## Friday, June 24, 2016

### Old School Problems

For your enjoyment, some algebra problems I found in a college textbook dating from circa 1893.

## Thursday, June 23, 2016

### Snapshots 2

Yesterday I posted some more snapshots to my Google+ profile. I figured I'd start using that outlet for pictures or little notes, while continuing to post blog-post-type things on this blog.

Anyway here's a link to the snapshots, if you want to see them—art and artifacts, buildings, street scenes, airports and hotels. A few of the pics in smaller format below.

## Monday, June 13, 2016

### Solution to Clock Puzzle

On a wall clock, the minute hand (black) and the sweep second hand (red) are the same length, while the hour hand (grey) is shorter. If the clock starts at noon, then at approximately what time after that do the tips of the three hands first become collinear?

During the first minute, the hour hand will barely move, and the minute hand will move a bit, so there arises a bit of angular separation between them. The line through their tips intersects the far edge of the clock somewhere to the left of the 6. Thus, there will be a time, somewhat after 30 seconds have passed, when the tips of the clock hands are collinear. Here's a picture of that moment:

If we take the hour hand to be three-fourths as long as the other hands, as suggested in the previous post, then we can give a more precise answer: collinearity first occurs after 33.6 seconds have passed.

Between noon and midnight, there are 708 different moments when the tips of the three hands are collinear.

(This count does not include the trivial instances of collinearity that occur when the minute hand and second hand coincide—there are 708 of those also.)

Here's a movie that animates all 708 of the moments when the hand-tips are nontrivially collinear.

If you want to know how I found these 708 cases, click here. (I think this is probably the first time I've ever factored a 1,438th-degree polynomial!)

## Sunday, June 5, 2016

### Clock Puzzle

On this wall clock, the minute hand (black) and the sweep second hand (red) are the same length, while the hour hand (grey) is shorter. At the moment of time shown, the tips of the three hands form a triangle.

Question: If the clock starts at noon, then at what time after that do the tips of the three hands first become collinear?

An approximate answer in the form of a sketch is fine. If you wish to approach the problem quantitatively, then let's agree that the hour hand is three-fourths as long as the other two hands.