Motion can cause force in opposite direction {friction}|, from surface-bump collisions as surfaces slide over each other or from electrical forces between close surfaces.
process: sliding
As one surface slides over another surface, surface molecules collide, forcing object backward and upward {kinetic friction}.
process: rolling
If one surface rolls over another surface, collisions do not push surfaces upwards, because surfaces have no sliding. A continuously changing rolling-surface part contacts stationary surface. Bumps and grooves in the surfaces mesh deeper, so contact is greater {static friction}. Static friction is greater than kinetic friction.
factors: force direction
Friction force is directly proportional to force perpendicular to surface. If surface is horizontal, perpendicular force is gravity. If surface is not horizontal, perpendicular friction force F is gravity g times sine of incline-to-horizontal angle A: F = g * sin(A).
factors: area
Contact area between two surfaces affects friction force only slightly, because more area makes pressure and force less, and less area makes pressure and force more.
factors: speed
Faster speed makes less sliding friction, because moving surface rides higher over stationary surface.
factors: electric force
Friction depends on hydrogen bonds and van der Waals electric forces between surface molecules. Smooth surfaces can be in close contact, and so have high electrical attractions and high friction. For example, two smooth glass plates or plastic pieces stick together tightly. Rough-surface molecules are farther apart on average, have smaller electrical attractions, and have less friction.
factors: lubricant
Oil, grease, and graphite can fill surface depressions and keep two surfaces separated, so surfaces have little hydrogen bonding or van der Waals forces, and friction is small. Heavier weight oil flows less easily and sticks to surfaces more, but stickiness causes the objects to have more friction. Lighter weight oil flows more easily and sticks to surfaces less but has less friction. Best-weight oil balances ability to stay in bumps with ability to flow easily. Multi-weight oil flows easily when cold and gets thicker as it gets warmer.
tires
When cornering, radial tires keep more tread on road, so force goes more into road, instead of going into tire side. Radial tires act the same as bias-ply tires while going straight.
Tires with greater radius keep more tread area on road, but this has little effect. Tires with greater radius have larger sidewalls, which can flex more and so become hotter and weaker. Tires with stiff and narrow sides stay cooler and stronger but have harsher ride.
Wider tires have increased area, but more area does not affect friction much. Wider tires can find dry or debris-free road parts, while smaller tires contact only sand or water. Wider tires can aquaplane more on water or snow, because fluid cannot leave treads fast enough.
Physical Sciences>Physics>Dynamics>Friction
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Date Modified: 2022.0224