# Gravity Bar

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## Introduction: Gravity Bar

This movement takes about 4 minutes--scroll along to see the results.

This is a simple test rig (mostly 3d printed) based on a design by John Walker http://www.fourmilab.ch/gravitation/foobar/
Gravity is a weak attraction between two objects (beyond that, things get pretty speculative). This "gravity bar" is a rod with lead weights on the ends. It is suspended from a ladder using 6 pound test fishing line. A "daggerboard" on the bottom of the rod sits in water to dampen rotation.

Parts needed:

Bar with fins to hold weights (3d print from file in next step or fabricate as shown by John Walker above).

Wire (I used about 24 inches of #26 magnet wire--most any wire will suffice)

About six feet of six pound fishing line

Small plastic cup to hold water

To follow the latest, go here.

### Teacher Notes

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## Step 1:

Print this file to create the gravity bar.

## Step 2:

Tie the wire through the end holes of the bar. These loops will join together above the bar and connect to the single strand of fishing line.

## Step 3:

Tie the fishing line from the gravity bar to the top of the ladder.

## Step 4:

Place the lead weights on the end of the rod--until the rod is balanced. Place water in a plastic cup and be sure that the fin is in the water while the fin and the bar do not touch the cup.

Let the rig rest for about 36 hours so that the fishing line will untwist and stretch.

Place the steel balls adjacent to the lead weights as shown. Very slowly (around 4 minutes), the rod will twist and the weights will move close to one another.

This rig needs to be in a room without a lot of vibration or air currents (garage or basement without climate control works well).

## Step 5:

For better results, I have made the bar longer. I've also included a "sphere holder" to support the steel balls.

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## 5 Discussions

Nice, have you tried it the other direction to be sure it's not due to another effect?

I have tried other directions and sometimes there is activity--sometimes not. So . . . I'm not at all certain as to what causes the movement, but it's interesting to watch and think about.

I had the same problem when I was trying to build a Foucault pendulum. Hard to tell how much affect the line and other imperfections in the apparatus. Very cool project though - I like testing the "laws of nature".

The steel balls provide concentrated mass at a close distance--moving the bar quickly (if you can call three or four minutes "quickly"). This setup is good for a demonstration. I have not at this time attempted any measurements; vibration and air currents are enemies to be dealt with.

Yes, your idea of a mirror and laser pointer is excellent for further study. For example, is there a shift over a 24 hour period as the moon changes its relationship to earth? (I don't know the answer to this question).

Wow- ignore my previous question about sophisticated measuring equipment, I didn't watch the video. I had no idea the effect was so large! I'm surprised it can overcome the elasticity of the fishing line.