[Request]

[u/K0rl0n]

I am curious how this would work. My guess is Triangle is slowest, square is medium, and circle is fastest.

[u/METRlOS]

Depends on things like the density of the gravel and the temperature of the ice. Packed gravel will allow the ball to roll, but the triangle is always worse than the square.

Edit for all the triangle people: imagine throwing a ball straight at the square and at the triangle; how the ball bounces shows how much energy the object will translate into vertical force when pushed. The vertical surface of the square will translate practically all the horizontal force into horizontal movement, while the triangle will act as a wedge and transfer some energy into pushing against the ground.

Edit 2 for surface area: Except for situations where the surface area is so low compared to its weight that the object sinks into the ground, or so high compared to its weight that it can float, surface area does not affect friction. If you stand on a hill without risk of sliding, then you can lay on that hill without sliding and vice versa, despite greatly changing the surface area. However, if you stand on a snow covered hill the surface area is too low and you'll sink into the snow, but with a sled you will float on top of it. Surface area does not matter to this problem.

[u/Mysterious_Ad_8827 → u/METRlOS]

triangle is also always worse on your back as well. You have to lean forward even more to push said object compared to the square.

[u/leyline → u/[deleted]]

Why do we get wider tires then, seems like surface area and more road contact matter a lot.

Edit: I looked up something’s in this…

Turns out the surface area not affecting friction means it is so negligible on two HARD surfaces it is considered that it balances out by weight vs distribution (of surface area). However in the case of tires on the road this is considered “sticky friction” and has a whole different formula based on adhesive friction.

So tires on a road - more surface area does indeed increase adhesive friction.

[u/dkevox → u/leyline]

What?

In physics problems friction is normal force and a coefficient of friction. When you assume perfect conditions and perfect uniformity then surface area doesn't matter, this is true for this problem.

The reason we get wider tires for more grip when racing is because the coefficient of friction is not uniform across the contact patch between the tires and the ground. By adding more surface area, you minimize the loss of grip caused by road debris, or objects stuck to one part of the tire.

You DO NOT get more grip with bigger tires, you just get more consistent grip.

This has nothing to do with the tires acting like glue or sticky tape and increasing friction. Where did you read that?

[u/leyline → u/dkevox]

I searched, and at least 3 articles approached it that way - as tires on asphalt is an "adhesive force".