Coefficient of friction error???

[dawilliams]

We did a simple test of inline and transverse dynamic coefficients of friction. We made a platform from plywood and wood plates. Screwed the wheels to it. Using a fishing scale, we measure the force of friction sliding the platform inline and transverse to the wheels. The weight on the wheels in the video was approx 140 lb (kid plus weights). We attempted to pull the platform at constant speed once it started moving.

Results? We recorded 12-15 lbs friction force in BOTH inline and transverse modes of dragging the platform. The flooring we used was Lowes sku# 8566, FRP with pebble finish. This means our coefficient of friction was about 0.1.

According to Kit of Parts the inline dynamic coefficient is 0.05, and the transverse dynamic coefficient is 0.1. Thus we should get twice the friction force pulling transverse.

See sample video:

http://www.youtube.com/watch?v=YUwjRcBckn0

Anybody have good phyics explanation for this? Or has the KOP given us bad info? Anybody else try this test?

[Daniel_LaFleur]

Originally Posted by dawilliams

We did a simple test of inline and transverse dynamic coefficients of friction. We made a platform from plywood and wood plates. Screwed the wheels to it. Using a fishing scale, we measure the force of friction sliding the platform inline and transverse to the wheels. The weight on the wheels in the video was approx 140 lb (kid plus weights). We attempted to pull the platform at constant speed once it started moving.

Results? We recorded 12-15 lbs friction force in BOTH inline and transverse modes of dragging the platform. The flooring we used was Lowes sku# 8566, FRP with pebble finish. This means our coefficient of friction was about 0.1.

According to Kit of Parts the inline dynamic coefficient is 0.05, and the transverse dynamic coefficient is 0.1. Thus we should get twice the friction force pulling transverse.

See sample video:

http://www.youtube.com/watch?v=YUwjRcBckn0

Anybody have good phyics explanation for this? Or has the KOP given us bad info? Anybody else try this test?

A couple of possibilities:
1> The 'Kid" weight is dynamic (IE moving) and therefore may not be an adaquate weight measurement
2> is the regolith on berber carpet? The compression into the carpet may influence the force required to move the robot
3> you need to pull with a consistant motion. Is it possible to figure out a more even system to pull the 'robot' ?


And I have a question for you:
Do you know what the CoF is for those wheels on the carpet edge?

[Jared Russell]

Many have arrived at the conclusion that the KOP data was incorrect. I have seen longitudinal and lateral coefficients in the range of 0.05 to 0.1 posted here, so your data fits in with what others have been saying.

[GeoffP]

excellent setup and experiment thanks for the info. I'll stick my neck out on this one. dynamic friction is dynamic friction. unless you have something to provide traction in one direction vs another then it should be the same in all directions because it is an intermolecular bond. in-line scratches from wear may develop which could provide some difference between the in-line and transverse but i doubt a 2-fold increase with 120lbs and this soft of a material. Do your test results suggest they meant static vs dynamic friction? I would expect a 2:1 ratio there. did you happen to measure the force required to initially move the cart from rest and then the force required to keep the cart moving at a constant velocity? I would be very interested in that. we are thinking of limiting our acceleration so we don't exceed the static frictional forces.
thanks again and great job setting that up.

[dawilliams]

We attempted to move at constant speed once we broke away from stand still.

There was no carpet involved.

[eugenebrooks]

Others who have measured the coefficient of friction have reported the same results. There is no physics reason for a different transverse figure, other than the possibility that the wheels that FIRST tested might have had some casting flash that caught the regolith when being dragged sideways. That casting flash will be gone in the first practice match or two.

Trust your data!

Eugene