Cycling UK Forum

Manc33 wrote:Now you've said this I can sort of grasp what you mean, but the fact is you are attached to the planet doing this speed, you're not an outside observer, you are moving at 67,000 MPH (plus or minus 1,040 MPH) and you're not off planet observing that, you're attached to it, so it doesn't come into it to say 67,000 MPH is only applicable to an outside observer, it isn't what they are experiencing as far as the physical movement goes.

Let's assume you're right so consider this:

So we're spinning at 1000mph and moving through space at 67,000mph. So that's 66,000 & 68,000mph right?

Suppose some nasty aliens decide to have fun and speed us up to 200,000mph. Well it still works since you've got 201,000 & 199,000 right? So the difference is still 2000mph.

Suppose they slow us down to 50,000mph. That's ok because we've got 51,000 & 49,000 yes?
What about 10,000. No probs because that's 11,000 & 9000.

So can we agree that the difference is always 2000mph? (If not why not?)

So what happens if they stop the earth.
Well the maths still works because you've got 1000 & -1000.

However, there's a problem. The earth is just a spinning ball, could be a beach ball, could be a spinning top, could be a bike wheel it's all the same for anyone stood on the periphery and spinning with the object.
But the minus sign says they're going backwards...
So if you're right then a person will go forwards on a spinning object for half a revolution and then go backwards...
Yet common sense tells you that a rotating object doesn't spin forwards and backwards - it only ever spins in one direction (unless a force acts to change it).

So how can it be that the maths says the object is changing direction twice every rotation?

The answer is simple, the maths you used assumed that you're stood next to it.
You were measuring the speed of the earth whilst effectively standing alongside it. The actual velocity through space is negated because you're also doing 67,000 mph so the speed of the earth relative to you is zero (as it is for someone sat on it), this is why it doesn't matter what velocity we pick the difference is always 2000mph.
By standing next to it and watching it spin you see people underneath you go past at 1000 mph and those on the 'back' of the earth go past at minus 1000mph but in actual fact they're really just doing 1000mph, there is no change in speed.

It's all to do with where you measure speed from, speed is relative not absolute.
I'm sat at my desk and I'm not moving. The desk, walls and building - the things I measure my velocity relative to are all travelling at the same speed as me.
If I stop spinning with the world and sit in space alongside it then I'm theoretically travelling around the sun at 67,000mph but the earth isn't moving relative to me so all I see is the equator going past at 1000mph.
If I stop moving with the earth and stand next to the sun I can see the earth going through space at 67,000mph.
And if I stop moving with the solar system and stay still relative to the galactic core then the earth is whizzing past at over 500,000 mph.


What I've tried to show here is the change in velocity of 2000mph isn't real. It's an artefact of how you measured the speed of the earth. If what you claimed were true it would also (as I showed above) be true when the earth wasn't moving at all and be true of anything that spins yet it's easy to show that it doesn't happen.

I've just remembered being given a record when I was a kid, featuring the peerless Peter Sellers. To my joy, it's up on the 'tube:
[youtube]xdHmzPhuoF8[/youtube]
As you will hear, Peter completely vindicates Manc's world view. Respect and enjoy!

Good grief! There's a 17th coming up just next month...

kwakers you're using the same argument people use to say "A fly can stop a train". Mathematically yes - there's a point where if the fly is going at 0 MPH the train "has to be" (sigh), but physically, the fly isn't stopping the train, or even causing any slowdown (that can be measured).

This to me is the same as saying "Earth can't be moving at 66,000 MPH on one side and 68,000 MPH on the other side because it would rip the Earth apart" or something. People have said this.

You might as well ask how can a Northern latitude be spinning around at half the speed of the equator, Earth would rip in half. How can a record spin way slower in the middle than around the outside, it would rip apart.

I know you never claimed this kwakers but it sounds similar to what you're saying, that you can't have one side moving at a different speed to the other.

Manc33 wrote:kwakers you're using the same argument people use to say "A fly can stop a train". Mathematically yes - there's a point where if the fly is going at 0 MPH the train "has to be" (sigh), but physically, the fly isn't stopping the train, or even causing any slowdown (that can be measured).

This to me is the same as saying Earth can't be moving at 66,000 MPH on one side and 68,000 MPH on the other side because it would rip the Earth apart or something.

You might as well ask how can a Northern latitude be spinning around at half the speed of the equator, Earth would rip in half. How can a record spin way slower in the middle than around the outside, it would rip apart.

I know you never claimed this kwakers but it sounds similar.

I don't follow what you're saying. Stuff spinning is all about angular velocity and nothing to do with absolute speed. If you stand in the middle of a record absolute speed is less than if you stand on the outside but the angular velocity in radians or degrees per second remains the same and that's why there's no stress on the record and why it won't fly apart or break. Every part of the record maintains it's structural position with regard every other.

Please explain why reducing the speed of the earth to zero still results in your +/- 2000 mph and how that's actually possible despite the fact that it's not observed on any other rotating object.
It's just basic maths, adding and subtracting and fully explained above.

If you spin a record (or CD) fast enough it will fail, and it will do really cool distortions before then...

[youtube]zs7x1Hu29Wc[/youtube]

[XAP]Bob wrote:If you spin a record (or CD) fast enough it will fail, and it will do really cool distortions before then...

Even better with grinding wheels, a lot more spectacular and massively scarier too!

That's all about centrifugal force though. No need to confuse Manc more than necessary. (I have been a bit lax with my mixing of 'speed' and 'velocity' but I'm guessing vector maths isn't his strong point...)

What happens if Earth stopped rotating and was still orbiting?

Is that what you're asking kwakers?

You'd be going at 67,000 MPH all the time, there's no rotating to reduce or increase the constant of 67,000 MPH.

Also in the spinning ball model, the core of the ball is always moving at 67,000 MPH, but because it is spinning at about 1,000 MPH to the East, one side has to be moving at 67,000 MPH minus 1,000 MPH (12PM, equator) and the other side at 67,000 MPH plus 1,000 MPH (12AM, equator).

This has nothing to do with:

- Centrifugal force
- Centripetal "force"
- Gravity
- Frames of reference
- Outside observers
- The notion that things moving should be ripping themselves apart

Manc33 wrote:Is that what you're asking kwakers?

Nope.

And on that, I'm out.
My tolerance for fwits has been exceeded.

Manc33 wrote:What happens if Earth stopped rotating and was still orbiting?

Is that what you're asking kwakers?

You'd be going at 67,000 MPH all the time, there's no rotating to reduce or increase the constant of 67,000 MPH.

.............
- Frames of reference

The frame of reference is important.
Is it tidal locked and does 1 rev per orbit. The sun does not move over one point so it looks like it not rotating but is.
Or is not rotating at all. A day/night lasts half an orbit so it looks like it's rotating but it's not.

What happens when you are in a car, or a train travelling at 70 mph? Outside of a bit of swaying and slight bumps now and again, a passenger can do most of the things that you do sitting at your desk, like using a computer. If you drop your pencil, it doesn't go flying back at 70 mph, does it? That's because it is in the same frame of reference. The pencil only flies away if it leaves the frame of reference, perhaps by being thrown out of the car. Sutff on the earth is the same way. A car on the M25 might be moving at 67031 mph, but what matters to our frame of reference is that it is moving at 70 mph, relative to the earth.

Manc33 wrote:What happens if Earth stopped rotating and was still orbiting?

Is that what you're asking kwakers?

You'd be going at 67,000 MPH all the time, there's no rotating to reduce or increase the constant of 67,000 MPH.

Also in the spinning ball model, the core of the ball is always moving at 67,000 MPH, but because it is spinning at about 1,000 MPH to the East, one side has to be moving at 67,000 MPH minus 1,000 MPH (12PM, equator) and the other side at 67,000 MPH plus 1,000 MPH (12AM, equator).

This has nothing to do with:

- Centrifugal force
- Centripetal "force"
- Gravity
- Frames of reference
- Outside observers
- The notion that things moving should be ripping themselves apart

It has everything to do with centripetal force (outside a rotating frame), centrifugal force (in a rotating frame), and frames of reference. Those two forces are descriptions of how the forces resolve depending on your reference frame...

You keep banging on about 66/7/8 thousand miles an hour - but never actually demonstrate the slightest understanding of the meaning behind those numbers.

What do you think is happening, what makes a sunrise/sunset?
What is so special about the earth that it is the only observable object that is large and flat?