What shape can substitute for a circle in one sitation?

Train motion puzzle.
Puzzle:

What part of a railroad engine is constantly moving backwards from the direcction the train is going?
Not in relative terms or in relationship to the train engine,but in absolute terms.

Puzzle.

What is the slowest moving part on most automobiles.

Puzzle.

What is the slowest moving part on most automobiles.

Re: What shape can substitute for a circle in one sitation?


Rotary engines use a triangle-shape with bulged out sides. Don't know the name of it, but each side is a circular arc having the opposite corner as it's center.

The portion of the tire that is in contact with the pavement is not moving, for a small percentage of the time. -- Regardless of how fast the car is driving.

Codeborg, Your quick on the draw.

Rotary engines do use the principle of the riddle figure. I don't think, however, that it would function as a roller.
Let's see some further responses and I'll get back to it.


The automobile tire is indeed stopped for a minute fraction of time.

But I'm asking 'What moving part is the slowest moving part on MOST automobiles." A big hint "most cars" -- newer cars don't have this moving part.

CORRECTION
CODEBERG "YOU'RE" NOT "YOUR"

The slowest moving part on most cars moves about 1/4" per 100,000 miles
It is the wheel in the odometer that indicates 100,000 miles.
Newer cars tend to have a digital readout.

Answer to the first riddle - a sphere.

cjhsa,

You said "sphere" and our Panel of judges agrees with you that it is a substitute for circle in this instance. You will be given full credit and your sum total is now $22,040.

However. there is a shape other than a circle/sphere that can act as a roller.

A roller that is not a circle:

However, the circle isn't the only curve of constant width. There is actually an infinite number of such curves, any one of which could form a manhole lid or the cross section of a roller that gives as smooth a ride as a cylinder.


Reuleaux triangle. The simplest such curve is known as the Reuleaux triangle, named after engineer Franz Reuleaux, who taught in Berlin during the late nineteenth century. One simple way to generate this figure is to start with an equilateral triangle, then draw three arcs of circles, with each arc having as its center one of the triangle's corners and as its endpoints the other two corners.
The resulting "curved triangle," as Reuleaux termed it, has a constant width equal to the length of the interior triangle's side. This shape, with rounded corners, may be familiar as the cross section of a bottle of NyQuil or Pepto-Bismol. Its most prominent and successful application may well be in the Wankel rotary internal combustion engine, which powers several types of cars manufactured by Mazda. The engine features a curved, triangular rotor turning in a specially shaped housing.

Like a circle, a Reuleaux triangle fits snugly inside a square having sides equal to the curve's width no matter which way the triangle is turned. Indeed, the rounded triangle can rotate freely inside the square without ever having any room to spare. .

Interestingly, as it rotates, the curved figure traces a path that eventually covers just about every part of the square (except for a little rounding at the corners). This property is the basis for an ingenious rotary drill that, constrained by a special guide plate, bores square holes.

Reuleaux curves based on the pentagon and heptagon. It's possible to construct a curve of constant width not only from an equilateral triangle but also from any polygon with an odd number of sides. Thus, one can readily obtain a curved pentagon, heptagon, and so on. Some coins have a rounded heptagonal shape that allows their use in slot machines designed for ordinary coins. Drills shaped like curved heptagons produce hexagonal holes.

The Reuleaux curves described so far have corners -- points where two sides meet at an angle. However, curves of constant width having rounded corners can be readily constructed from the angular forms. Moreover, a curve of constant width need not be symmetrical or even consist of circular arcs. So there's an unlimited number of curves of constant width, and the Reuleaux triangle happens to be the family member of least area.

Why can't Reuleaux polygons be used in place of wheels? The trouble is that these polygons don't have a fixed center of rotation. The hub of a circular wheel, for example, stays a fixed height above the ground, allowing smooth, horizontal motion. In contrast, the center of, say, a Reuleaux triangle wobbles as the curve rotates. That doesn't matter for rollers laid down on a surface to ease the passage of a heavy load, but it does matter if the roller or wheel has a fixed axis. That's also why the drill for cutting square holes requires a special "floating" chuck to hold the drill.