High School SMILE Meeting
1997-98 -- 05-06 Academic Years
Mechanical Friction

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.


First Case:  the car is at rest on a horizontal table:
Car on level table

The normal force of the table (upward) and the weight of the car (downward) are balanced: N = W. So far so simple!
Second Case:  the car is at rest on an incline:
car on tilted table
The static friction force f acts upward along the table, the normal force N acts perpendicular to the table, and the weight W acts straight downward. The three forces sum to zero. If qis the angle of inclination , we then have
N = W cos q
f = W  sin q
tan q = f / N = m
m = coefficient of friction
Third Case: Identify all the forces when the car is moving up the table at constant speed.  Interestingly, the force diagram is the same as in the static case, since the net force must be zero for motion at constant velocity.  However, the frictional force f is kinetic, rather than static.  Furthermore, when the wheels are being driven by the power source, the friction force is different from that with sliding friction. In fact, we verified that the car would sit at rest for a table inclination of 40o or less, corresponding to the coefficient of static friction ms = tan 40o = 0.84.  By contrast, when the car drives up the table at an inclination of 45o, one must have a coefficient of friction m = tan 45o = 1.00. These little cars keep getting better for showing physics.  Great, Ann!

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!