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FW: friction frustration



I have some nice pasco carts with low friction wheels that would give
less than 0.1, but I instead decided to use some older carts that have
large metal wheels. Seems kind of obvious now that if you spin the
wheels while you hold the cart in your hand, they stop rather quickly
due to friction. So there is some baseline amount of friction present
regardless of the incline. If I really wanted to show a decrease in the
amount of friction with increase in angle, I probably should have had
them allow blocks to slide down the incline.
Thanks,
Matt




-----Original Message-----
From: Rick Tarara [mailto:rtarara@SAINTMARYS.EDU]
Sent: Tuesday, November 12, 2002 4:08 PM
To: PHYS-L@lists.nau.edu
Subject: Re: friction frustration

What kind of carts are you using? If they have even half-way decent
axles,
the frictional forces should be much smaller than what you indicate.
Calculate the coefficients of friction necessary for these effects. If
the
wheels are really rolling you should be less than 0.1. This looks to me
to
be something else going on rather than friction.

Rick

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
********************************************************
Free Physics Educational Software (Win & Mac)
www.saintmarys.edu/~rtarara/software.html
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----- Original Message -----
From: "Matt Harding" <Harding.Matt@ICCSD.K12.IA.US>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, November 12, 2002 4:05 PM
Subject: friction frustration


I teach several sections of high school physics, and ran in to the
same
problem consistently during a recent activity. I was doing a lab with
carts on an inclined ramp. The intent was to have the students
predict
the acceleration for the cart based on the angle of the incline and
the
weight of the cart. They were to then find the actual value for
acceleration using Mac Motion with sonic ranger motion detectors.

I was hoping that as the students increased the angle of the ramp,
they would have better agreement between predicted and actual values
for
acceleration due to the decrease in friction (decrease in normal force
resulting in less friction). However, when I tried to have them work
backwards from the actual acceleration to eventually arrive at a value
for friction, they consistently found that the amount of friction was
increasing as the angle increased.

Here's how the calculation worked.

Cart mass = 1.26 kg

Incline = 3.1 degrees

x-component of force = 0.67 N

ideal acceleration = 0.53 m/s/s



Analyzing the acceleration graph in mac motion gave an average
acceleration of 0.27 m/s/s

Then I had them work backwards to find the force that would cause that
amount of acceleration,

1.26kg*0.27m/s/s = 0.34 N



I assumed that the difference between the forces ( 0.67 - 0.34 = 0.33N
)
would more or less be attributable to friction.


However, when I had them increase the angle (using same cart and ramp)
they got numbers like this...

Incline=10.56 degrees

x-component of force = 2.26N

ideal acceleration = 1.79 m/s/s



...and mac motion returned an average acceleration of 1.37 m/s/s,
meaning the force was 1.72 N. The difference would be (2.26 - 1.72 =
0.54 N). So it would appear that increasing the angle resulted in more
friction. OOOPS!



My goal was to have an extension to the lab where they would work with
a
fourth angle, but calculate the amount of friction beforehand based on
coefficient of friction and normal force (I had hoped to return
consistent values for the coefficient of friction for the three
initial
angles, but the coefficient of friction was apparently increasing) in
order to have a more accurate prediction for acceleration.



I'm assuming the main source of friction that we're dealing with is
occurring where the axle meets the bearings (we're using some older
wooden carts with less-than-"frictionless" bearings).

1. Should that amount of friction not be decreasing with an
increase in angle for the ramp?





2. Am I fundamentally flawed in my assumptions for friction in this
lab?





I'm fairly confident in the mac-motion values, the acceleration graphs
ended up having obvious horizontal regions, and the students were able
to analyze those sections to get a solid average value. The only
problem I can see with the data collection comes from the small data
collection period. The ramps were only 1m long, and 0.5 m of that is
useable because of the detectors personal buffer space.



Help?



Thanks,

Matt Harding

Iowa City West High School









This posting is the position of the writer, not that of SUNY-BSC, NAU
or
the AAPT.

This posting is the position of the writer, not that of SUNY-BSC, NAU or
the AAPT.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.