MICROGRAVITY CONCEPTS

SOME NOTES AND ACTIVITIES by Mike Roach

Microgravity is a term to describe a very low gravity environment. The word "micro" comes from the Greek word meaning small. Weightlessness is a similar term used to describe (nearly) zero-gravity .

Astronauts in earth orbit are still being pulled by gravity and so they still have weight, but the astronauts (and their space shuttles and space stations) are "falling "around the Earth. They no longer feel the effects of gravity and their weight. They"'feel" weightless! Their legs and their pelvic region no longer support their insides, and their organs respond as though gravity was not present. They are in a state of apparent weightlessness. The blood in their bodies is no longer pulled down to their legs by gravity as on Earth. Their blood "floats", and so distributes evenly in their bodies, giving the astronauts much fuller round faces than back on Earth. Everything about living in a weightless environment is different to being on Earth.

CAN YOU FEEL WEIGHTLESS ON EARTH?

Yes of course you can! All we need to do is bungy jump, fall off a roof, or dive from a high tower into a swimming pool. However the Earth's gravity accelerates us faster and faster towards it the longer we fall. We know that objects fall 5 metres in the first second of fall, but they travel 15 metres in the next second, 25 metres in the next and so on. The ground soon comes to meet us!

NASA has Drop Towers to study the effects of weightlessness, but even these huge structures 145 metres in height, do not provide much more than 5 seconds of experimental data.

To establish microgravity conditions for any length of time so the astronauts can become accustomed to the effects of weightlessness it is necessary for them to take a flight in a specially prepared airplane (the KC-135) which flies a parabolic path. As the plane comes over the top of its path the trainee astronauts experience 22 seconds of weightlessness each loop. They will do this about 20 times each flight, and they experience alternating cycles from around two times gravity force (2g) as the plane climbs up to microgravity (almost zero g), at the top.

The plane is a commercial sized Boeing 707 with most of its seats removed. Padded walls protect the people inside. Although these planes cannot achieve microgravity conditions of as high quality as those produced in drop towers ( as they are never completely in free fall as air friction is considerable) at least the experimenters can ride along with the experiments.

If scientists want to investigate weightlessness effects on various experiments they will put them aboard the Space Shuttles (for between 8 to 16 days) or on the Space Station (Mir) for longer periods of time.

During the year 2000-2001 many countries (including the USA, Russia, Japan, Canada, Italy, England, France and Germany ) will begin building the International Space Station. Scientists will then have a permanent home in a weightless environment.

TWO ACTIVITIES THAT DEMONSTRATE THE EFFECTS OF WEIGHTLESSNESS

1. THE FALLING CUP.

1. Take a polystyrene cup ( or maybe to be environmentally friendly, you could substitute an empty coke can)

2. Fill it to near the top with water.

3. Go outside on a balcony, or above a stair well with a rubbish bin for your cup to fall into. At least stand on a desk, so students get a good view .

4. Announce that you have water in the cup, and that you are going to punch a hole with your biro in the side of the cup (or can) about 2 cm from the bottom. What do the students think will happen?

You can draw attention to the parabolic shape of the water as it falls towards the ground

5. Place your finger back over the hole and question them on why the water fell out?

They will probably come up with the word gravity if you ask them what made the water fall.

It is gravity that makes you feel your weight. If you go to the moon there is only one sixth gravity compared to Earth. The water would take much longer to reach the ground.

6. Announce that you are going to briefly let more water fall out of the hole, and then you will quickly count one, two, three drop so they don't miss seeing the falling cup.

7. Now remove your finger, say "1,2,3 drop" and drop the cup!

The students should notice that no water flows from the cup during "free fall". It was as if the water had no weight to cause it to fall! "The water became weightless". Now that does not mean that gravity turned off, because the cup still fell to the ground. But so did the water, they were both falling at the same rate towards the Earth, so no water fell out of the cup.

Falling objects are in a state of apparent weightlessness.

2. FALLING MAGNETS

1. Build your own plastic cylinder using one overhead transparency. Roll it up so that the cylinder has a diameter that loosely fits a bar magnet (around two centimetres). Wrap masking tape around the cylinder once at top and once near bottom to secure it. Form a bottom by overlapping two pieces of masking tape at one end. Now you have an open cylinder.

2. Place one of the magnets in the cylinder so that it slides to the bottom. Identify the pole that is facing up. Place the other magnet with a like pole to the magnet facing up in the cylinder.

The magnets may push apart, but more likely they will touch, as the weight will overcome the repulsive force.

3. Now tell the students that you will put the magnets in a weightless situation by dropping the cylinder. This time you need not stand on the table, but it does get the class attention, as the effect is quite visible immediately. Tell the students that you will count 1,2,3 and drop, and the drop the cylinder and magnets into a waste paper basket or some other soft landing (if you wish to use the same apparatus again).

This activity shows that completely different conditions exist for astronauts on the space shuttle. How could powerful magnets move things around in space?

WANT MORE ? Try http://microgravity.nasa.gov/oeK-12.html

FALLING AROUND THE EARTH

If you drop a stone from waist height it will fall in a straight line towards the centre of the Earth (to the ground). If you throw the stone outwards (horizontally) from your body it will fall in a curved path towards the ground (Diagram 1). If you throw it harder, the stone will travel faster, and the stone will land further from you, but still falling in a curved path (called a parabola).

Now imagine yourself to be on a "very small earth". You could throw the stone fast enough to make its curved path match the curvature of the small earth. (Diagram 2). You could throw the stone just right, and it will follow a circular ORBIT. Gravity keeps the stone attracted to the earth but it is going fast enough to do a complete orbit.

EXPERIMENT TWO: Swinging the Billy.

This is the way that cattle drovers and bushmen in the outback used to treat their billy-can tea before they poured it?

We will not use boiling water, for obvious safety reasons, but the experiment is the same.

1. Get a bucket and put a litre or so of water in it.

2. Swing the bucket in a circle vertically above your head and towards the ground

If you swing the bucket fast enough you will not get wet, and the water will stay in the bucket.

If you slow down or stop the bucket above your head the water will fall out.

EXTENSION QUESTIONS and DISCUSSION:

What is the slowest speed ( in metres per second ) necessary to swing the bucket so that the water in bucket is travelling fast enough so it does not fall on your head?

Did the old bushmen know about microgravity and weightlessness? Or was there another reason?

The reason they did this was so that the tea leaves would be pushed to the bottom of the billy can, and even this has a very important space science application for future space stations.