2015 AMC 10A Problems/Problem 14
The diagram below shows the circular face of a clock with radius cm and a circular disk with radius cm externally tangent to the clock face at o' clock. The disk has an arrow painted on it, initially pointing in the upward vertical direction. Let the disk roll clockwise around the clock face. At what point on the clock face will the disk be tangent when the arrow is next pointing in the upward vertical direction?
The circumference of the clock is twice that of the disk. So, a quarter way around the clock (3:00), the point halfway around the disk will be tangent. The arrow will point to the left. We can see the disk made a 75% rotation from 12 to 3, and 3 is 75% of 4, so it would make 100% rotation from 12 to 4. The answer is .
The rotation factor of the arrow is the sum of the rates of the regular rotation of the arrow (360° every 360° rotation = 1) and the rotation of the disk around the clock with twice the circumference (360° every 180° = 2). Thus, the rotation factor of the arrow is 3, and so our answer corresponds to 360°/3 = 120°, which is 4 o' clock.
The arrow travels a path of radius 30 (20 from the interior clock and 10 from the radius of the disk itself). We note that 1 complete rotation of 360 degrees is needed for the arrow to appear up again, so, therefore, the disk must travel its circumference before the arrow goes up. Its circumference is , so that is traveled on a arrow path. This is a ratio of 1/3, so the angle it carves is 120 degrees, which leads us to the correct answer of 4 o' clock.
Suppose that the small disk also had a clock face on it, and that both disks were toothed wheels, free to rotate around their centers. The part of the picture where they engage would look like this: The small cog has half the radius, and therefore half the circumference. If the large cog turns anticlockwise (i.e. 1 hour), the small cog turns clockwise (i.e. 2 hours). However, in the original problem the large cog does not rotate; it stays where it is. Therefore we must turn the whole diagram above clockwise to see what happens when the small cog rolls around it. It turns out that, when the point of tangency moves clockwise (one hour), from our point of view the small disk rotates clockwise (three hours) around its center. Thus, for the small disk to perform a complete rotation of (twelve hours) around its center from our point of view, the point of tangency must move round four hours. So the answer is
Just imagine the smaller circle rolling in your head. With enough imagination, you can see that it will end up as
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