Activities for Freefall

1. The pendulum and the non-pendulum.

A pendulum on earth is compared to a pendulum on the International Space Station.


An astronaut in the space station and a scientist on the ground each posses a pendulum made from a fishing weight and a string about 20 cm long.

To Do and Notice

They both pull the pendulum to the side until it is horizontal in the reference frame of the camera. The audience is asked to predict what will happen when the pendula are released. The experimenters release their pendula. The earth based pendulum swings back and forth. the one on the space station remains horizontal.

What's Going On?

The Space station is in freefall.

In the freely falling frame of reference gravity does not cause the pendulum to accelerate relative to the space station. They are both freely falling together.

Note that the force of gravity on the astronauts is only slightly less than the force of gravity on the webcast studio audience. In an orbit 200 miles above the surface of the earth the force of gravity is 90% of its value on the surface.

2. The slinky and the astronaut spine

A slinky on earth is compared to a slinky in freefall.
The comparison shows us a bit about astronaut spines.


An astronaut in the space station and a scientist on the ground each possess a slinky.

To Do and Notice

The bottom of each slinky is held in one hand the top is held in the other hand.

The slinky is oriented vertically. (In the frame of the camera.)

The scientist releases the top of the slinky, it remains compressed in his hand.

The scientist inverts the slinky, one end falls to the ground.

the audience is asked what will happen when the astronaut releases the top of his slinky.

The astronaut releases the top of his slinky. The slinky expands a little.

The astronaut inverts his slinky, it remains stretched just as before.

The scientist holds the slinky horizontal and compressed between his two hands like an accordion and releases it. It falls to the ground and expands while it is in freefall to the same length as the slinky in the space station.

What's Going On?

On the earth gravity pulls down on the slinks of the slinky compressing them into the hand of the scientist. When the slinky is inverted gravity stretches the slinks down toward the ground.

In freefall the slinky and the hand are falling together. There is no compression of the slinky between gravity down and the hand up so the slinky assumes its natural uncompressed length.

So What?

Astronaut spines are like slinkies.

On earth they are compressed by gravity pulling down on the head and the spine itself. In space they expand. This makes fitting space suits difficult.

Astronaut stories. How much did you grow?

3. Oil and Water

Oil and water don't mix on earth, what happens in freefall?


Two squeeze bottles with squirt nozzles one with water and one with vegetable oil. (optional replacement for vegetable oil: mineral oil.)

Soda Straws.

Waxed dental floss.

A wet string.

On Earth: isopropanol.

To Do and Notice.

Gently squeeze the water bottle to make a ball of water at least 2.5 cm in diameter. (Under 5 cm in diameter.)

Notice that you can blow this ball of water around with the straw.

You can also stretch one or more strands of dental floss between your fingers and push on the ball with the waxed strands.

Bring the ball to rest.

Make an approximately equal size ball of vegetable oil.

Blow on it or push it with a wet string to bring it to rest.

Ask the audience what will happen when the ball of oil collides with the ball of water.

Accelerate the ball of oil into the ball of water, notice what happens during the collision.

What's Going On?

The water and the oil are each held in a spherical shape by surface tension.

Oil and water do not mix.

At low velocities the oil and the water balls deform during the collision then bounce away from each other.

Going Further.

Put the nozzle of the oil squirter into the ball of water.

Ask the audience to predict what will happen when oil is injected into the water.

Inject a ball of oil into the water.

The oil will form a ball and remain in position in the water.

And Further

The scientist on earth can do similar experiments with a mixture of water and isopropanol and oil. The water mixture should be adjusted to have the same density as the oil. Then the oil can be suspended as a spherical drop in the water mixture or the water mixture can be suspended as a spherical drop inside the oil.

So What?

In space liquids assume spherical shapes.

In space liquids of different densities can be mixed and will not separate.

In freefall the liquids fall together and do not separate.

4. Meteorite impact

Collide water drops into water balls, what happens can model large meteorite impacts on planets.


A squeeze bottle with a squirt nozzle full of water.

A squirt gun.

To Do and Notice

Make a ball of water 2.5 cm in diameter.

Squirt the ball of water with the squirt gun.

Try hitting it with water streams at different distances and speeds.

Notice the collisions.

What's Going On?

At low speeds the incident streams join the ball of water causing it to oscillate but adding to its mass.

At higher speeds the incident stream can cause the ball of water to eject water balls.

At high speeds meteorites striking planets behave like fluids, splashing rock into space and causing the planetary body to oscillate.

5. Electrostatic ping pong

In freefall, a charged balloon can be pushed back and forth between to like-charged paddles.


Two plastic ping pong paddles

An inflated balloon

Wool Felt.

To Do and Notice

Rub the balloon with the wool felt.

Rub the paddles with the wool felt.

Toss the balloon toward an astronaut with a paddle. the astronaut uses the paddle to repel the balloon el;electrostatically, the balloon comes to rest and then accelerates away from the paddle.

The original astronaut then uses his paddle to repel the balloon back again.

What's Going On.

The plastic and the balloon both get negative charges when they are repelled.

Like charges repel.

The balloon in freefall does not drop to the floor of the space station and so it can be repelled back and forth at leisure by the paddles of the astronauts.

6. Superballs and No-bounce balls.

That's the way the ball bounces, or doesn't, on earth and in space.


Two superballs and two no-bounce balls.

To Do and Notice

The astronaut and the scientist both hod superballs in their hands about a meter above the "floor."

They both drop their balls at the same time.

The ball of the scientist on earth falls to the floor and bounces back up almost but not quite to hand height.

The ball of the astronaut just stays there.

The astronaut then throws his ball gently toward the floor, it bounces up and continues on past his hand to the "ceiling." where it bounces down again.

It continues to bounce off the floor and ceiling losing a little energy with each bounce.

Now take the no-bounce balls.

The scientist on earth drops his ball, it falls to the ground and does not bounce.

The astronaut throws his no-bounce ball gently to the floor. It collides with the floor and ...what do you think will happen?

It bounces slowly off the floor.

the scientist throws his ball into a wall and notices that it does bounce off the wall.

What's Going On?

The superball loses a little energy on each bounce, so on earth it bounces almost to the height from which it was dropped. In the space station it keeps bouncing back and forth between floor and ceiling going slower and slower as it loses energy.

The No-bounce ball loses a lot of energy when it collides with the floor. On the earth it loses so much energy it cannot bounce off the floor again.

In space, in freefall, even the smallest amount of kinetic energy will allow the ball to bounce off the floor. So even the no-bounce ball has enough energy remaining after colliding with the floor to move away from the floor at a constant speed.

Scientific Explorations by Paul Doherty

© 2000 The Exploratorium

21 November 2000