Head-on this morning on MVT

[lordofthemark]

I bike relative to an absolutely fixed ether. None of this relativistic biking for me.

[LeprosyStudyGroup]

@notmarian 142088 wrote:

I’ve thought about bypassing MVT taking Eads or the secret road up through Crystal City, but that doesn’t get me around this particular grief point, and I’m not a huge fan of that trail connection.

You can bypass those shittiest parts of mvt around gravelly point by continuing past the crystal city water park entrance up crystal drive, turning right on to long bridge drive/boundary channel drive to go north around the pentagon, and then right turning into a parkinglot to cross a wooden bridge which leads into the park/marina parkinglot. That then leads into a loop-around over lil MVT humpback bridge and onto the 14th st bridge. Try it sometime it is a good route which seems to only be used by experienced commuters.

[mstone]

Great, now the pathletes are going to start having delusions about their relativistic speeds.

[sjclaeys]

@Steve O 142082 wrote:

Actually, the slower ones are morons.

Aren’t morons one of the newly discovered basic particles of the universe?

[Vicegrip]

@Amalitza 142092 wrote:

This is an approximately true statement.

This is where you are completely wrong.

There’s no such thing as an absolute zero speed or an objective velocity. Velocity only has meaning relative to something else and it totally doesn’t matter how you define your zero point. If dkel and I are moving towards each other at a relative velocity of 30mph, it does not matter one iota whether you have
– defined your motionless fixed point as me, meaning we view dkel as moving at 30 mph towards me, or
– define dkel as the motionless fixed point meaning that I’m moving 30 mph towards him, or
-define the surface of the earth as the fixed point and we’re each moving 15 mph (or dkel 10 and me 20, or whatever) , or
– define the center of the earth as the fixed point and dkel is moving at 1000mph while i’m moving at 1030mph or
-define that squirrel over there as the fixed point and we’re both moving highly erratically in a weirdly choreographed fashion that somehow results in us coming steadily closer to each other at 30mph.

The physics are the same. We still smash into each other at 30mph. Not 15mph. Not 1000mph. Not whatever the heck that squirrel is doing. 30mph, the total combined speed at which we’re approaching each other.

Yes the Combined speed is 30 but the number of impactors is 2 so division is required. The example has same size vehicles and riders. The amount of energy in a single 15 mph to 0 impact is N. N is 1/2 the energy of two 15 mph impacts. Two impacts into each other is the same energy as 2 into a wall. Where does the energy come from for both vehicles to absorb a 30 mph impact come from? Inorder for one veicle to sustain the energy of a 30 mph impact the other vehicle has to sustain the impact energy of 0.

[peterw_diy]

I’m going to warn my children that failure to pay close attention in physics class may lead to trouble on Web forums…

[dkel]

What Vicegrip said. Also, when we’re done with this argument, we can go back to the tire-pressure-on-the-moon argument. Any takers?

[DismalScientist]

@lordofthemark 142093 wrote:

I bike relative to an absolutely fixed ether. None of this relativistic biking for me.

Actually Amalitza’s argument is highly non-relativistic. If two objects are approaching a fixed point each at a velocity approaching the speed of light, that does not imply that their relative approach speed exceeds the speed of light.

[Vicegrip]

That one is easy and I deal with it often*. on earth at sea level we have 14.7 pounds of atmospheric air pressure pressing on us. Most things that read air pressure like a tire pump, air compressor or tire pressure gauge read pressure as “gauge”. Guage only counts pressure above atmospheric. In a vacuum the pressure used is “absolute” and absolute does not need to take into account the existing pressure. So 100 psi “gauge” at sea level is 114.7 absolute. On the moon a tire filled to 100 psi on earth would feel and ride like a tire filled to 114.7. Given that your mass is being pulled towards the moon with 1/6 the gravity as on earth you can let some air from the tire for a more cushy ride. Have to say I think riding on the moon would be the single best reason to own a fat tire bike. Here on earth I continue to struggle with the desire to get one. On the + side an increase in “N” is always good but the number of days a fat tire bike is the go to per year is too few for me.

*sadly for reasons other than being on the moon.

[Vicegrip]

@DismalScientist 142107 wrote:

Actually Amalitza’s argument is highly non-relativistic. If two objects are approaching a fixed point each at a velocity approaching the speed of light, that does not imply that their relative approach speed exceeds the speed of light.

If it did shining 2 flashlights at each other would cause a real mess!

[Anonymous]

@Vicegrip 142100 wrote:

Yes the Combined speed is 30 but the number of impactors is 2 so division is required. The example has same size vehicles and riders. The amount of energy in a single 15 mph to 0 impact is N. N is 1/2 the energy of two 15 mph impacts. Two impacts into each other is the same energy as 2 into a wall. Where does the energy come from for both vehicles to absorb a 30 mph impact come from? Inorder for one veicle to sustain the energy of a 30 mph impact the other vehicle has to sustain the impact energy of 0.

Yes, I understand that. That’s why the one rider hitting the wall at 15mph is roughly the same (from the perspective of the rider) as two riders hitting each other at 30 mph. Pretty much all the impact force is being absorbed by the one rider while the wall just sits there absorbing negligible amounts of force. It’s not because the wall wasn’t moving when you hit it. It’s because the wall is a wall, and not very good at dissipating force. Riding at 10 mph into a wall that was moving towards you at 5 mph would have the same effect. Because closing speed matters.

[dkel]

@Amalitza 142110 wrote:

the wall just sits there absorbing negligible amounts of force.

The wall isn’t just sitting there, it’s exerting an equal and opposite force, otherwise it would be collapsing or otherwise retreating from the rider. This is why closing speed is not the best way to describe the impact.

[dkel]

@Amalitza 142110 wrote:

Riding at 10 mph into a wall that was moving towards you at 5 mph would have the same effect.

If the wall is on wheels, it’s speed would diminish upon impact, which is why closing speed doesn’t matter.

[peterw_diy]

@dkel 142112 wrote:

If the wall is on wheels, it’s speed would diminish upon impact, which is why closing speed doesn’t matter.

While we’re being super pedantic, may I complain that you used the contraction “it’s” when you should have used the possessive pronoun “its”?

[KLizotte]

Is there a physicist in the house?!?

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