Help finding friction force of block moving across horizontal atwood's machine


by Littledude
Tags: atwood's machine, friction, hanging mass, lab report, physics
Littledude
Littledude is offline
#1
Apr23-12, 06:33 PM
P: 5
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.

If I was to find the force applied by the hanging mass how would that relate to finding the friction force, is that the right step to be taking?
Phys.Org News Partner Science news on Phys.org
SensaBubble: It's a bubble, but not as we know it (w/ video)
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Microbes provide insights into evolution of human language
Doc Al
Doc Al is offline
#2
Apr23-12, 06:48 PM
Mentor
Doc Al's Avatar
P: 40,905
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?
Littledude
Littledude is offline
#3
Apr23-12, 06:53 PM
P: 5
Quote Quote by Doc Al View Post
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?
Sadly, I don't know, either I was absent or just not paying attention when we went over tension. I don't remember the word ever being mentioned

also, I'm sorry if I put this in the wrong forum section, I just noticed the homework help section of the forums.

Doc Al
Doc Al is offline
#4
Apr24-12, 04:34 AM
Mentor
Doc Al's Avatar
P: 40,905

Help finding friction force of block moving across horizontal atwood's machine


Quote Quote by Littledude View Post
Sadly, I don't know, either I was absent or just not paying attention when we went over tension. I don't remember the word ever being mentioned
Well, time to crack open the textbook! (See: Horizontal Pulley with Friction)

What forces act on the hanging mass? (Hint: Only two.) What must they add to? (The hanging mass is also moving with constant speed.)
also, I'm sorry if I put this in the wrong forum section, I just noticed the homework help section of the forums.
I will move it.
Littledude
Littledude is offline
#5
Apr24-12, 06:40 PM
P: 5
Quote Quote by Doc Al View Post
Well, time to crack open the textbook! (See: Horizontal Pulley with Friction)

What forces act on the hanging mass? (Hint: Only two.) What must they add to? (The hanging mass is also moving with constant speed.)
Gravity and tension and they must equal 0?
Doc Al
Doc Al is offline
#6
Apr24-12, 06:41 PM
Mentor
Doc Al's Avatar
P: 40,905
Quote Quote by Littledude View Post
Gravity and tension and they must equal 0?
Right. (They must add to zero.)
Littledude
Littledude is offline
#7
Apr24-12, 07:24 PM
P: 5
Ok, so tension would 9.8 while gravity is -9.8

so yeah, I'm really clueless as to what to do next, it's extremely frustrating that the concept just won't click.

I appreciate that you are trying to help me actually learn the concept of this, but the lab report is already late and I'm failing the clss until I hand it in.
If it's possible for you to just explain how to do this clearly I'd be grateful.
Littledude
Littledude is offline
#8
Apr25-12, 07:12 AM
P: 5
Thanks for all the help, I talked to some other people and I believe I know how to do it now.
Doc Al
Doc Al is offline
#9
Apr25-12, 07:20 AM
Mentor
Doc Al's Avatar
P: 40,905
Quote Quote by Littledude View Post
Ok, so tension would 9.8 while gravity is -9.8
The weight of the hanging mass is mg, so the string tension must equal mg. And now that you have the string tension, how does that relate to the friction force? (Review post #2.)
Doc Al
Doc Al is offline
#10
Apr25-12, 07:21 AM
Mentor
Doc Al's Avatar
P: 40,905
Quote Quote by Littledude View Post
Thanks for all the help, I talked to some other people and I believe I know how to do it now.
Cool. (Sorry I didn't get to this sooner.)


Register to reply

Related Discussions
Finding a Mass of a Block on an Atwood's Machine Introductory Physics Homework 5
Finding acceleration of a block being pushed up a ramp by a horizontal force Introductory Physics Homework 9
Moving a block with kinetic and static friction to find the net force? Introductory Physics Homework 2
Work done by a friction force, block moving up a ramp--question Introductory Physics Homework 12