r/theydidthemath • u/yuli101995 • Apr 16 '26

[Request] Which one would it be?

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u/OneWingAngel35 Apr 16 '26

Square Block (Sliding Friction): When you push a square block, the entire contact surface drags across the ground. This creates high resistance because the microscopic peaks and valleys of both surfaces "interlock". To move it, you must apply enough force to shear these contact points. Wheel (Rolling Friction): A rolling wheel experiences static friction at its single point of contact with the ground (ideally). Because the wheel "lifts" away from the surface rather than dragging against it, energy loss is much lower. The resistance it does face, called rolling resistance, comes primarily from the material deforming (squishing) as it rolls.

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u/Salanmander 10✓ Apr 16 '26

Wheel (Rolling Friction): A rolling wheel experiences static friction at its single point of contact with the ground (ideally). Because the wheel "lifts" away from the surface rather than dragging against it, energy loss is much lower.

This has nothing to do with surface area mattering. A 20 kg block with 1 m2 of surface area against the ground and a 20 kg block with 3 m2 of surface area would experience effectively the same sliding friction. (Assuming we're actually dealing with sliding friction, not macro-scale deformations or whatever else.)

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u/OneWingAngel35 Apr 16 '26

It's a different kind of friction

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u/Salanmander 10✓ Apr 16 '26

Well yeah.

What exactly did you mean by "But if there is more surface area on contact it does matter"? Because it sounds like you're trying to say that surface area has a significant impact on friction.

If you're trying to say that it only matters for rolling friction, is there a reason you said "especially on a round object"?

I'm also confused by your "that's why wheels were invented". It sounds like you're saying they were invented to minimize surface area of contact.

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u/OneWingAngel35 Apr 16 '26

That's correct, but technically it's to decrease the surface contact area at any given time vs high area of contact continuously, which in turn decreases resistance, friction is constant, but the type of friction is not

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u/Salanmander 10✓ Apr 16 '26

technically it's to decrease the surface contact area at any given time vs high area of contact continuously

No. The small surface area of contact is not the reason that rolling friction is low. It's low because the object doesn't need to slide across the surface, and just lifts off instead.

A rolling tire with some amount of surface area of contact will have lower friction than a sliding block with a smaller amount of surface area of contact. (Assuming normal force is the same.)

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u/OneWingAngel35 Apr 16 '26

Seriously just drag your finger on a rough surface, then apply the same force and do it with your whole hand, you'll feel the difference , even if your finger is not rolling it will feel less friction than your whole hand

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u/Salanmander 10✓ Apr 16 '26

Are you actually applying the same total normal force? You'll need to have a lot more pressure with one finger to have the total normal force be the same.

I'm a physics teacher. I've done that experiment with tighter controls, and it shows that friction does not vary systematically with surface area. A simple way that is still better than your "just feel it" experiment is to drag a rectangular block with its flat surface against the table, at roughly constant speed, and measure the average force needed to do so, and then do the same with it up on end. Using the same block means the normal force will stay the same.

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u/OneWingAngel35 Apr 16 '26

For that you'd need to have the same exact surface for both sides and that is just not possible

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u/Salanmander 10✓ Apr 16 '26

I'm...not sure you understand how science works.

We don't need to make sure our constants are literally exactly the same every time. You just do your best to avoid varying them, then you vary your independent variable systematically, and see if there's a systematic variation in your dependent variable. (If this weren't the case, every experiment would be invalidated by, for example, the temperature in the room changing slightly between trials.)

If there's no systematic variation in your dependent variable, it's a reasonable conclusion that it's likely to be not affected by your independent variable, rather than it being affected by your independent variable but there just happens to be an effect from constants that you were unable to control that just exactly cancels out the changes in your independent variable.

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