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Cool Physics Video
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This whole video is fascinating but at the 3:00 min mark it starts to get me thinking that a rolling hill TT course should be faster than a flat one but that never seems to be the case.

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Re: Cool Physics Video [Nolegs] [ In reply to ]
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I was probably using Best Bike Split wrong but it always showed a hilly course being faster for me.

Despite that, even though I put out more power (197 watt AP=2:39 time, 2200' gain) on 56 mile hilly courses compared to 56 mile flat courses (188 watts=2:23 time, 787' gain), I have always had slower bike splits on hilly courses.

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Re: Cool Physics Video [Nolegs] [ In reply to ]
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I can't watch this until later, but I've trouble believing there's a valid argument for a rolling course being faster for the same power.
What are they proposing? You can match up interval efforts with climbs and resting on descents but can't see it being possible to do better than a constant effort on a flat route.
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Re: Cool Physics Video [Ai_1] [ In reply to ]
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no, they are just showing that if you are coasting down a steeper hill or larger hill that you go faster. uh, wow, who knew?!

edit: um, let's see, we are comparing different potential energy states and then being shocked when the kinetic energy state is different
Last edited by: jeffp: Jun 28, 18 7:59
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Re: Cool Physics Video [Nolegs] [ In reply to ]
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I would suspect that for the theory to work on a bike course the rollers would have to be shallow enough that the momentum from one downhill would take you to the top of the next hill.
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Re: Cool Physics Video [jeffp] [ In reply to ]
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That's fascinating... I wasn't expecting the balls to be at the same time because acceleration is constant and one distance is shorter
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Re: Cool Physics Video [Nolegs] [ In reply to ]
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Is there any way to see the video without a Facebook account?

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Re: Cool Physics Video [lacticturkey] [ In reply to ]
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if teh track wasn't specifically designed to accomplish that, teh results would be different, ie on a constant slope, the results would not be the same
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Re: Cool Physics Video [DarkSpeedWorks] [ In reply to ]
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This chrome link works for me without logging in: https://www.facebook.com/...ts/1498536866917512/
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Re: Cool Physics Video [JoelO] [ In reply to ]
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JoelO wrote:
I would suspect that for the theory to work on a bike course the rollers would have to be shallow enough that the momentum from one downhill would take you to the top of the next hill.

That doesn't require shallow hills, it requires magic. [Edit]...or for the 2nd hill to be lower ;) [End of Edit]

If we're talking physics, perhaps I might mention entropy?
Last edited by: Ai_1: Jun 28, 18 8:21
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Re: Cool Physics Video [Nolegs] [ In reply to ]
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An ideal case where this could apply would be an out and back time trial, where you put the start/finish and the turnaround at the tops of two hills, versus on a flat road.
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Re: Cool Physics Video [brando] [ In reply to ]
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Yes, that would work because you're using the downhill for acceleration and the uphill where you would otherwise be braking. If the hills were short enough it would help, otherwise no.
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Re: Cool Physics Video [jeffp] [ In reply to ]
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I think the PE vs. KE is the key observation. In the ball rolling tracks, he is showing two different paths where PE+KE is constant (ignoring friction losses), but one of those paths more time is spent with a bigger KE component, resulting in a larger avg. velocity. At the other extreme to his rolling track would be a perfectly flat track where the ball should never move.

So would this help you on a rolling TT vs. a flat TT? Sure, it could in theory, but the problem is that we can't ignore friction losses. The friction loses from wind go up as v^3, which sucks. And the PE=>KE gains from those downhills are not that big compared to the energy needed to overcome the friction losses. Otherwise you could just roll down a hill and stay at a constant speed on the next flat without having to pedal at all. So the net result on a rolling course is that it is slower. Unless you are in a vacuum, then you definitely want a rolling course.
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Re: Cool Physics Video [Nolegs] [ In reply to ]
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Clearly, the ball that's faster going up & down the hills, has a little motor in it

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Re: Cool Physics Video [Nolegs] [ In reply to ]
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Beware drawing analogies from physics experiments like this. If we are going to play around in pure physics land (no energy in or out), the fastest possible bike course is going to be one where I drop you straight down and you perfectly elastically bounce back to the start. :) That is assuming you don't care about covering horizontal distance.
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Re: Cool Physics Video [jbank] [ In reply to ]
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According to this video 180km would take as long 40km if it's all downhill and start at the same elevation?
Last edited by: lacticturkey: Jun 28, 18 10:10
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Re: Cool Physics Video [lacticturkey] [ In reply to ]
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Yes - if there are no losses

But there are. And they are BIG
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Re: Cool Physics Video [Ai_1] [ In reply to ]
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What are the losses that prevents it?

You mean like wind resistance becomes a limiter, stuff like that?
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Re: Cool Physics Video [Nolegs] [ In reply to ]
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Nolegs wrote:
This whole video is fascinating but at the 3:00 min mark it starts to get me thinking that a rolling hill TT course should be faster than a flat one but that never seems to be the case.

https://www.facebook.com/...os/1498536866917512/
This is great information for the next time I find a TT that is done only by coasting.
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Re: Cool Physics Video [lacticturkey] [ In reply to ]
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lacticturkey wrote:
According to this video 180km would take as long 40km if it's all downhill and start at the same elevation?

That is not generically true, but would be true if the course was along a circular shaped profile. The guy from the video was trying to make some cool points about pendulums. And one of the super cool things about a pendulum is that the period (to a very good approximation) is purely a function of the length of the pendulum (which is why they are good in clocks). A ball (or bicycle) riding along a circular profile is just like a pendulum.

Let's play with a toy problem. The formula for a pendulum's period is T = 2pi sqrt(L/g), where L is the length of the pendulum arm, g is gravity and T is the period. We get our smallest T by having the shortest L. Conveniently for our 40km/25mi TT course a half-circle profile with a radius of ~8mi works out almost exactly to 25mi and would fit our criteria of shortest L that doesn't go past vertical at the ends. So how long does this course take in our ideal no friction world. L=8 mi. g=32 ft/s^2. T=2pi*sqrt(8mi * 5280ft/mi / (32 ft/s^2)) = 228 seconds. Not bad. And if you started halfway down for a 20km TT, it would also take 228 seconds, cause of the whole pendulum thing having a constant period. If we picked a "flatter" course, like 25mi section at the bottom of a circle with a 16 mile radius, the period of the pendulum would be longer, so we would be slower. In the limit, a flat course corresponds to a pendulum with infinite radius and our time goes to infinity as well (no pedaling of course).

How fast would you be going at the bottom? Well, from conservation of energy, the PE at the top is the same as the KE at the bottom. So we have mgh=1/2 mv^2. Simplifying, we have v=sqrt(2gh). h here is our radius L. So v=sqrt(2 * 32 ft/s^ * 8mi * 5280 ft/mi) = 1644 ft/s. That is fast, 1121 mph.

Lets see why friction sucks. If we were really riding the course I described, we wouldn't hit 1121 mph. We'd be pretty lucky to top out at 80 mph if you have a really good aero tuck on your bike. So that downhill segment instead of taking us 228s/2=114s, takes us optimistically 12.5mi/80mph, or around 10 minutes, not that bad. Now we have to ride up a 12.5 mile quarterpipe starting at <80mph instead of rolling in at 1121 mph. Even discounting riding up the cliff face at the top, I'm going to say doing more than say 10 mph avg (even at say 5 w/kg) is gonna be tough on that second half (8 mi of elevation in 12.5 mi of riding), which is going to mean the second half takes more than 75 minutes, for a total of 85 minutes. Quite a bit slower than the 228 seconds without friction. And certainly slower than a flat TT would have been given the effort required to climb the halfpipe.
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Re: Cool Physics Video [jbank] [ In reply to ]
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That's a fantastic analysis! Thanks

Now I'm just wondering why a bike course against the earth's direction of rotation isn't faster than with j/k
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