Coin Rotation Paradox Calculator
Calculate coin rotation paradox instantly with our math tool. Shows detailed work, formulas used, and multiple solution methods.
Formula
Rotations = R/r + 1 (outside) or R/r - 1 (inside)
When a coin of radius r rolls around a stationary coin of radius R, the total rotations equal the circumference ratio R/r plus 1 (for outside rolling) or minus 1 (for inside rolling). The extra rotation comes from the orbital revolution around the stationary coin.
Worked Examples
Example 1: Classic SAT Problem - Radius 3 and 1
Problem: A coin of radius 1 rolls around the outside of a coin of radius 3 without slipping. How many times does it rotate?
Solution: Stationary coin radius R = 3, rolling coin radius r = 1\nNaive answer (circumference ratio): R/r = 3/1 = 3 rotations\nActual answer (with revolution): R/r + 1 = 3 + 1 = 4 rotations\nThe rolling coin travels a path of circumference 2pi(R+r) = 2pi(4) = 8pi\nIts own circumference is 2pi(1) = 2pi\nCenter path / own circumference = 8pi / 2pi = 4 rotations
Result: 4 rotations (not 3 as naively expected - this was the 1982 SAT error)
Example 2: Equal Coins Rolling Inside
Problem: A coin of radius 2 rolls inside a coin of radius 2. How many rotations does it make?
Solution: R = 2, r = 2, rolling inside\nRotations = R/r - 1 = 2/2 - 1 = 0 rotations\nThe rolling coin translates in a circle without rotating!\nCenter path circumference = 2pi|R-r| = 2pi(0) = 0\nThe center of the rolling coin stays at the center of the stationary coin.
Result: 0 rotations - the coin translates without rotating (special case)
Frequently Asked Questions
What is the coin rotation paradox?
The coin rotation paradox is a counterintuitive phenomenon that occurs when one coin rolls around another without slipping. When a coin with radius r rolls completely around the outside of a stationary coin with radius R, it makes R/r + 1 rotations about its own center, not R/r as most people expect. The extra rotation comes from the fact that the rolling coin also orbits around the stationary coin once, contributing one additional self-rotation. For example, when two identical coins are used (R = r), the rolling coin makes 2 full rotations, not 1. This paradox gained widespread attention in 1982 when a question about it appeared on the SAT, and all the provided answer choices were wrong because the test makers had overlooked the extra rotation.
Why does the rolling coin make an extra rotation?
The extra rotation occurs because there are two separate contributions to the rolling coin's rotation. First, there is the rotation due to the rolling contact between the two surfaces: the coin rolls along an arc equal to the stationary coin's circumference, producing R/r rotations from friction alone. Second, there is the rotation due to revolution: as the rolling coin orbits around the stationary coin, it makes one complete loop, which contributes exactly one additional rotation relative to an external observer. Think of it this way: if you carried a non-rotating coin around a circle and brought it back to the start, it would have rotated once relative to the ground even without any rolling. The total rotation is the sum of both contributions.
What mathematical curves are traced by the coin rotation paradox?
When a point on the rolling coin is tracked as it rolls around the stationary coin, it traces a curve called an epicycloid (for external rolling) or a hypocycloid (for internal rolling). The specific curve depends on the radius ratio. For external rolling with R/r = 1 (equal coins), the traced curve is a cardioid. For R/r = 2, it is a nephroid. For R/r = 3, it is a three-cusped epicycloid. For internal rolling, R/r = 3 produces a deltoid (three-pointed star), and R/r = 4 produces an astroid (four-pointed star). These curves have important applications in gear design, where epicycloidal gear tooth profiles provide smooth, constant-velocity power transmission. Spirograph toys create these curves mechanically.
How is the coin rotation paradox used in mechanical engineering?
The principles behind the coin rotation paradox are fundamental to planetary gear systems (epicyclic gears) used in automatic transmissions, bicycle hub gears, and wind turbine gearboxes. In a planetary gear set, planet gears roll around a central sun gear, and the rotation count follows the same math as the coin paradox. The gear ratio depends on whether the ring gear, sun gear, or planet carrier is held fixed. Wankel rotary engines also rely on this principle: the triangular rotor makes epicycloidal motion inside the housing, with the rotor spinning at one-third the speed of the eccentric shaft due to the internal rolling geometry. Understanding the extra rotation is essential for correctly calculating gear ratios in these systems.
Can this paradox be generalized to non-circular shapes?
Yes, the coin rotation paradox generalizes to any convex shape rolling around another. The key insight is that the total rotation equals the rotation from contact (related to the arc length ratio) plus the rotation from revolution (related to the total turning of the path). For a coin rolling along a straight line, there is no revolution, so the rotations equal the distance divided by the circumference, as expected. For a coin rolling around any closed convex curve, it gains one extra rotation per complete trip, regardless of the curve's shape. For a coin rolling around a polygon, the extra rotation comes from the turns at the vertices. A coin rolling around a triangle (total exterior angle 360 degrees) still gains exactly one extra rotation, split among the three vertex turns.
How can you demonstrate the coin rotation paradox physically?
The easiest demonstration uses two identical coins (such as quarters). Place one coin flat on a table and roll the other coin around its edge without slipping, marking the starting orientation of the rolling coin. After one complete trip around the stationary coin, the rolling coin will have rotated twice, not once as intuition suggests. For a more controlled experiment, use cardboard circles with marked reference points and roll them carefully. You can also demonstrate the inside case by cutting a circle in a piece of cardboard and rolling a smaller disk inside it. Digital demonstrations can be created using geometry software like GeoGebra, which allows precise tracking of rotation angles. The physical demonstration is particularly compelling because seeing the two full rotations challenges most people's intuitive prediction of just one.