Friction and the science fair

The Boy wants to do a science fair experiment related to friction. I'm trying to think of GRM related aspects that can be scientifically observed by a 10 year old. Basic concept to measure would be to put an object onto a surface that can be tilted, and measure the angle that corresponds to the intiation of movement by the object. Pinewood Derby is all about friction. Metal that is bare/painted/waxed should have different coefficients of friction.

Any other ideas, either for cool test setups or things to test?

Slot cars.

Rubber wheels vs plastic on the same track.

better to stick with sliding friction rather than rolling friction. hockey puck on teflon cookie sheet versus hockey puck on sandpaper, etc. big differences show better than subtle differences.

Connect a rubber band to a block of 2x4.

Take a board about 2' long and wrap it with various things... wax paper, sand paper... and then demonstrate friction in terms of how much the rubber band stretches when the block is dragged across the different surfaces.

Spin stuff really fast!

http://en.wikipedia.org/wiki/Friction_welding

Using the stationary object on a tilting platform model, test the lubricating effectiveness of various substances to overcome the dry friction tilt necessary to move the object.

Make it 10 year old fun by testing stuff like peanut butter, pudding, milkshake, jello, etc.

bludroptop wrote: Make it 10 year old fun by testing stuff like peanut butter, pudding, milkshake, jello, etc.

+1 add water, veggie oil, wd-40

Here's something that might boggle your mind:

Friction is independent of surface area.

Why does tire width make a difference in observed g-loads?

scardeal wrote: Here's something that might boggle your mind: Friction is independent of surface area. Why does tire width make a difference in observed g-loads?

Have to check, but I'm pretty sure that coefficient of friction is independent of surface area. If your statement is correct, that could be cool to test (wood block with solid bottom vs. wood block with slatted bottom.

I also thought, that surface area was a factor of tire pressure and weight. The tire will deform until there is enough surface are to balance the pressure of the air. (e.g. A 3000lb car with tires inflated to 30lbs/sqin would end up with 100sqin of contact patch.)

I thought the difference was that wider tires had a different shape of contact patch, that was more stable under side load. Also, same height of sidewall, but lower ratio means more lateral stability and more progressive break away, with the same ability to absorb linear force.

Just my (probably completely wrong) hypotheses.

salanis, you're close. it is because friction is not independent of temperature. larger tire has more heat sink and more convective surface, so runs cooler for a given input load, therefore takes longer to get thermally saturated and start down the greasy side of the grip vs temp curve.

AngryCorvair wrote: salanis, you're close. it is because friction is *not* independent of temperature. larger tire has more heat sink and more convective surface, so runs cooler for a given input load, therefore takes longer to get thermally saturated and start down the greasy side of the grip vs temp curve.

Then why do larger tires offer greater grip for auto-x?

How would they be running cooler? Yes, they have greater heat-sink, but they also have to bleed off more heat to run at the same temperature (temp =/= heat). Greater mass at the same temperature = greater heat.

Maybe they'd gain temperature slightly slower, but since their contact patch is greater, they're going to build more heat (thus similar temp) in the same time.

Seems much more likely that it's a factor of stability of the contact patch. A race tire is constantly dealing with minor upsets, and a wider tire offers a greater surface that will be less bothered by a single imperfection, and will recover from these faster. At their instantaneous peak, they might offer the same traction, but the mean traction of the wider tire will be greater. It would be interesting to do a controlled test on a perfectly flat smooth surface and see the results.

I'm pretty friction is independent of temperature, but I am sure the stickiness of the tire's compound isn't independent of temp. Too cold and they won't stick as much. Too hot and they won't stick as much.

The argument is that a larger tires contact patch isn't any greater than a smaller tire, it's just shaped differently - wide rather than long. But bigger tires do grip more. Now possibly the wider contact patch puts more side to side friction and less fore to aft.

friction and adhesion are not the same thing

When cars run larger tires, do they also run lower pressures?

Friction is independent of surface area only for non-deformable objects, like a block of steel sliding on a block of aluminum. When you introduce a deformable object like a tire, you get into a situation where the tire starts filling in the visible and microscopic grooves in the surface of the road and you get direct mechanical forces on the tire as well as friction. This is why wider tires give more grip (you have more mechanical interaction of the tires with grooves) but as you continue to increase the width of the tire you get less gain in benefit because the mass forcing the tire into the pavement is spread over a larger area so the tire doesn't get forced as deep into the grooves.

Bob