By Scott Kim|Wednesday, October 01, 2003

Get Physical
It's time to throw some weight around

Learning physics from a textbook is like learning to ride a bicycle from a manual. To master the subject, you have to try it yourself. But physical experiments can be time-consuming, complicated to set up, and even dangerous. Interactive Physics ( lets students perform repeatable virtual experiments using simulated masses, springs, gears, and other physics-lab staples. The puzzle below is adapted from one of the experiments.

Three blocks are stacked as shown and then released, causing the blocks to topple. The directions in which the blocks fall depend on their weight. If all the blocks weighed the same, they would, surprisingly, barely topple to the right. The blocks in diagram 1, which are labeled with their weight in pounds, are shown half a second after leaving the starting position. The three-pound red block is just heavy enough to cause the one-pound orange block to tip to the left, while the heavier, nine-pound blue block resists tipping either way.

The remaining five diagrams also show the same three blocks half a second after the starting position. But the weight assigned to the individual blocks—one pound, three pounds, or nine pounds—varies. Can you figure out the weight of the blocks in each diagram? Hint: If the red block is lighter than the orange block, both blocks will fall to the right.

David Roy is an artist in Ashford, Connecticut, who has always been fascinated by motion and mechanics. He earned an undergraduate degree in physics and then, inspired by his wife, Marji, an artist, started to create kinetic sculptures. Today, Roy crafts one-of-a-kind and limited-edition kinetic wooden sculptures sold through the couple's company, Wood That Works (

A drawing of a recent work, Tangle, appears above. A spring powers the lower mechanism that pulls lines to control the upper mechanism. There, a central wheel spins about its hub and three darker spirals spin around three pivot points on the wheel. As the wheel turns and the spirals rotate, Tangle takes on different forms, which can be described in terms of angle measurements. For example, if you turn the gray wheel so that the pivot point for the red spiral falls on the 30 degree mark, you could describe the wheel as being 30 degrees from its starting position. Now imagine moving the degree markings so that they center on the red spiral's pivot point. In the spiral's starting position (0 degrees), the hub of the wheel, the pivot point, and the tip of the spiral fall in a straight line. Rotate the red spiral a quarter turn around its pivot point, and the spiral would be 90 degrees from its "straight" position.

Can you match each set of angles below with its corresponding drawing below? For example, drawing C matches angle set 1: The gray wheel has been turned clockwise 15 degrees, carrying the spirals along. The red spiral is still in a straight, 0 degree position, the blue piece has been rotated 90 degrees clockwise, and the purple piece has been turned 180 degrees.

1. 15°, 0°, 90°, 180°
2. 180°, 180°, 180°, 180°

3. 120°, 210°, 300°, 150°

4. 60°, 0°, 240°, 120°

5. 240°, 300°, 210°, 150°

6. 0°, 60°, 150°, 300°

A heavy rock takes more strength to hold up than a light one, so a heavy rock should fall faster, right? If you answered yes, you're in good company. Aristotle believed that heavier objects fall faster, and that belief persisted for centuries.

In the 16th century, Galileo and others took a fresh look at the problem and came to the surprising conclusion that the speed of a falling object depends only on the strength of Earth's gravity, not the weight of the falling object.

According to legend, Galileo simultaneously dropped balls of equal size but different weights from the Tower of Pisa, and they hit the ground at the same time. Although Galileo's conclusion was essentially correct, there are practical problems with the experiment that suggest that the story is apocryphal. Can you think of three reasons why two spherical balls of equal size but different weight would hit the ground at different times if dropped simultaneously from a height of 50 meters above Earth's surface? Hint: Read the problem carefully.


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Let us know what you think of Bogglers: E-mail alternate solutions, comments, and suggestions to

Got new solutions for the puzzle? Want to see other people's solutions? Talk to the puzzle master in his discussion forum at

There are more puzzles and inventions by Scott Kim on his Web site,

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