Why train tracks are laid on top of stones?

A sight like this image above is pretty common. But have you ever wondered why there are stones in between and underneath those rail tracks?

Trains can be extremely heavy, this weight is focused upon the fairly small area of their wheels. The stones are actually called track ballast and help to spread the huge force from the train's wheels out over a larger area of ground. Without this ballast the ground underneath might sink unevenly. The sleepers (cross ties) of the tracks are not directly attached to the ballast which allows the track to have a little movement (e.g. as the track expands and contracts due to changes in temperature).

Stones are a good choice for this role because they are relatively cheap when compared to alternate materials, they can resist compressive loads well and require very low maintenance.

Also of note: Ballast is itself built upon a foundation of earth (the subgrade) that helps to raise the track and further distribute load.

Other functions: Stops plants growing around the tracks, allows water to drain away. Thus it is essential in maintaining the stability of the ground.

Are rocks a significant derailment hazard? Not really -- trains are massive and move quickly -- this enormous momentum means they smash right through most anything in their path. Rocks are typically turned to dust by the wheels of the train or thrown out of the way by the tremendous pressure of the wheels against the track.

 

Why does it take 5 seconds for banks to take money off your account via credit/debit cards but it takes 5 days for the bank to refund it?

5 Seconds after you complete your transaction: the money leaves your account.

5 secs to 2 days after: The bank is holding onto the money while they check all the information/credentials, make sure its all legit, go through federally mandated safety measures, etc. It might be more accurate to say that 5 seconds after you complete your transaction, an authorization hold is placed for the value the merchant submitted. Your ledger balance isn't updated until the completed transaction posts, but the "available balance" your bank gives you includes the value of that hold.

2 to 3 days after: The merchant actually receives the money.

Now, just apply that in reverse for refunds. 5 seconds after the transaction, the money has left the merchant's account, then the bank does a day or two of processing, and then, 3 days later, you finally get the money.

It only LOOKS like its done in 5 seconds for payments, and takes 3 days for refunds, because you're only seeing your end of things. You're not seeing the middle part of processing or the merchant's end of things.

How do blinking sequential Christmas lights work?

It's that time of the year when Christmas lights seem to be everywhere. They manage to catch our attention even captivate us by blinking in patterns. They can even blink in several patterns where the patterns can be changed by just a push of a  button. Now when I see those lights, the engineer in me tries to decipher how they work. Using my knowledge on basic electricals, I try to design a blinking light circuit but with no avail. 

 

A little bit of poking around & online research gave me answers that I was looking for. Let's start with the basics. The lights are usually made of 2.5V incandescent bulbs. But how can we plug a 2.5V bulb to a standard 240V supply? 240/2.5=96, Hence 96 bulbs  are connected in series to form a bunch of lights(Voltage adds up in series). If more bulbs are required, then another bunch of 96 lights are connected in parallel with the original bunch. So, if you purchase a Christmas light bunch, the voltage rating of each bulb can be calculated by dividing supply voltage by no. of bulbs in an individual loop . Eg: 240/96 = 2.5V. Before you do this ensure that you're able identify number of parallel loops. Typically the no. of bulbs in each loop is rounded off to 100. 

 

 

Now coming to the blinking. Let's consider a simple pattern where a bunch consisting of four loops where each loop goes on & off periodically. Each loop has a blinker bulb whose filament contains a bi-metallic strip. In simple layman terms, a bi-metallic strip can be said to bend in one direction on heating & in the opposite direction on cooling. Google would do a better job on explaining the working of a bi-metallic strip than me. This strip in the filament closes the circuit when cold & opens it when hot. This opening & closing of loops leads to the 'blinking' pattern of Christmas lights. 

 

 

And about the more sophisticated patterns. In this case the loops are interleaved than being connected one after another(Consider 4 loops). The controller box consists of integrated circuit having four triacs/transistors - One to control each loop. The IC simply turns on a triac to light one of the four loops. By sequencing the triacs appropriately, we can create all sorts of patterns. More on this here.

 

New Shepard Vs Falcon 9

Blue Origin's New Shepard suborbital booster and SpaceX's Falcon 9 suborbital booster may look alike at first glance. However, the Falcon 9 carries a staggering 130 tons at launch while New Shepard carries only 5. The two vehicles are, in fact, completely different designs for two completely different purposes. Blue Origin's booster is designed to carry a single crew capsule to space (100 kilometers up), while SpaceX's Falcon 9 booster (and the Falcon 9 upper stage) bring payloads to orbit. This requires about four times as much total energy as New Shepard's 100.5km hop into space.

New Shepard's trajectory brings it up and back down with only two burns: a launch burn and a landing burn. The Falcon 9, on the other hand, has to deliver (with the upper stage) a payload to orbit. This means that the Falcon 9 has to angle itself towards the horizon during launch; at stage separation (when the upper stage detaches to complete the mission), the Falcon 9 booster is travelling faster horizontally than vertically. This velocity is reversed with three more engine restarts (four including the launch).

The boostback burn brings the vehicle's trajectory back towards land. However, the booster will reenter the atmosphere at a speed far too fast to survive. So at an altitude of about 70km, three of the engines are restarted to slow the vehicle down before it hits the thicker, lower parts of the atmosphere. When the entry burn ends, the Falcon 9 booster is only 40km above the ocean, on a trajectory that will drop it harmlessly into the ocean in case of a failed landing burn.

The third, critical, burn the Falcon 9 performs is the landing burn. This is a single-engine burn, starting about 30 seconds before touchdown. The single center engine reignites a final time as the vehicle plummets towards the ground; the grid fins reorient the vehicle and push it towards the landing pad, and about 5 seconds from touchdown, the landing legs unfold and the booster touches down on the landing pad.

Another major advantage New Shepard has over the Falcon 9 is its deep-throttleable BE-3 engine. Most rocket engines have a very limited throttle range (and some can't throttle at all). The Merlin 1D, nine of which are used on the Falcon 9 booster, can throttle from about 65% to 100%, and that is very, very deep throttling for an engine of its size. New Shepard's BE-3, on the other hand, can throttle down to at least 40%. This means that New Shepard can hover while landing, which gives them more time to line up and touch down softly. The Falcon 9 cannot hover at all and must reach zero velocity the moment it touches the pad or it will begin to rise up again. This necessitates more accurate and advanced control systems.

In closing, it's unfair to compare the two vehicles. New Shepard is designed for several minutes of microgravity, while Falcon 9 is designed to carry satellites into orbit. The vehicles perform their respective tasks admirably.