# Nailing Down Gravity

## We know everything there is to know about gravity, right? Wrong.

By Michael DiSpezioOct 1, 2003 5:00 AM

Scientists are analyzing the velocity of drifting spacecraft and uncovering irregularities that appear to challenge our theory of universal gravitation. So scientists may need to more closely examine the possibility that gravity behaves differently in various regions of space.

### Observing Gravity's Beat

As objects fall to Earth, their speed accelerates. That's an effect of gravity that is usually easy to see. In this activity, you won't be watching the effect. Instead, you'll mostly observe acceleration by listening to impact tempos created as strings of washers fall into an aluminum pie pan.

Metal washers or nutsRulerKite stringAluminum pie panScissorsTimer

1. Obtain a length of string about one meter long. Use your ruler and pen to mark off the string at 10-centimeter lengths.

2. Starting at one end of the string, tie a metal washer (or nut) at the position of the first 10-centimeter mark. Secure the washer with a single simple knot.

3. Tie a second washer at the 20-centimeter position.

4. Continue attaching washers so that a washer is positioned at each of the marks for the length of the string.

5. Secure a washer to the ends of the string as well. Hold the string by one end above the center of the pie pan. The bottom-most washer should be touching the pan's surface. Make a prediction. Will the tempo of the impacts change as the string falls into the pan? If so, how?

6. Now, release the string. What happens?

7. Undo the knots and release the washers. Now consider a different pattern of washer distances. Suppose the pattern reflected a geometric progression and the washers were positioned sequentially so the "internut" distance was 2, 4, 8, 16, 32 centimeters. How would this affect the observed "beat?" Make a prediction, and then test it through experimentation.

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