What Causes Weightlessness?

A Simple Guide to the Physics of Weightlessness

It's not a result of being far from Earth!

Skip the long lesson and "cut to the chase" -- What causes weightlessness in space?

Astronauts train for shuttle mission STS-114 in NASA's KC-135 aircraft. Photo courtesy of NASA.

Weightlessness is the absence of weight. But what is weight?

Weight is the force of an object pushing against its support mechanism. For example, if you are sitting in a chair, the force of your bottom pushing against the chair is weight, which is balanced by the force of the chair pushing against your bottom. Take away that force, and you are weightless.

An instrument for measuring the force of weight is the scale, such as a bathroom scale. The scale has a spring inside. The higher the weight, the farther the spring is stretched.

The scale tells me that I weigh about 165 pounds (75 kg). However, my weight is not a constant. For example, if I were to travel to to the top of Mt. Everest and stand on the same scale, I would weigh about half a pound less because of the weaker gravity several miles farther from the mass of the Earth. If I were to travel to the surface of the Moon and stand on the same scale inside a Moon lander space ship, I would weigh only one-sixth as much, about 28 pounds (13 kg), due to the Moon's weaker gravity.

My weight can also vary due to motion. Suppose I travel back to Earth and stand on the scale. I start hopping up and down a little bit. My weight fluctuates!

Imagine that I make bigger and bigger hops on the scale. The fluctuations get larger and larger.

Now imagine that I hop so hard, I jump completely into the air and my feet leave the scale.
The scale goes to zero. I'm weightless! The weightlessness I experience in my jump is exactly the same the weightlessness experienced by astronauts.

Well, almost the same.
My weightlessness is rather brief, just a fraction of a second while I'm in the air. The astronauts experience weightlessness much longer. Also, I feel a small breeze as I move through the air. The astronauts don't feel any breeze because the space ship and its air are moving along with them. However, in principle, my weightlessness is the same as what the astronauts feel. Any person or object that is falling freely, without being held up by anything, is weightless. 

Later on in this article, I'll suggest some weightlessness experiments you can carry out yourself in your back yard while jumping off of a picnic table or chair.

Next: Weight and Mass

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2009 Gray Chang