1997-98 -- 05-06 Academic Years Mechanical Friction |
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07 November 2000: Ann Brandon (Joliet West HS)
held up three 6 inch wood blocks with cup hooks on end, and a
small
spring scale. She hung one of the blocks from the scale, which read a
weight mg
= 1.3 N. Then Ann used the scale to pull one of the blocks
across the
table horizontally at a steady speed. The pull on the scale read 0.2
N.
When she stacked another block on top of the first, the pull went up to
0.4 N,
and with three blocks, pull was 0.55 N. It was clear to see that
the
pull, which was due to sliding friction, went up nearly linearly with
the
combined weight of the blocks. But repeating the experiment did not
produce the
same results! The pull went from 0.4 N, to 0.9 N, to 1.5
N as
the number of blocks stacked up went from 1 to 3.
Clearly, the friction was much higher the second time. Why? Ann showed us the reason. The second time, she had turned over the block in contact with the table, and it had a coating of silicone sealer on that surface; the first time its other (non-coated) surface was in contact, so friction was lower. She said that Roy Coleman - years ago - had showed that a silicone sealer coating drastically increases friction. Even though the block with the sealer had been used for many years, it still worked. She placed the non-coated block on the surface of a chair arm (desk), and tilted the arm up until the block just began to slip. Then she turned the block over with its coated side on the arm, and the chair arm had to be tilted at more than twice the angle before the block slipped, showing the high coefficient of friction. What a great way to get students' attention, and to learn something about friction! Thanks, Ann!
19 March 2002: Bill Shanks (Happily Retired Physics Teacher) --
Friction
Bill placed a pine block on a pine board, and by tilting it up from
the
horizontal, he showed that the block would slide smoothly down the
board at an
angle of q = 27°, corresponding
to a
coefficient of static friction m=
tan q
= 0.51. He took a paper/plastic sign left over from recent
political
campaigning, and showed that it slid quite readily down the board
when
placed on the board, but that when the reverse side was placed on the
board, it
would slide only at a tilt angle close to 90°, an uncommonly
high
coefficient of friction!. So, that's how you find the smooth side
of
politicians, Bill!
14 September 2004: Ann Brandon [Joliet West HS,
Physics]
45o or Bust
Ann has discovered a 2-speed toy car, which is available at
TOYS R US
http://www.toysrus.com/. The car,
called
a Fast Line -- Power Cranker, operates at low speed (4
wheels
powered) and high speed (2 wheels powered), and it
requires two
batteries (AAA). The car is advertised as being able to
climb a 45o
incline at low speed. We first tested the vehicle on the
classroom floor,
and found that it traveled across the room (about 6 meters) in
about 16
seconds at low speed, and in about 8 seconds at high speed.
Furthermore, it traveled in a rather straight path. Then
we tested
it on an inclined wooden board, and found that it would climb up the
board when
the board was tilted at 45o above the horizontal. A
very
nice gadget indeed!
Ann then used the car to discuss the forces acting on the car in various cases.
Roy Coleman pointed out that when a board is coated with Silicone Sealant (bathtub caulk), a viscous friction force is produced. An object will then slide down the board at fixed speed, where its speed increases as the slope of the inclined board is increased. Roy was urged to show us the physics at a future meeting.
01 November 2005: Bill Blunk (Joliet Central HS,
retired)
Friction
Bill started with the well known connection between the frictional
force
F and the normal force N: F= mN,
where
there is a different coefficient m for
stationary objects (static friction) and moving objects (kinetic
friction). The static coefficient can be calculated by placing a
wood block
on a wooden ramp and increasing the angle (q
with
the horizontal) of the ramp until the block just begins to slide. The
static m
is equal to tan q. Bill then
used
the job of a roofer to illustrate the coefficient of friction. The
steepness of
the pitch of the roof has to be matched to the coefficient of friction
of the
material of the roof to allow the roofer to walk without
slipping. For
example wet plywood or plywood with sawdust on it requires a much less
steep
pitch than material with a higher coefficient of friction
This summer Bill bought some great things from Amazing Toys in Great Falls, MT: http://www.amazingtoys.net/. In particular, Bill obtained Fun Slides Carpet Skates, which also may be used to illustrate friction. They were slipper type shoes with a special low friction sole that allows sliding on carpets. With these you can also determine the coefficient of friction. Larry Alofs volunteered! First we measured Larry's weight (157 lb). Larry put on the slippers and Bill pulled him along the carpet with a spring scale to measure to force needed to slide him along the carpet at constant speed (24 lb). Therefore the coefficient of (kinetic) friction m for this experiment was F/N or 24/157, or about 0.15. Next we made an incline with Larry and the same system to check the kinetic coefficient of friction. The m of 0.15 is the tangent of a 9° angle. Bill angled the ramp a little more than 9° and, sure enough, an initial slight push started Larry down the ramp at a constant speed. Absolutely terrific! Thanks, Bill!
Roy Coleman mentioned that a great science fair project is to use this technique with the ramp coated with a silicone sealer. At angles of increasing magnitude the object will proceed down the ramp with increasing (constant) speed. That is, its coefficient of friction changes with the angle! How come? Interesting, Roy!