Try This

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One push. Then it falls.

Not to scale. The ground is a line so the idea can be seen.

You throw a stone. It leaves your hand and falls. Something that keeps pushing can leave. What changed?

Eve can say this.

You throw a stone. It leaves your hand and falls. Something that keeps pushing can leave. What changed? Blow up a balloon. Let it go.

Words Eve says

You throw a stone. It leaves your hand and falls. Something that keeps pushing can leave. What changed? Blow up a balloon. Let it go.

You can try this.

A balloon, then a stone

Balloons and a soft throw. No stones at people or windows. A grown-up nearby is wise.

Blow up a balloon. Do not tie it. Let it go. Watch which way it goes.

Then throw a small stone or a ball. One push. Then it falls.

Do not aim at a face. Do this outside or in a hallway with a grown-up nearby.

What if the balloon never ran out of air?

Your question. Not ours. It stays on this device.

Look closer

Notice

The stone only has the push you gave it. Then your hand is gone.

After that, gravity is in charge, and air drags a little.

If something keeps pushing, it can keep changing speed after it leaves the hand.

What do you think will happen?

A balloon full of air. You let go. Which way does the balloon go?

Change one push, or keep pushing.

Keep looking

What must be true?

A thrown stone has one push. Then it coasts, and gravity pulls it down.

Something that keeps pushing, by throwing mass behind it, can keep speeding up.

It does not need air to push against. Throwing mass backward is the push.

The keep-pushing has a name.

Thrust is a push made by throwing mass out the back. Newton’s third law again: if you throw gas backward, the gas throws you forward. A stone cannot do that once it leaves your hand. A balloon can, until the air is gone. Rockets work in empty space for the same reason. They are not paddling in the sky.

Eve can read this, if you want.

You throw a stone. It leaves your hand and falls. A thrown stone only has the push you gave it. Then gravity is in charge. Something that keeps pushing, by throwing mass behind it, can keep changing speed. It does not need air to push against. Try a balloon. Let the air out. Watch which way it goes.

Words Eve says

You throw a stone. It leaves your hand and falls. A thrown stone only has the push you gave it. Then gravity is in charge. Something that keeps pushing, by throwing mass behind it, can keep changing speed. It does not need air to push against. Try a balloon. Let the air out. Watch which way it goes.

The push that does not need air

A common guess is that a craft needs air to push against, the way a swimmer needs water.

That guess fails in a vacuum, and it fails with a balloon in a room: the push is the stuff going backward.

The stone is honest about a single push. The balloon is honest about keeping it up.