Delta

Hi,

I'm in the middle of a physics homework due tomorrow, and I'm stuck on this specific problem. The result I'm getting doesn't make any physical sense, afaik.

The problem is the following:

There is a 1.0 kg mass at the edge and on top of a rotating disk. The disk is revolving at 40 rpm and has a diameter of 1.2 m. What is the minimum coefficient of friction required for the mass to remain on the disk?

Here's what I've written so far: http://i.imgur.com/LgmCygE.jpg

Here's how I defined the variables I use in the above document:

F_fic is the fictitious force that pulls the mass opposite the center of the the disk,

f is the friction,

N is the normal force,

µ is the coefficient of friction (this is what we're looking for),

m is the mass that's on top of the disk,

g is the standard gravity,

a_c is the centripetal acceleration,

a_ref is the acceleration of the reference frame,

v is the tangential speed,

R is the radius of the disk,

ω is the angular speed.

In a nutshell, here's my reasoning:

The normal force is equal and opposite the weight of the mass, therefore:

N = mg

The friction is proportional to the normal force:

f = µN

In reference to the disk, the mass doesn't accelerate, therefore:

ma = ∑F ⇒ 0 = µmg - fictitious force ⇒ µ = F_fic / mg

The fictitious force is equal and opposite the mass multiplied by the acceleration of the referential, the latter being the centripetal acceleration. Therefore :

F_fic = -m*a_ref = -m*a_c = -mv²/R

The value of the tangential speed isn't given, but we know the value of the angular speed, therefore:

v = Rω, and ω = 40 rpm*2π/60s = 4π/3 rad/s

If we put all this together we get this:

µ = F_fic/mg = (-mv²/R)/mg = (-m(Rω)²/R)/mg = -Rω²/g

If we put the numbers in:

µ = -Rω²/g = -(0.6 m)(4π/3 rad/s)²/(9.8 m/s²) = -1.07

When I put the numbers in the formula, I get a result of -1.07, which seems impossible for a coefficient of friction (how can it be negative?). Strangely enough, several other classmates got the same answer through different methods and no one could figure out what was wrong.

Could someone please explain what I don't get or what I did wrong?

Thanks :)

Hi,

I just got my Nexus One, and everything works fine *but* the calendar.

From what I understand, it's suppose to sync automatically with Google Calendar.

But it doesn't work. If I open the Calendar app on my Nexus One, none of my calendars show up.

In fact, if I try to add an event, an error message appears, telling me that I have no calendar. But it doesn't give the option to add one.

Why am I not able to add events to the default Calendar app, and why won't it sync?

My website is hosted at HostGator.

I'd like to write my website in Python so that my webpages are generated by a Python script.

However, despite Python being installed on HostGator, I am not able to run any Python scripts. I always see them as plain text.

Since HostGator is a fairly popular webhost, I guess that there's someone out there who knows how to solve my problem.

So, please, if I want to run Python script (e.g. a simple "Hello World") what do I do?

Bonjour,

Je dois de toute urgence remettre à mon professeur de français le sujet d'un exposé oral portant sur la période allant du Moyen Âge au 18e siècle.

Or, je dois admettre que je ne connais rien à cette époque, encore moins côté littérature. Et je n'ai pas eu la brillante idée de me placer en équipe.

Le but de cet exposé est de présenter une oeuvre littéraire, autant que possible, et son auteur, entre le Moyen Âge et le 18e s. Toutefois, des sujets plus larges (exemple : la censure) sont permis.

Les sujets faciles comme "Le Cid" de Corneille ou "Tartuffe" et "Le Malade imaginaire" de Molière sont déjà pris.

Idéalement, j'aimerais trouver une oeuvre littéraire représentative de l'époque et qui soit facile à lire d'ici 1 semaine ou 2, et dont on peut facilement trouver des informations sur la vie de l'auteur. Ou encore, si vous songiez à un sujet général touchant cette époque et dont il serait facile de traiter, votre idée est aussi la bienvenue.

Merci pour votre aide, je l'apprécie, je ne connais rien à cette période en littérature.

I have finally beat Super Metroid.

I see the "See You Next Mission" screen with my completion percentage.

But nothing happens. Do I have to turn off the console? Or is there a way to save my progress (in some Metroid games, you can save after completion to unlock certain things or to show a symbol next to your save game)?

If you've played the game, I'd like to know whether I must turn off the console or if there is a button or something I can press, and if my progress is saved.

10 points à celui ou celle qui a la bonne réponse.

I want to order somthing online. I live in Canada, and the item will be shipped from the United States. It's the first time I order something from outside Canada.

On the website, it is written that the item will not be declared as a "gift" to the customs. Does that mean that thre will be duties to pay? If yes, how much, approximately?

Thank you !

Hi,

If a website says it accepts MasterCard, can I use a MasterCard Prepaid Card to make a purchase?

On the MasterCard website, it's written that Prepaid Cards are accepted "everywhere Debit MasterCard is accepted", but I don't know if that means it can be used to make an online purchase like if it was a MasterCard credit card.

Do you know if can use it online? If yes, what are the limitations?

Thank you!

Mon frère a des amis peu recommandables. Récemment, mon iPod de 500 $ a disparu, et ce, alors que mon frère avait laissé l'un de ses amis (un kleptomane) tout seul dans la maison.

Je n'ai aucune preuve, si ce n'est qu'il manque : mon iPod, la PlayStation Portable de mon frère, un bijou appartenant à ma mère. De plus, les circonstances sont accablantes : mon frère le laisse seul dans la maison, et tous ces objets sont depuis introuvables. Finalement, lorsque interrogé, il ne nie pas avoir quelque chose à voir avec ça, il dit tout simplement qu'il « ne sait pas » s'il a à voir avec ça.

Que devrais-je faire ?

Merci !

P.S. : Bien que je sais avec certitude que mon iPod était dans ma chambre, l'ayant utilisé peu avant sa disparition, je n'écarte pas la thèse de l'avoir égaré. Ce iPod me tient à cœur, car il est personnalisé, contient énormément de chansons, il me servait de disque dur externe donc contient des documents importants, et il est discontinué, bien que ce soit l'un des meilleurs modèles de iPod jamais conçus.

Hi,

I'm writing a program in Python, and I need a little help.

First of all, my code fetches a Web page:

import urllib

f = urllib.urlopen("http://www.poil.ca/%22)

s = f.read()

f.close()

print s

This prints the source code of the Web page.

Then, I need to find within the source code of the page all the occurences of id="voteup?????" in the source code, such as id="voteup25051":

import re

m = re.findall('id="voteup....."', s)

print m

This prints this list:

['id="voteup20524"', 'id="voteup20516"', 'id="voteup20517"', 'id="voteup20526"', 'id="voteup20525"', 'id="voteup20528"', 'id="voteup20527"', 'id="voteup20518"', 'id="voteup20512"', 'id="voteup20511"', 'id="voteup20510"', 'id="voteup20509"', 'id="voteup20507"', 'id="voteup20501"', 'id="voteup20496"', 'id="voteup20495"', 'id="voteup20492"', 'id="voteup20494"', 'id="voteup20488"', 'id="voteup20484"', 'id="voteup20483"', 'id="voteup20485"', 'id="voteup20480"', 'id="voteup20482"', 'id="voteup20481"', 'id="voteup20471"', 'id="voteup20461"', 'id="voteup20470"', 'id="voteup20466"', 'id="voteup20469"']

Now, I need something to keep only the numbers in this list. I want the output to be:

['20524', '20516', '20517', '20526', '20525', '20528', '20527', '20518', '20512', '20511', '20510', '20509', '20507', '20501', '20496', '20495', '20492', '20494', '20488', '20484', '20483', '20485', '20480', '20482', '20481', '20471', '20461', '20470', '20466', '20469']

And I want to store that in a variable.

How could I do this?

Thanks!

Is it dangerous if I take Dimenhydrinate (GRAVOL®) to help me cope with the nausea that is sometimes associated with an overdose of Dextromethorphan Hydrobromide?

Thank you.

P.S.: I'm not trying to commit suicide by overdose, just to get high on DXM.

I'd like to become a nark (an undercover narcotics agent). What can I do?

I know it's unlikely, but, in theory, is it possible?

In other word : if a person with copper hair used hydrogen peroxide, could it turn red or would it turn blonde ?

En d'autres termes : une personne aux cheveux brun clair ou cuivrés pourrait-il se ramasser avec une tête de poil de carotte suite à l'usage de peroxyde d'hydrogène ? Ou se retrouverait-il avec des cheveux blancs ou blond ?

As a casual drug user, I know it's possible to get "high" simply by listening to music. I compare the effect I get while listening to music to the one caused by a small dose of a dissociative drug.

However, a friend of mine, a regular drug user, claims that music is the most powerful drug.

What do you think? Can you get high on music? Do you think there are some factors that enhance the dissociative effect caused by music? Is the effect more intense if you actually play music instead of just listening?

Thanks for sharing your opinion.

Bonjour,

Je suis québécois. Éventuellement, j'aimerais abandonner ma vie merdique ici et déménager en France.

Je n'ai jamais voyagé. Je n'y connais rien à l'immigration.

Comment pourrais-je obtenir du travail en France ? Y poursuivre mes études ? Quel capital devrais-je avoir de côté pour pouvoir subvenir à mes besoins ? Comment obtient-on la nationalité française ? Etc.

Merci ! Vos réponses sont appréciées !

Pour les Montréalais :

Je cherche un endroit où je pourrais acheter à Montréal des chargeurs à crème fouettée et un "cracker" pour en extraire le gaz N2O.

Merci !