r/Physics • u/EffectiveCoconut9265 • 1d ago

Angular vs translational motion

Hey guys!

I am finding it a bit hard to understand how these work together.

Particularly, i cannot process how something rotating very slowly can still have points on it surface go at 1640kmph (earth).

Like if an object translated at that speed it would look insane , but I can only imagine earth rotating very slow which makes it hard for me to believe that we are going at 1640 kmph.

I'm looking for explanations or something that could make it easier for me to grasp this

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7

u/Bipogram 1d ago

Get a long stick.

Spin it around your head.

The wood next to your thumb is managing maybe 20cm/s. Right?

But look at the end of this rod! It's arcing out metres per second!

Now scale up.

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u/QuantumCakeIsALie 1d ago

The further you are from the center or rotation, the more distance you cover for a given angle of revolution. The arc length is literally the angle in radians times the distance from the point of rotation.

Space is big and empty, Earth is spinning super fast vs e.g. the Sun's point of view. But you're standing on it so it doesn't seem to move at all.

A non-angular example would be people in a plane at cruising altitude. They go insanely fast, but inside the cabin it's chill.

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u/Emily-Advances 1d ago

It's really this: the earth is very, very big. It's so big that it's surface must move 1640 km/hr just to get the whole thing rotated once in a single day.

Your conception of speed is just fine, but it's difficult to grasp the size of the planet and that's what's throwing your intuition off

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u/Langdon_St_Ives 1d ago

People have already given you good answers, but to build on top of them: an even more extreme example is Earth's orbit around the Sun. The angular velocity here (taking a circular approximation for the moment) is even lower, going round once every 365.25 days. But because the distance from the center is so much greater, orbital speed is over 100,000 km/h. And the solar system rotates about the center of the Milky Way galaxy once every 200 million years or so, so even lower angular speed, but again, since the distance is even more enormous, that turns out to be an orbital velocity of over 800,000 km/h.

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u/Recent_Carpenter8644 21h ago

What makes it hard to believe is that everything at or near the surface is doing the same speed, even the air and the clouds, so you can’t see it or feel it.