If you watch anything about space travel on TV, they often talk about things being "weightless" in space. In fact, they still have weight. For example, equipment on the International Space Station still weighs about 90% of their weight on the surface of the Earth. So how can astronauts and other things float around inside the ISS or any other spacecraft? This week, we will find out.
You will need
A disposable plastic cup. I found a Styrofoam one worked well.
Something to make a hole in the plastic cup with.
Water
An outdoor area, ideally over a garden or some grass.
A bucket
What to do
Make a hole in the side of the cup, near the bottom.
If you used a Styrofoam cup, this is easy. If you are using a thin plastic cup, you may need an adult's help.
Hold the cup so your thumb covers the hole.
Fill the cup with water.
Hold the cup up as high as you can.
Uncover the hole. The water will squirt out in a steady stream. Try not to aim it at anyone, unless they have annoyed you.
Try to predict what will happen if you let go of the cup. As the cup falls, will the water squirt out more quickly, more slowly or at the same rate?
Let go of the cup. What happens?
What's happening?
You should find that when you let go of the cup, the water stops flowing out almost instantly. As it drops, the water will stay in the cup until it hits the ground.
When you were holding the cup, gravity was pulling down on the cup and the water. The cup didn't move, because you were holding it. Gravity also pushes the water down against the bottom of the cup. The water at the bottom of the cup squirts out because of the weight of the water above it pushing it against the bottom of the cup.
When you let go of the cup, gravity accelerates the cup and the water inside it equally, so they fall at the same speed. Since they are falling together, the water doesn't feel a force pushing it against the cup any more, so the water doesn't squirt out.
The same thing happens inside a spacecraft. When an astronaut is sitting inside a spacecraft on the ground, gravity pushes them down into their seat. Once the spacecraft has launched and the engine is turned off, the only force acting on it is gravity. The astronaut appears to be weightless because gravity accelerates the astronaut and the spacecraft equally, just like the water and the cup. The astronaut doesn't feel any force pushing them against the seat, just like the water didn't feel a force pushing it against the cup.
A better term for the way things in spacecraft seem to be weightless is "free fall", because both the spacecraft and the astronaut are falling without anything stopping them.
Applications
Astronauts can't use normal cups in free-fall. Since gravity wouldn't pull water to the bottom of the cup, the first time they tried to take a drink they would splash it all over their faces. Astronauts drink from squeeze bottles instead.
On Earth, water and other liquids always flow downhill. In free-fall there is no downhill, so spacecraft designers need to use pumps to move liquids around.
Make a hole in the side of the cup, near the bottom.
Hold the cup up high and let the water flow out. If you hold a book in your left arm, you can pretend you're the Statue of Liberty.
When you let go of the cup, the water stops pouring out of it.
