Can someone explain this....................?

getprg

Here goes...............

When I am out riding the bike the road stands still whatever speed I go (usually slow!)

So doesn't that mean that the part of the tyre in contact with the road is also static. It follows that if the bottom of the tyre is static then so is the top and of course the rest of it. If I am doing 5 mph or 50 mph (in my dreams) my wheels are standing still - aren't I going to fall off!!!!?

Come on all physicists - please explain this to me in words of one syllabubble (or less).

Kieran_Burns

Go look at a gyroscope....

gtvlusso

WTF are you on about?

rick_chasey

getprg wrote:

Here goes...............

When I am out riding the bike the road stands still whatever speed I go (usually slow!)

So doesn't that mean that the part of the tyre in contact with the road is also static. It follows that if the bottom of the tyre is static then so is the top and of course the rest of it. If I am doing 5 mph or 50 mph (in my dreams) my wheels are standing still - aren't I going to fall off!!!!?

Come on all physicists - please explain this to me in words of one syllabubble (or less).

Relative to the ground, the point of contact on the wheel between the wheel at the road is stationary, at the point directly opposite that on the wheel is going twice as fast as the bike is going in total.

Mr Plum

Kieran_Burns wrote:

Go look at a gyroscope....

+1

janm399

I'm not a physicist, but bike riding cannot have anything to do with gyroscopes.

Suppose that staying upright on a bike relies on the gyroscopic effect of the rotating wheels. This means that the gyroscopic effect has to be fairly strong. But that would mean that it'd be very difficult to steer. So, it can't be the gyroscopic effect of the rotating wheels.

I think it has more to do with preservation of momentum. Once you get a mass moving, it will--without any further inputs--keep moving in the same direction. So, once we get moving on a bike past certain speed, we stay upright because it would take too much force to topple us.

As for the wheel, I don't want to get into calculus (not to mention that the editor lacks symbols for the definite integral); suffice to say that at some speed v > 0, an epsilon-neighbourhood of a point on the wheel is still relative to the road for some delta-neighbourhood of any point in time t; where epsilon and delta are *very small* numbers.

Kieran_Burns

janm399 wrote:

I'm not a physicist, but bike riding cannot have anything to do with gyroscopes.

Suppose that staying upright on a bike relies on the gyroscopic effect of the rotating wheels. This means that the gyroscopic effect has to be fairly strong. But that would mean that it'd be very difficult to steer. So, it can't be the gyroscopic effect of the rotating wheels.

I think it has more to do with preservation of momentum. Once you get a mass moving, it will--without any further inputs--keep moving in the same direction. So, once we get moving on a bike past certain speed, we stay upright because it would take too much force to topple us.

As for the wheel, I don't want to get into calculus (not to mention that the editor lacks symbols for the definite integral); suffice to say that at some speed v > 0, an epsilon-neighbourhood of a point on the wheel is still relative to the road for some delta-neighbourhood of any point in time t; where epsilon and delta are *very small* numbers.

I know all this....

I wanted him to go look at a gyroscope 'cos they're really cool and it'll keep him occupied and stop asking stupid bloody questions 8)

jimmypippa

janm399 wrote:

I'm not a physicist

janm399 wrote:

As for the wheel, I don't want to get into calculus (not to mention that the editor lacks symbols for the definite integral); suffice to say that at some speed v > 0, an epsilon-neighbourhood of a point on the wheel is still relative to the road for some delta-neighbourhood of any point in time t; where epsilon and delta are *very small* numbers.

I take it that you mean that you are not a physicist by profession.

I bet you studied physics to at least A-level.

For clarity, as I kept misreading the red bit, "still" means "stationary" in the above quote...

I kept thinking you were meaning "remaining relative" and having a WTF... :oops:

Jay dubbleU

getprg wrote:

Here goes...............

When I am out riding the bike the road stands still whatever speed I go (usually slow!)

So doesn't that mean that the part of the tyre in contact with the road is also static. It follows that if the bottom of the tyre is static then so is the top and of course the rest of it. If I am doing 5 mph or 50 mph (in my dreams) my wheels are standing still - aren't I going to fall off!!!!?

Come on all physicists - please explain this to me in words of one syllabubble (or less).

It just does right ? - now go and play with the gyroscope

Jay dubbleU

getprg wrote:

Here goes...............

When I am out riding the bike the road stands still whatever speed I go (usually slow!)

So doesn't that mean that the part of the tyre in contact with the road is also static. It follows that if the bottom of the tyre is static then so is the top and of course the rest of it. If I am doing 5 mph or 50 mph (in my dreams) my wheels are standing still - aren't I going to fall off!!!!?

Come on all physicists - please explain this to me in words of one syllabubble (or less).

It just does right ? - now go and play with the gyroscope

EKE_38BPM

Squirrel!

SimonAH

EKE_38BPM wrote:

Squirrel!

Haha!
Perfect EKE

PBo

you are taking a "static" snapshot of a "dynamic" system.

If you did start to fall off, then by definition, the clock must have started again, and therefore your wheel rolls again - so you'll be fine!

Simples!

PBo

janm399 wrote:

I'm not a physicist, but bike riding cannot have anything to do with gyroscopes.
.

bet you're a bl00dy mathmo though - epsilon neighbourhoods indeed.

whilst the relative impact/size of the gyroscopic effect on a bike may not be as important as once thought, fact remains the front wheel is a gyroscope and therefore MUST have something to do with riding a bike....

rolf_f

Could be worse - with railway wheels, part of the wheel is actually moving backwards!

EKE_38BPM

Ferris wheels are stationary, but part of them go up!!!! How does that work?

Squirrel!

shouldbeinbed

But is the road static if you factor in the rotation of the earth and its orbit of the sun? Where's that squirrel?

Dog Breath

I see what the OP is saying though. I'm no physics expert, but one thing that still confuses me (and related to the original question) is that if you look at the caterpillar track of a bulldozer, the point of contact of the track with the ground does not move - it stays on the same point on the ground. And yet the top of the track (the non-contact part) must be moving very fast to overtake the bottom part, So how can the same track be moving and not moving at the same time :?

EKE_38BPM

Question for you, Dogbreath.
A train is heading north at 125mph. A fly is heading south at 12.5mph.
The fly and train collide with each other and the fly is squished on the windscreen of the train and the train driver and the passengers don't feel a thing. But, at one point in time the fly goes from 12.5 mph (southerly) to 0 mph as it hits the windscreen and then to 125 mph (northerly) as a smear on the train's windscreen.
At the instant that the fly is stationary it is in contact with the windscreen, so the windscreen must also be stationary.
If the windscreen is stationary, is the rest of the train also stationary?

PBo

Dog Breath wrote:

I see what the OP is saying though. I'm no physics expert, but one thing that still confuses me (and related to the original question) is that if you look at the caterpillar track of a bulldozer, the point of contact of the track with the ground does not move - it stays on the same point on the ground. And yet the top of the track (the non-contact part) must be moving very fast to overtake the bottom part, So how can the same track be moving and not moving at the same time :?

think of a small loop of string. imagine you've put your finger in it and pressed down at a point to trap the loop onto the table. Now put 2 fingers from your other hand in the loop and spread the fingers out to tension the loop. If you move both of those fingers left and right as far as you can go, you are moving a point on the top of the loop, even thought a bottom point is fixed - it's just like you are making a different shape with the slack string. So even though it's got wheels in it, a caterpillar track is doing the same thing - it's just taking slack from behind the fixed point and moving it forward over the top.

walkingbootweather

jimmypippa wrote:

janm399 wrote:

I'm not a physicist

janm399 wrote:

As for the wheel, I don't want to get into calculus (not to mention that the editor lacks symbols for the definite integral); suffice to say that at some speed v > 0, an epsilon-neighbourhood of a point on the wheel is still relative to the road for some delta-neighbourhood of any point in time t; where epsilon and delta are *very small* numbers.

I take it that you mean that you are not a physicist by profession.

I bet you studied physics to at least A-level.

For clarity, as I kept misreading the red bit, "still" means "stationary" in the above quote...

I kept thinking you were meaning "remaining relative" and having a WTF... :oops:

So I go into the pub and say to the barman "can I have a glass of orange please?". "Is that still orange?" asks he. "No, I've changed my mind, you had better make that a beer"

Cycling makes the world go around. If I don't get out on my bike every day, the world will probably stop spinning. Don't think this is related to any gyroscopic effect, however if I were made redundant, I could cycle more and that might be described as a giro-scopic effect.

PBo

EKE_38BPM wrote:

Question for you, Dogbreath.
A train is heading north at 125mph. A fly is heading south at 12.5mph.
The fly and train collide with each other and the fly is squished on the windscreen of the train and the train driver and the passengers don't feel a thing. But, at one point in time the fly goes from 12.5 mph (southerly) to 0 mph as it hits the windscreen and then to 125 mph (northerly) as a smear on the train's windscreen.
At the instant that the fly is stationary it is in contact with the windscreen, so the windscreen must also be stationary.
If the windscreen is stationary, is the rest of the train also stationary?

EKE, the windscreen is still moving at 125mph even for that split second that the flies velocity is 0 - unless you are looking at it from the windscreen's pov - in which case the windscreen's velocity is always 0.

tx14

EKE_38BPM wrote:

Question for you, Dogbreath.
A train is heading north at 125mph. A fly is heading south at 12.5mph.
The fly and train collide with each other and the fly is squished on the windscreen of the train and the train driver and the passengers don't feel a thing. But, at one point in time the fly goes from 12.5 mph (southerly) to 0 mph as it hits the windscreen and then to 125 mph (northerly) as a smear on the train's windscreen.
At the instant that the fly is stationary it is in contact with the windscreen, so the windscreen must also be stationary.
If the windscreen is stationary, is the rest of the train also stationary?

you assumed the velocity of the fly is continuous. ie it goes from 12.5 to -125 taking every value in between. but the force from the impact means the fly's acceleration jumped so v' for the fly is discontinuous which doesn't actually say anything about the velocity of the fly.
i lost myself.

PBo

tx14 wrote:

EKE_38BPM wrote:

Question for you, Dogbreath.
A train is heading north at 125mph. A fly is heading south at 12.5mph.
The fly and train collide with each other and the fly is squished on the windscreen of the train and the train driver and the passengers don't feel a thing. But, at one point in time the fly goes from 12.5 mph (southerly) to 0 mph as it hits the windscreen and then to 125 mph (northerly) as a smear on the train's windscreen.
At the instant that the fly is stationary it is in contact with the windscreen, so the windscreen must also be stationary.
If the windscreen is stationary, is the rest of the train also stationary?

you assumed the velocity of the fly is continuous. ie it goes from 12.5 to -125 taking every value in between. but the force from the impact means the fly's acceleration jumped so v' for the fly is discontinuous which doesn't actually say anything about the velocity of the fly.
i lost myself.

no - the flies values of v have to be continous, even if changing very rapidly.......you did indeed lose yourself.

where's other physics geeks when you need them....

KB?

Kieran_Burns

getprg wrote:

Here goes...............

When I am out riding the bike the road stands still whatever speed I go (usually slow!)

So doesn't that mean that the part of the tyre in contact with the road is also static. It follows that if the bottom of the tyre is static then so is the top and of course the rest of it. If I am doing 5 mph or 50 mph (in my dreams) my wheels are standing still - aren't I going to fall off!!!!?

Come on all physicists - please explain this to me in words of one syllabubble (or less).

Being serious - you're confusing dynamic motion with a static moment.

At any given time the wheel is stationary but if the wheel is stationary then so is the rest of the body. How can anything fall when it is stationary?

You can't look at a single event and deduce motion from it

Kieran_Burns

PBo wrote:

tx14 wrote:

EKE_38BPM wrote:

Question for you, Dogbreath.
A train is heading north at 125mph. A fly is heading south at 12.5mph.
The fly and train collide with each other and the fly is squished on the windscreen of the train and the train driver and the passengers don't feel a thing. But, at one point in time the fly goes from 12.5 mph (southerly) to 0 mph as it hits the windscreen and then to 125 mph (northerly) as a smear on the train's windscreen.
At the instant that the fly is stationary it is in contact with the windscreen, so the windscreen must also be stationary.
If the windscreen is stationary, is the rest of the train also stationary?

you assumed the velocity of the fly is continuous. ie it goes from 12.5 to -125 taking every value in between. but the force from the impact means the fly's acceleration jumped so v' for the fly is discontinuous which doesn't actually say anything about the velocity of the fly.
i lost myself.

no - the flies values of v have to be continous, even if changing very rapidly.......you did indeed lose yourself.

where's other physics geeks when you need them....

KB?

Same as I've said above - you are looking at a dynamic system as series of non-related events.

At a given point of time (take note that point means an infinitely small period) the fly is stationary regardless of anything else going on.

As is the train.

but this does not take into account the period of time before, or after. You MUST view a dynamic system in dynamic terms to understand the CHANGE.

In this case the fly doesn't actually instantaneously go from 12.5mph to -125mph it decelerates massively quickly but NOT instantly. The fly is NOT one body. Its arse will still be doing 12.5mph while its head will be doing -125mph.

PBo

thanks KB - confirmed my own diagnosis.....I knew i could rely on you.....





















geek

EKE_38BPM

So, in the case I outlined, whats the last thing to go through the fly's head?


























Its ar$e!

Thanks a lot folks, I'm here all week.
Try the prawn cocktails and don't forget to tip your waitress.

Shaky

PBo wrote:

tx14 wrote:

EKE_38BPM wrote:

Question for you, Dogbreath.
A train is heading north at 125mph. A fly is heading south at 12.5mph.
The fly and train collide with each other and the fly is squished on the windscreen of the train and the train driver and the passengers don't feel a thing. But, at one point in time the fly goes from 12.5 mph (southerly) to 0 mph as it hits the windscreen and then to 125 mph (northerly) as a smear on the train's windscreen.
At the instant that the fly is stationary it is in contact with the windscreen, so the windscreen must also be stationary.
If the windscreen is stationary, is the rest of the train also stationary?

you assumed the velocity of the fly is continuous. ie it goes from 12.5 to -125 taking every value in between. but the force from the impact means the fly's acceleration jumped so v' for the fly is discontinuous which doesn't actually say anything about the velocity of the fly.
i lost myself.

no - the flies values of v have to be continous, even if changing very rapidly.......you did indeed lose yourself.

where's other physics geeks when you need them....

KB?

I have no idea what any of you are talking about. However, I'm pretty sure that the last thing to go through the fly's mind is his @ss......

Shaky

EKE_38BPM wrote:

So, in the case I outlined, whats the last thing to go through the fly's head?


























Its ar$e!

Thanks a lot folks, I'm here all week.
Try the prawn cocktails and don't forget to tip your waitress.

Beat me to it by a minute

EKE_38BPM

Shaky wrote:

Beat me to it by a minute

Thats what my gf always says to me!