Friction

Friction is presented as a loss in most of this subtest, and half the questions about it are about cases where it is the point.

What friction is

Enlarge

Zoom in

Friction is the force resisting sliding between two surfaces in contact.

It always opposes the direction of motion, or of attempted motion. It cannot drive anything; it can only resist.

It converts kinetic energy to heat, which is why brakes get hot, why rubbing your hands warms them, and why a machine's efficiency is never 100 percent.

Static against kinetic

Static friction is greater, which is why a heavy crate takes a hard shove to start and then slides more easily. The push that was needed at the moment it broke free is more than the push needed to keep it going.

That difference is the most asked fact in the topic, and it explains several practical things:

• Antilock brakes work by keeping the tire rolling rather than sliding, because a rolling tire grips with static friction and a skidding one with kinetic.
• A skidding vehicle takes longer to stop than one braking at the threshold of a skid, for the same reason.

Rolling friction is smaller still, which is why a wheel is such an old and good idea. Sliding a load is far harder than rolling it.

What changes friction

Two things:

The materials in contact. Rubber on dry asphalt grips hard; steel on ice barely at all. This is captured by a coefficient of friction, and the subtest asks about it comparatively rather than numerically.

How hard the surfaces are pressed together, which is the normal force. A heavier object presses harder and experiences more friction.

And one thing that does not change it:

The contact area. A brick slid on its wide face and on its narrow edge experiences the same friction, provided the weight and the materials are the same. Spreading the same weight over more area reduces the pressure at each point in exactly the proportion that it adds points of contact, so the total is unchanged.

That is counter-intuitive and is asked directly. Wide tires improve grip for reasons involving heat, wear and deformation rather than this simple model, and at the level this subtest asks, area does not change friction.

Reducing friction

• Lubricate with oil or grease.
• Use bearings to replace sliding with rolling.
• Smooth the surfaces, up to a point.
• Reduce the load, which reduces the normal force.
• Choose materials with low friction against each other.

Bearings and seals

Enlarge

Zoom in

A bearing supports a rotating shaft with low friction.

A shaft seal keeps lubricant in where a rotating shaft leaves its housing.

In an engine, the oil pump supplies pressurized lubricant to the moving parts and the oil filter removes particles from it. Oil is changed at intervals because it breaks down and collects contaminants.

Wanting friction

Half the questions in this topic are about friction being useful.

A world without friction is one where nothing can be gripped, fastened, stopped or walked on. A question asking which of four things would fail without friction usually has all four failing, so read for the one where friction is the whole mechanism.

A screw's thread friction is what stops it unwinding under load, which is the point the screws lesson makes and a good example of friction doing the job rather than obstructing it.

What you can skip

Across the 66 questions on this topic in our bank:

• The laws of friction by name and the angle of repose never appear.
• Fluid and viscous friction come up once. The friction questions are about solid surfaces sliding and rolling.
• Coefficients beyond the arithmetic. The coefficient comes up 12 times, and every use is friction force = coefficient x normal force. There are no tables of coefficients to memorize.

Where people lose points

Saying kinetic friction is larger. Static is.

Thinking a larger contact area means more friction. It does not, at this level.

Saying friction can cause motion. It opposes it; what it does is let another force act without slipping.

Forgetting friction produces heat.

Treating friction as only a loss. Brakes, tires and fasteners depend on it.

Thinking a smoother surface always means less friction. Extremely smooth surfaces can grip harder, which is beyond scope but worth not asserting the opposite of.

Work one in under a minute

Why does a vehicle stop in a shorter distance when its wheels are on the verge of locking than when they are fully locked and skidding?

A rolling tire grips the road with static friction, because the contact patch is not sliding relative to the surface at the instant it touches.

A skidding tire grips with kinetic friction, which is smaller.

So the skidding vehicle has less retarding force and travels further. That is the whole reason antilock braking exists, and it is the static-against-kinetic fact doing practical work.

Where this leads

Friction is the reason every machine falls short of its ideal advantage, and it is the mechanism behind brakes, clutches and belt drives in Auto and Shop.