#reduced speed

[diagnozabam]

Viteza minimă pe drumurile din România: Ce spune legea și ce sancțiuni riscă șoferii care circulă prea încet?

Pe drumurile din România, reglementările rutiere stabilesc limite maxime de viteză pentru a preveni accidentele, însă există și situații în care legislația prevede sancțiuni pentru conducerea cu o viteză excesiv de redusă. Iată cum se aplică aceste reguli și ce trebuie să știe șoferii. Viteza minimă și accesul pe drumuri de mare viteză Conform Art. 74, alin. 1 din legislația rutieră: „Pe…

[chriskotiesen]

[vampirehizzies]

I'm beginning to think we're a lot alike, you and I.

[vagueconfusion]

source

[venacoeurva]

Tip: fucking hate lining the outside of whatever you’re drawing for whatever reason that particular day? Lasso tool with border layer setting then brightness to opacity baby

[thnksfrthmmrs]

[thenotoriousscuttlecliff]

Magneto had a really great character arc in the 80s which just got completely chucked in the bin because the new editor taking over the title thought that he should always be a one-note maniacal villain, and I'm disappointed to see X-Men '97 doing the same because the ex-showrunner wanted to recreate his favourite issue.

[tea-time-terrier]

We...don't have a broad jump.

[thetimelordbatgirl]

Look, I'm not saying Eternals is the ruler of gay rep in the MCU, but the fact that it actually showed two husbands together in scenes together and even kissing but got nothing versus fans celebrating being queerbaited by Agatha All Along basically...

[network-rail]

Why are obstacles on the tracks a problem?

Previously, I mentioned that when a train encounters an obstacle on the line such as a tree branch, what happens is a complicated physics process that results in the train pushing the branch along the line. Here I will explain that process, but be aware that complicated physics things are about to happen. There are some pretty diagrams to look at though, so if you want you can look at those and then skip to the end for a summary. They're even in color!!

First of all, some basic setup (before putting some numbers in):

We have a train travelling at an initial velocity u, with mass M, and an engine capable of producing a constant power P (we will use this to restore the train's velocity to u if it decreases for some reason).

The train encounters an obstacle on the line, such as a tree branch of mass m. We will assume (for now) that the collision is elastic - that is, no energy was lost (for instance, as sound).

We are also assuming a frictionless vacuum, cylindrical tree branch, and rectangular train.

To start with, we look at conservation of momentum (figure 1):

Since the train has elastically collided with a branch, its speed is reduced, given as Vtrain . As trains are typically much heavier than a single tree branch, we take M >> m, and so Vtrain ≈ u.

However, this is somewhat unrealistic, as when a train hits an obstacle, energy is lost –as a crunch sound, for instance– so it may be more appropriate to assume an inelastic collision. Since I said that the branch sticks to the train (and I am right), we should assume a completely inelastic collision, where as much kinetic energy as possible is lost.

Again, we look at conservation of momentum (figure 2):

In this case, if we again assume M>>m, we still get v ≈ u.

Since we know from reality that problems will happen if the train collides with the branch, this tells us that we have made an unrealistic assumption somewhere. In this case, it must be the assumption that the train's mass, M, is large enough that the branch's mass m can be ignored. So, without this assumption, we look at how long it takes the train to get back to its initial speed, using the equations for motion under constant power (equations derived from Taylor, 1930 and shown in figure 3):

To find how much energy is used in each case, simply multiply the time by the power.

By now, you may be wondering what the point of all this is – after all, I haven't actually shown you if this is meaningful. So let's add some numbers to this and see how reasonable all of our assumptions were!

If we take the train's mass to be M=30 tonnes (30,000kg), its power P=1500kW, and its initial speed u=40 m/s (144 km/hr) respectively; and assume the branch has a mass m=5kg (that seems reasonable, right? Trees are mostly water, which is 1kg per cubic meter, and if it has a radius of 0.5m and a length of 1.435m, it should be about that much), we can calculate all the various things we need:

First, the final velocity of the train and branch in the inelastic case (see figure 2 for the equation):

v≈39.99m/s which is pretty close to the initial velocity.

The time taken to return to speed (fig. 3) for the train/branch system is:

t≈0.0053s

This is quite fast, but hold on: the energy used to do this is about 8000 joules, which is probably quite expensive at current electricity prices, but those are given in kWh and I really don't feel like converting between them. (8000 is a big number, right?)

For the elastic case, things are a little bit more complicated, as we have two different velocities to calculate (figure 4):

If you were just looking at the pictures and are upset that the last two have been equations, don't worry, the next one isn't.

Vbranch ≈ 79.99 m/s

Vtrain ≈ 39.98 m/s

The time taken to return to speed:

t≈0.0094s

This is almost double that of the inelastic case, resulting in the energy used increasing to the enormous –and probably expensive– value of 14 kJ. (I even needed to use an SI prefix this time! And one of the ones that makes things bigger!)

However, both of these cases also reveal some interesting things about the situation: the elastic case has the tree branch launched away from the train at 80m/s, which is about 288 km/hr. Since the train and branch are likely irregularly shaped, the branch probably won't be pushed along the tracks at 290km/hr, and could instead be launched into the air space towards you. Nobody wants to be in the situation where a tree branch is flying towards you at almost 300 km/hr. I could do some math to see how much it would hurt, or if you could reasonably expect to dodge it, but I think we can just assume it will be quite painful.

Historically, trains avoided flinging branches at nearby passengers at almost 500km/hr (that's half the speed of sound) by employing a triangular device on the front of the train to deflect objects such as cows off the tracks. These were particularly common in North America, where lineside fences have yet to be discovered outside of the Northeast Corridor and it is easy for things to wander onto the tracks. However, thanks to innovations by the Budd company and others, more recent american trains are basically indestructible, rendering obstacle deflectors unnecessary. The effects of the obstacle deflector are shown below (figure 5):

This device is known in America as a burgerizer, since it can provide an easy meal for the train crew –two of the five ingredients for a cheeseburger right on the front of the train, more if you're lucky– although since usually the obstacle is shoved off the track, the British name of "cowcatcher" is misleading, especially if you hit a truck instead. The burgerizer's physics can easily be calculated using conservation of momentum, but this involves vectors, and I don't want to deal with vectors right now is left as an excercise for the reader.

In the inelastic case, we note that the branch sticks to the front of the train. Since the inelastic case is more realistic (I will not justify this statement), this means that other things will also stick to the train. By the time the train reaches the end of the line, the mass of the things stuck to it may end up not being negligible (figure 6):

If the train is electric, this will strain the power grid and could lead to power cuts elsewhere as more energy is given to keep the train running at speed. If the train is diesel, it will be unable to provide constant power and could slow down (an electric train has access to every power station in the country if the need arises, a diesel train just has its onboard generator or motor AND a limited amount of fuel).

This mess is also difficult to clean up, and could damage the track as it is pushed along. Also, although we have been ignoring friction (since trains have very little rolling resistance) this pile of stuff will cause friction to be very noticeable, and could even obstruct the driver's visibility – potentially leading to more collisions.

–//–

Now that you have read through all of the calculations (or looked at the pictures and skipped the rest), you should have a thorough understanding of why we have to stop trains to clear things off the line, and can't just plow through them like in the movies. (I assume this happens in movies, I have not checked)

TL;DR: When the train hits a branch, either the branch goes flying towards you really quickly, at basically 1000km/hr, which is approximately the speed of sound; or it sticks to the front of the train and becomes part of a massive pile of things that gets in the way and slows down the train.

Finally:

I put the images together using the shapes in my computer's word processor (except the various rail logos); while the equations of motion under constant power are from this paper by Lloyd W. Taylor (published in 1930, I believe). Also thanks to @cosmos-dot-semicolon for peer reviewing this, any errors are not my fault.

[hijolehijola]

Me, driving on a small two-lane road with some foot traffic: Damn it's crazy how going the speed limit will piss so many people off like

The carbrains on Tumblr: Um I GUESS if you want to go the "speed limit", you fucking cop, or whatever that's on you but stay out of the left lane (there is no left lane) or the person behind you will be morally correct in committing vehicular homicide!!!!!!

[the-pea-and-the-sun]

THIS COLLEGE SHIT IS EASY

[lightblueminecraftorchid]

Today was the last DND session for a three-year campaign and I am big sad :(

One of the players brought whiskey for a last toast, which was very kind of them. I had never had whiskey before. Turns out I do not like whiskey. Unsurprising.

It tastes like slightly nicer hand sanitizer.

[ivy-meshle]

[vounoura]

oh I didn't notice the siphoning strikes change either dbjhjbngdf

[lynne-monstr]

I went for a run this morning and now there is nothing else that requires my attention for the rest of the day. how wonderful