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Scale 100standing still is 100Standing still.

You are standing still. Gravity is pulling you down. Why don’t you fall through the floor?

Eve can say this.

You are standing still. Gravity is pulling you down. Why don't you fall through the floor? Jump. Watch the number. Then try a small hop on a real scale.

Words Eve says

You are standing still. Gravity is pulling you down. Why don't you fall through the floor? Jump. Watch the number. Then try a small hop on a real scale.

You can try this.

Try it on a bathroom scale

A small hop on a bathroom scale is enough. Hold the sink if the scale is slippery.

Stand still. Notice the number. That is the floor pushing up as hard as Earth pulls down.

Bend your knees and jump. The number jumps too, then drops while you are in the air, then jumps again when you land.

You do not need to jump high. A small hop is enough.

What if the floor were ice? Sand? A trampoline?

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

Look closer

Notice

The floor is not a hole. You stay where you are.

If nothing pushed up, you would accelerate downward, the way you do when you jump off a chair.

Something has to be balancing the downward pull. Look at the scale.

What do you think will happen?

If you bend your knees and jump, what will the scale do just before you leave it?

Change how hard you push down before you leave.

Keep looking

What must be true?

While you stand still, two pushes match: gravity down, the floor up.

To jump, you push the floor harder. The floor pushes you harder. That extra push is what sends you up.

In the air, the floor is gone. The upward push is gone. The scale reads zero. Gravity is still pulling.

The floor’s push has a name.

Physicists call it the normal force. “Normal” here means perpendicular, straight out from the surface, not “ordinary.” Gravity pulls you toward Earth. The floor pushes back. When those two match, you stay. You are not being held down. You are being held up.

Eve can read this, if you want.

You are standing still. Gravity is pulling you down. So why don't you fall through the floor? Something has to be pushing up, as hard as gravity pulls down. Jump on a scale. Watch the number. That jump in the number is the floor pushing back. Try it.

Words Eve says

You are standing still. Gravity is pulling you down. So why don't you fall through the floor? Something has to be pushing up, as hard as gravity pulls down. Jump on a scale. Watch the number. That jump in the number is the floor pushing back. Try it.

The push that had to be there

For a long time, people could say “things fall” without asking what the ground was doing.

Isaac Newton’s third law is the spare version of what you just saw: if you push on the floor, the floor pushes on you. Same strength. Opposite direction.

He was not the only person thinking about force. The useful move is the one you just made: if you are not falling through, something must be pushing back.