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ELI5 Yahaaa

You would come back

What If You Fell Through a Hole in the Earth?

You would fall to the far side, arrive at walking pace, and fall straight back — forever.

1 Step in2 Speed up allthe way tothe middle30Slow down tothe far side4 Arrive atzero speed.Fall back.
Nobody catches you at the other end. You do not need catching.

  1. Step 1 of 5

    You would speed up all the way to the middle.

    Falling through the centreabout 28,000km/hA rocket leaving Earthabout 40,000km/h escapespeedA rifle bulletabout 3,000 km/h

    For the whole first half, everything below you is pulling you down and there is nothing to slow you but air. You would pass the centre at about 28,000 kilometres an hour — faster than any spacecraft has ever gone.

  2. Step 2 of 5

    Gravity gets weaker as you go down, not stronger.

    At the surface All of Earth is belowyouDeep inside Rock pulling upcancels rock pullingdown

    This is the surprising part. Once you are underground, the rock above you pulls up while the rock below pulls down, and those two cancel out more and more as you descend. Halfway to the centre you weigh about half what you did.

  3. Step 3 of 5

    At the very centre you weigh nothing at all.

    You, at the centrePulled up by everything abovePulled down by everything below
    Weightless and fastest, at the same instant.

    There is now an equal amount of Earth in every direction, so every pull is matched by an opposite one. You would be weightless — and moving faster than at any other moment of the trip.

  4. Step 4 of 5

    You arrive at the far side at exactly zero speed.

    Fall0Centre:fastest Rise Stop, exactlyat thesurface
    A pendulum, with the whole planet as the string.

    Past the centre, gravity is behind you, slowing you down by exactly as much as it sped you up. You would rise up the far shaft and come to a dead stop with your fingertips at ground level. If nobody grabbed you, you would fall back and do it again forever.

  5. Step 5 of 5

    It takes 42 minutes — for any hole you dig.

    Straight through thecentre 42 minutesA short chord near thesurface Also 42 minutes

    London to Sydney, or a short chord from Paris to Berlin: both take the same 42 minutes. A shorter tunnel has weaker pull along it, and the two effects cancel exactly.

    Real Earth would stop you long before: the centre is about 5,000°C, and the air in the shaft would burn you up. The 42 minutes is what the physics says, not what the rock allows.

The short version

Fall through the Earth and gravity gives back exactly what it took: you arrive at the far side at a standstill, 42 minutes later.

Try it yourself

Swing a pendulum. It rises to exactly the height you released it from, never higher. The hole is the same trick with a planet.

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