Material

A high mounting point which can hold 1000 pounds (500 kg).(Such as the anchor for a handrail 2 stories above the ground. The perfect height is 6 m or 18 feet.)

A mountain climbing rope, static line is best.

A mountain climbing harness.

two locking carabiners.

4 x 4 foot square plywood. (1 m x 1 m)

2 x 4's to create a triangular support structure for the plywood.

Assembly

For the moon.

Attach the rope to the high strong anchor point. To achieve lunar gravity of 1/6 earth gravity the rope attachment point should be 6 times higher than the distance from the attachment wall to the waist of the person.

Make an inclined plane with the plywood and the 2 x 4's.

An inclined plane for a moonwalk tips in one part for every 6 parts of rise.

Make it slope in 1 foot for every 6 feet of rise.
(If you use the 4 foot high board make it slope inward 8 inches.)

Place the inclined plane as close as possible to the wall beneath the anchor point.

To Do and Notice

Have one person put on the climbing harness. (I sometimes use a belay seat which is cheaper)

Attach the harness to the rope by tying it in or by using the carabiners.

The person stands on the inclined plane and jumps gently up and down or turns sideways and takes a step or two.

They will feel what it is like to walk under lunar gravity.

What's Going On?

When you walk on level ground the full force of the earth's gravity pulls you down into the ground.

If you hang from a rope that hangs straight down with your feet touching a vertical wall there will be no part of the downward pull of gravity tugging you toward the wall.

However if you are standing on an inclined plane with the rope holding a part of your weight up against gravity, then only a fraction of the earth's gravity force will be tugging you toward the inclined plane.

Math Root

If the inclined plane slopes inward one meter for every six meters of rise then the gravity force toward the surface of the incline is 1/6 of the force of gravity on earth, the actual gravity force on the moon.

Earth gravity pulls you down, the tension in the rope holds you up and the inclined plane pushes out on you with 1/6 gravity.

The forces sum to zero. They form a right angle triangle.

The sum of the forces on you when you are at rest is zero. Since the tension force acts along the direction of the rope, and gravity acts downward there is another small force required to make the forces sum to zero, this is the force of the inclined plane on the person. It is called the normal force because it is normal to, or perpendicular to, the inclined plane.

The right angle triangle for the forces is similar to the right angle triangle of the inclined plane and the right angle triangle made by the rope and the legs of the person. Since the rope and the inclined plane both have a ratio of 1/6 between their sides, so does the force triangle. And so the force between the inclined plane and the person is 1/6 gravity.

Optional

You can also use a swingset to experience lunar gravity. Build the inclined plane as above and nail it into the ground beneath the swingset so that when the feet of the participant touch the inclined plane the ropes of the swingset are parallel to the surface of the inclined plane.

Going Further

For the ultimate lunar walk experience the inclined plane can be replaced with the frustum of a cone with the rope hung from the position of the vertex of the cone. This allows the participant to turn sideways and walk around and around the cone.

Math Root

The ratio of F_normal/F_gravity is the cosine of the angle at the bottom of the inclined plane. When this angle is near 0 degrees the sin is near 1 and you fell full earth gravity. When the angle is near 90 degrees the sin is nearly zero and no force pulls you in toward the wall. For lunar gravity you want an angle whose sin is 1/6, about 81 degrees, for mars gravity you want an angle whose sin is 0.38, about 68 degreees.

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Scientific Explorations with Paul Doherty

© 2002

10 July 2002