Falling off a roof

My dad, and probably everyone else’s dad, has said that falling off the roof doesn’t hurt and it’s only when you hit the ground that the pain begins.

Hitting the ground also causes any remaining air in the lungs (after all the screaming on the way down) to leave the body with an “ooof” sound. The ‘f’ part is probably the start of another sound, but by then there’s no air in the lungs.

That hitting the ground pain is fairly intense and makes you long for that time when you were still falling through the air.

Back then you had time to consider how strongly the Earth pulls you towards it. And yet, it isn’t all a one way street.

While you close in on the ground at an accelerating rate the Earth is moving towards you. Not by much, granted, but it is.

Certified genius Isaac Newton worked it out years ago when he watched an apple falling from a tree. He saw the apple. Saw the Moon. Put two and two together to come up with ‘G’ for gravity (and genius).

Gravity is amazing and more than a bit mysterious. Boffins know what it does, but there seems to be a lack of consensus about what causes it.

Graviton particles, frame acceleration, warped spacetime, gravity fairies? But one thing they agree on is that it is incredibly weak.

I mean, a rocky ball 12,000km in diameter can’t pull a fridge-magnet off the fridge. Pathetic.

One of my first cars was so underpowered my mates said it couldn’t pull the skin off a custard. But I reckon it could deal with a fridge magnet, no worries.

So, let’s agree that gravity is pathetic. But try telling that to yourself as you plummet from the roof towards the ground while preparing for a 100km/h impact.

Question: You are at the edge of the roof of a tall building, ready to drop a bucket full of water. Before you let go you push a cork to the bottom of the bucket with a stick. When you do let everything go, the cork is free to move in the water. But what does the cork do as the bucket falls? Float to the top, stay at the bottom, hover around the middle or pop out of the water and fall separately?


1 Comment on "Falling off a roof"

  1. Bruce Hollow | December 8, 2021 at 1:17 pm |

    The cork will stay at the bottom of the bucket and not float up at all during the fall, since the cork, the bucket and the water all fall at the same acceleration g (neglecting air resistance as all good physics questions do). The buoyant force of the water does not push the cork upward in this case since in free fall the buoyant force is zero. In other words, a cork only floats up when the water around it is denser than the water above it. In free fall the water is weightless, as is the cork and bucket, so there is no force to push the cork to the top of the bucket.

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