Why are the rear wheels of buses not provided at the extreme rear, but at about 1/4th of the distance?
Curiosity at its best!There are two parts of the question.Why wheels are not at extreme ends?Why rear wheels are more inside than the front wheels?For a moment, forget the bus.Imagine a beam 12 meters long and carrying a symmetrical uniformly distributed load, as shown in the figure below.Now let's add some support to this beam. There are at least two cases of doing. The best one will give lesser shear forces and bending moments.Case 1: Supports at extreme endsThe above configurations will give the following Shear forces and Bending moments.Case 2: Supports away from extremes.The above configurations will give the following Shear forces and Bending moments.Okay now scroll back a bit, and check the values of both Shear forces and the moments. You will easily notice the values are lesser for the second case.This implies the overall strength of the structure increases for the same load and the same beam. Also it implies, that for the same beam, you can load it more, in the second case. This answers the first query.Edits: In response to one of the comments on if the uniform loading assumption true or not, this is the more accurate picture of the exact/typical loading.2. Now depending upon the position of engine, seating arrangements, slight modification and shifting can be done, to increase the overall stability.Rear Engine busFront engine busApart from this, it adds to other benefits like lower turning radius etc, as pointed out by Shree Nath. Secondly you would notice that this thing is not completely exclusive to buses, it is seen in many cars too, though in lesser amount.Also see, for another important aspect for this.László Arany's answer to Why are the rear wheels of buses not provided at the extreme rear, but at about 1/4 th distance?Source for the SFD and BMD DiagramsBending Moment and Shear Force Diagram CalculatorKeerthi Vasan - Thanks a lot for A2A. Amazing question!You may like this too - A beautiful history of the bicycle
How does a railway engine overcome slippage on track?
The steel wheel of a locomotive (loco) doesn't get as good friction on steel rail as rubber tyre on tar roads. Further, best value of friction between rail and the wheel occurs at about 3-5% slip in good conditions of rail-wheel contact and weather (i.e. wheel must be moving faster than the locomotive-note that Circumference*rotations per hour gives you speed of the wheel and should be speed of the loco in non-slipping conditions). As the condition of the rail wheel interface deteriorates (say there are fresh leaves on the rail)-wheel need to slip more to have highest possible adhesion (or friction). As the rail wheel interface deteriorate wheel needs to slip more to get best adhesion. In modern locomotives there is a adhesion optimiser which search for the minimum slip needed to deliver best possible tractive effort (Tractive Effort = mass_Loco*adhesion). There is need to know the vehicle's ground speed-either by absolute measurement (say by a doppler radar) or mathematical estimation (using speed of all axles) and then control slip accordingly. In case there is a non driven wheel, that would give true ground speed.The picture shown above shows what happens when the slip is uncontrolled-the railwheel acts like a grinding wheel and does the rest as shown in the picture.
How are smart and innovative DRDO and ISRO engineers/scientists?
I believe I am the perfect guy to answer this question as I have been selected at both ISRO and DRDO. However since I am working at ISRO my answer would be related to ISRO only.It has been one month since I joined ISRO. Contrary to common perception that scientists at ISRO are too smart and innovative, I believe that the people working here are just average. I will start with myself. I have always believed myself to be an average guy. I have met some super smart people in my life but at ISRO every person that I have met till now is good or average. I haven’t met any such person who I feel is an intellectual genius. Obviously I haven’t met many people but still I hold my view.Now the obvious question is:How could an organisation with average minded people have achieved so much?There are many factors for this. The most important is that people here after working for some years become really good in their domain. The other important thing is teamwork. I believe that the success of this organization is mainly because of the brilliant effort put by the average people. Some other things that I have liked at this place is the humbleness of the people working here. I still remember the day when the new joinees were to meet the director of VSSC. He was such a great person. He had this great sense of humour and I felt at ease during the whole process. My boss who is a scientist F never makes me feel that she is superior. Many times I have doubts and she makes sure I understand everything clearly. I have met numerous people here and it is hard to distinguish who is which level of scientist from their behaviour. This is the thing that I have liked the most at ISRO.I hope that the rest of the journey is as good as it is now.Thank you.
Why can’t NASA's Curiosity rover rescue Opportunity?
Mars is big, and we don’t build rovers to get repaired (or do repairs).Curiosity is nearly half the planet away. I’ve seen estimates of 8000 km. Keep in mind that Opportunity herself holds the distance record for Mars, around 45 km over 15 years. (Curiosity is able to drive faster, and has trekked a bit more than 20 km in 6 years). Even considering that a rover going at full speed is faster than one that stops to do science, it would take years for Curiosity to get to Opportunity’s landing site, assuming the terrain is passable at all. Curiosity is also past her warranty of ‘1 Martian year’, so no one would guarantee she’d make it.Plus, the direct route is basically over highland terrain, rather than the flat plains. So either one goes around, or one has to remotely pilot a vehicle over slopes they weren’t designed to traverse. (And keep in mind that orders are sent daily thanks to a speed of light delay: if Curiosity gets stuck or on-board computer decides the path is unsafe, she stops and tells Earth the next day that she didn’t drive.) This will come up pretty quickly as we landed Curiosity in a crater, so she’d have to be able to climb up the walls.Then when she gets there, it’s not like Curiosity is carrying jumper cables. She might be able to brush off Opportunity’s solar panels, as ‘brushing away dust’ is a thing she can do with her arm, but if anything inside Opportunity is broken (and it has been spending years out in the cold: something is probably broken), Curiosity can do nothing to help.Plus, we fund Curiosity to do science. Every day spent driving all out for Meridiani Planum from Gale Crater is a day when very little science can be done. So not only did we lose a rover doing science, but we’ll lose use of another.(That is one of the things that gets lost on the Martian movie[1]: Mars is smaller than Earth, but not small. Mark Watney took weeks of driving at much faster speeds to get to Pathfinder, and months of driving to get to the site where equipment for the next mission was dropped. In the book, he considers dropping by Opportunity’s site on his last drive, but decides he can't chance the detour for what amounted to tourism and maybe a chance to cannibalize more spare parts.)[1] Or is represented by Watney’s beard growth. Doing ‘and then a bunch of uneventful time passed’ is hard.
What was the reason NASA, et al, did not use “tank treads” on lunar and Mars rovers? Is there an inherent advantage that wheels have over treads? I’m thinking of the Curiosity rover that got stuck in loose Martian soil.
Weight, bulk, mechanical breakdown.Tank treads are great when they have access to regular repair facilities. They are also far heavier than wheels.Tank treads give you 2 places of mechanical breakdown. 4–6 independent wheels made of lightweight material are just fine for a rover exploring a planet or moon with less than half the gravity of earth. 6 wheels, means 2 or 3 can fail and the rover can still maneuver around.
If we are able to send a few people to Mars, could they survive on the planet (barring a storm) with the same quality of life as the characters in The Martian, using the technology we currently have?
Aside from the storm at the beginning - and some nonsense about the final rendezvous with the rescue ship in orbit - the book and movie are really quite accurate.There are a couple of unknowns - one is whether humans can survive healthily with so little gravity for so long. It would take at least six months - maybe 8 of zero g just to get to Mars. Once you’re there - you can’t start back for about 2 years because of the orbital periods of Earth and Mars - and then you have another 6 to 8 months of zero g. Nobody has ever spend more than a little over a year in less-than-earth gravity…and we really don’t know what that might do to our astronauts.So if Mars gravity isn’t enough to be completely healthy - there is a serious concern about whether they’d survive long enough for the return trip.Another concern is equipment reliability.The people on this mission would be absolutely at the mercy of equipment failures. If something goes wrong with (say) the oxygen recycler on the ISS, you can survive for many days before you have to climb into the Soyuz emergency crew return vehicle - and sending spares up to help you keep the backup recycler working shouldn’t be too difficult.But if on Mars, you find that the perchlorates that people tramp into the habitat on their boots corrodes some gasket or other - then you’re dead.With an enforced 2 year stay on the planet…there are a hell of a lot of things that can go wrong.
Why is the theory of relativity so difficult to grasp?
Why is the theory of relativity so difficult to grasp?Because the people who don’t understand it don’t explain it correctly. They say things like “light curves because it follows the curvature of spacetime”, but it isn’t true. It curves because the speed of light is spatially variable. See what Einstein said in 1920:“Second, this consequence shows that the law of the constancy of the speed of light no longer holds, according to the general theory of relativity, in spaces that have gravitational fields. As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable”.Light curves downwards rather like sonar waves curve downwards in the sea:Image from FAS and the US Navy, see course ES310 chapter 20It’s also rather like the way sound waves in air tend to curve downwards at night:Image from Rod Nave’s hyperphysics, see refraction of soundAlternatively imagine you’re driving a car. If there’s mud on the left hand side of the road, the car pulls to the left. That’s because the mud slows down the wheels on the left. It’s similar for tank tracks. You steer a tank to the left by slowing down the track on the left.Once you understand why light curves, it’s easy to understand why matter falls down. All you need to know about is the Einstein-de Haas effectwhich “demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics”. So just think of an electron as light going around and around. Then simplify it to light going around a square path. Like this:Now imagine it’s in a gravitational field. The vertical parts of the path stay vertical, but the horizontal parts bend down a little. So the electron falls down:That’s it. It’s that simple. If you’re interested in more complete details see my “physics detective” articles on the nature of time,the speed of light,how gravity works,and the principle of equivalence and other myths.