont have a bloody clue where the hell you got that from!!

by **elbanaan** on Thu Oct 23, 2008 9:44 am

the first you can derive from the navier-stokes equation.

just google poiseulle's law.

2nd is the general law of drag, learned it in car-dynamics. Smile

by Mulgrave on Thu Oct 23, 2008 11:36 am

Ahaha Got a C at AS Physics, dropped it this year.

by **Big Boy Bulley** on Thu Oct 23, 2008 3:03 pm

Thanks elbanaan, we handed it in today (or not) since only 1 of 26 of us had done it!!

by **jerryGFL** on Thu Oct 23, 2008 3:32 pm

let me guess that was you?

by **Big Boy Bulley** on Thu Oct 23, 2008 3:51 pm

No, i still dont have a clue how to answer it, and elbanaans answer cant be right since he uses things not even given to you!

by **elbanaan** on Thu Oct 23, 2008 4:16 pm

the answer is right! the method maybe isn't, but I don't know what "the method of dimensions" translates into Sad

.

by **Big Boy Bulley** on Thu Oct 23, 2008 4:44 pm

I'll have to wait to see what our teacher tells us after half-term!