We can find an ordinary pen, which can write normally. There is no doubt that we can write with our heads down, but if we experimenters lie down and try with our heads up, can we write?
You can write with the pen facing up, which proves that even a pen with negative acceleration of gravity and opposite direction can write. What's more, gravity is zero In fact, writing on the vertical wall with a pen can basically simulate the use of the pen in the weightless state of the space capsule.
The space capsule here refers to the space capsule with normal atmospheric pressure, and astronauts don't need to wear pressurized spacesuits. It's not a transition cabin for spacewalking.
In a space without atmospheric pressure, it is estimated that a small bubble in the pen will spray all the liquid in the pen, and even the pen tube will burst. However, this can't be a reason why you can't use a pen in the space capsule where astronauts work and live normally.
The reason why pens are not used in space is estimated to be that excessive acceleration during rocket launch will make pen liquid flow out. If the pen liquid accidentally leaks in the space capsule, it will drift around and cause a short circuit. The air pressure in the capsule cannot be guaranteed to be constant, and the pen is not the lightest writing tool. For various reasons, pens are not used in space, not because pens can never write in a weightless space capsule with air pressure.
Recently, I watched an Indian film. Three fools talked about the problem that the pen can't be used in the space capsule. In the final finale, the film mentioned that the pencil can't be used in the space capsule either, because the lead of the pencil will break, and the broken refill will fly around in the weightless space capsule, causing injuries to astronauts and wiring problems.
So nothing is as simple as everyone thinks.
This question is raised here to remind everyone of a train of thought. Some fallacies can be proved by simple experimental methods, so don't follow suit.