ITT: Littledude asks a forum about physics that 90% can't help with... again

[Littledude]

So yeah, this time my question involves friction.
Basically we did a lab where you attach a pulley to the desk with a clamp and then have a friction block (three types: cork, plastic, and felt) attached to a string which is attached to a hanging mass. This basic setup is called an atwood's machine. The purpose of the lab was to identify the coefficient of friction by determining what hanging mass is needed to move the block at a constant speed.
We did multiple trials with each block, adding different weights to the block and then adding weights to the hanging mass in order to pull the block.
The steps I need to take in order to find the coefficient of the friction (we're trying to find the kinetic friction by the way) are, I believe, find the normal force (which I found), then find the frictional force, then the coefficient using those two.
My problem is finding the frictional force, I do not know any formula's which would allow me to find it without the coefficient of friction, which I need the friction force to find.

I vaguely remember somebody saying that the frictional force is equal to the hanging mass, but I'm not sure that makes sense. Could someone confirm that?

If the data is needed to explain this to me just let me know.

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[Aces]

You're using different kinds of blocks and adding different weights along with that?

Or are you using different measuring thingies that weigh stuff?

[Khepri]

http://www.ehow.com/how_5200551_calculate-coefficient-friction.html

[Littledude]

You're using different kinds of blocks and adding different weights along with that?

Or are you using different measuring thingies that weigh stuff?

three different blocks and six trials each block, each trial adds 100 grams (90g-weight of block, 190, 290,390,490,590)

[Littledude]

ok hold on, if I find the force that the hanging mass applies (simple enough to do) what would the force of kinetic friction be, relative to that?
I've managed to start a very slow conversation on the physics forums with  Doc Al, who apparently has a PhD in some sort of physics. I believe him to be the creator of the forums or at least some kind of moderator.
A bit annoying that he doesn't want to outright tell me what to do and instead wants to nudge me towards figuring it out myself.

[Littledude]

Triple posting in order to gain attention because the guy on the other forums isn't responding right now.
His last post was this:

Since the speed of the block is constant, the net force on it must equal zero. The only horizontal forces acting on the block are the friction force and the string tension, so they must be equal (and opposite). How does the string tension relate to the weight of the hanging mass?

can anyone answer the question he was trying to get me to answer?


what sucks is I'm tired and don't want to be doing this, but if I hand it in tomorrow it will already be 2 days late, I can't really allow it to get 3 days late.
The points I'd be getting for it wouldn't even be worth the trouble assuming he would deduct more points than he would for it just being late in general, really depends on the mood he's in when I hand it in.
He'd probably be in a good mood since all of the juniors in the class will not be there tomorrow or wednesday or thursday since they have standardized testing

[Aces]

Hm..

You're using 2 changing variables, which is the weight and the type of block.

Is it okay to do this? It might be hard with 2 manipulated variables. Atleast that's what I guess.