What depends on what?

By Irum Naz

Everything is either dependent or independent in Physics. Let me explain with three simple thought experiments.

1. Pencil and Ink Pen: Dependency on Gravity

“Be a pencil in a world of ink pen”

Imagine two people in a room where there is no gravity and they both have to write their names. One of them has an ink pen and another has a pencil. Which of them will be able to write their name?

The person with the pencil.

A pencil writes by leaving graphite on paper, so it does not need gravity. A normal ink pen or ballpoint pen needs gravity to pull the ink down onto the paper. If you hold it upwards then it stops writing. In this way, the ink pen is dependent on gravity while the pencil is independent.

This is not just theory. When humans first went to space, astronauts used pencils but later, scientists found that broken graphite could float and damage equipment so they invented the Fisher Space Pen which is a pressurized pen that is independent of gravity. It can write anywhere even underwater.

2. Voice and Radio Waves: Dependency on Air

“You don’t need only a voice to communicate”

Now imagine a second room where there is no air at all. Two people are inside with oxygen cylinders to breathe but they are unable to talk to each other. Why can’t their voices reach each other?

Because sound is dependent on air.

Sound is a mechanical wave. It needs a medium like air, water, or a solid to travel. If there is a vacuum with no air, there is no medium, so the voice cannot travel from one person to another.

This is exactly what happens in space. Space is a vacuum, so astronauts cannot shout to each other even if they are just a few feet away.

Astronauts use radio waves and microwaves because these are electromagnetic waves and are independent so they do not need air to travel. They can travel through a complete vacuum.

3. The Moon and the Sun: Dependency for Light

“Not everything has their own light”

There is a third example right above us every night.

The Sun produces its own light and It is independent. It creates energy on its own through nuclear fusion in its core.

While the Moon has no light of its own. The light we see from the Moon is actually sunlight reflecting off its surface. If the Sun disappeared then the Moon would become dark. So the Moon is dependent on the Sun for its glow.

In the end, physics teaches us a life lesson. Some things, like an ink pen, sound, and the Moon, are dependent. They need gravity, air, or sun to work. Other things, like a pencil, radio waves, and the Sun, are independent. They work on their own in any condition.

In a world full of dependencies, be like the pencil, the radio wave, and the Sun.