bellbottom25 Posted May 30, 2008 Share Posted May 30, 2008 Hi, I never understood locomotive traction! It was in my study. How come metal made wheels of locomotive are made to stop on metal rails!?? I prompted my lecturer, he told me about magnetic brakes! But i didn't got the answer that i really wanted! Metallic heavy locomotive with a huge momentum is made to stop on rails which have no friction between them! Or is the magnetic brake just an alternative in clamping down the speed of the wheel movement! If you ask me, the locomotive should never stop! ------------- eric Link to comment Share on other sites More sharing options...
YT2095 Posted May 30, 2008 Share Posted May 30, 2008 Hi,Metallic heavy locomotive with a huge momentum is made to stop on rails which have no friction between them! who said there was no Friction? Link to comment Share on other sites More sharing options...
insane_alien Posted May 30, 2008 Share Posted May 30, 2008 rub two bits of metal together, plenty of friction. now do it with several tonnes sitting on it. unless there is lubricant(decomposing leaves on the track for example) there is plenty of friction. Link to comment Share on other sites More sharing options...
bellbottom25 Posted May 30, 2008 Author Share Posted May 30, 2008 Huh, but not the required friction to stop a huge locomotive! The wheels would need more than friction to even start rolling on the metal rails, then how come they stop that easily! ------------------ eric Link to comment Share on other sites More sharing options...
insane_alien Posted May 30, 2008 Share Posted May 30, 2008 (edited) oh theres more than enough friction. trains can start and stop faster than they do on the conventional brakes. they also have sand dispensers for emergency braking. i'll look up the friction coefficients and prove to you that there is enough friction available to stop the train. EDIT: here we go, Friction coefficients for steel/steel static: 0.74 (S) dynamic: 0.57 (D) source: CRC Handbook of Physical Quantities. Boca Raton, FL: CRC Press, 1997: 145-156. Static friction (Fs) Fs=S*N ; N=normal force Dynamic friction (Fd) Fd=D*N N=mass * g mass of a train ~200 tonnes g=9.81 m/S^2 as the wheels should not be slipping we use the static friction equation. Fd = 0.74*9.81*200000 = 1451880 N this is the maximum force of friction this train could take before the wheels slip. F=ma -> a=F/m a=1451880/200000 = 7.26 m/s^2 this is the acceleration at maximum friction. if a train accelerated or braked with this acceleration you would be flung out of your seat without being strapped in. Edited May 30, 2008 by insane_alien Link to comment Share on other sites More sharing options...
Edtharan Posted May 30, 2008 Share Posted May 30, 2008 How come metal made wheels of locomotive are made to stop on metal rails!?? Have a look at what the brake disks in your car are made from... Metal And also have a look at what they clamp onto... Also Metal. Metal on metal clearly has enough friction to stop objects. With a train, it also has a lot of wheels, and braking is applied to all the wheels when a train needs to stop. This means that even though each wheel only provides a small surface area for braking, the total surface area because of all the wheels is quite large and provides a lot of friction used for braking. Link to comment Share on other sites More sharing options...
swansont Posted May 30, 2008 Share Posted May 30, 2008 Trains stop easily? Link to comment Share on other sites More sharing options...
insane_alien Posted May 30, 2008 Share Posted May 30, 2008 Ed: the area of wheel contacting the track doesn't come into it. it just depends on the normal force. swansont: news to me as well, i just let it slip. Link to comment Share on other sites More sharing options...
bellbottom25 Posted June 2, 2008 Author Share Posted June 2, 2008 I have been thinking too much about generalised concepts! Equations in physics might be solving every querry. But ask yourself a huge metal wheels of a locomotive if turned by the axil of an engine, then the force might have been produced with accumulating weight which combines with the momentum of the locomotive! Then when it comes to stopping the train, might be that the axils were applied pressure for lowering down the speed! But if not then the locomotive should be going forward on its own weight! Might be that the friction that we are discussing would be even negligible! Then how come the locomotives stop and even start so smoothly! ------------- eric Link to comment Share on other sites More sharing options...
insane_alien Posted June 2, 2008 Share Posted June 2, 2008 i explained it, the wheels can apply a breaking acceleration of 7.26m/s^2 this takes into account momentum and such. just because you can't imagine it having enough friction does not mean that they don't. trains start and stop so smoothly because they do not apply anywhere near the maximum force before wheel slip occurs. they do it smoothly because the drivers are trained to do it smoothly. if you went in a train and jammed the throttle open instantly then i assure you it would not be a smooth ride. Link to comment Share on other sites More sharing options...
YT2095 Posted June 2, 2008 Share Posted June 2, 2008 here, try a simple experiment to prove it to yourself, get a telescopic antenna, the sort you get on little portable radios and such. now extend and collapse the antenna very quickly several times (being careful not to snap or bend it), do this about 10 times as quickly as you can, you`ll notice that the metal has gotten quite hot! this is due to metal on metal friction. Link to comment Share on other sites More sharing options...
bellbottom25 Posted June 2, 2008 Author Share Posted June 2, 2008 i explained it, the wheels can apply a breaking acceleration of 7.26m/s^2 this takes into account momentum and such. just because you can't imagine it having enough friction does not mean that they don't. trains start and stop so smoothly because they do not apply anywhere near the maximum force before wheel slip occurs. they do it smoothly because the drivers are trained to do it smoothly. if you went in a train and jammed the throttle open instantly then i assure you it would not be a smooth ride. You are just trying to reason out with your calculations! What i am trying to ask is perhaps way over your type of heads! --------------- eric Link to comment Share on other sites More sharing options...
insane_alien Posted June 2, 2008 Share Posted June 2, 2008 i thought your question was that you couldn't understand how trains could stop on metal tracks? if we have misinterpreted then please restate your question and try to make it clearer. i understand english is not your first language so it may be a language barrier. Also, one little thing about my calculations i should have mentioned, that is for all wheels being powered/braked if not all wheels are powered/braked then the acceleration will be proportionally smaller. Link to comment Share on other sites More sharing options...
bellbottom25 Posted June 2, 2008 Author Share Posted June 2, 2008 What you'll again reply with such absurdness if i even restate my question! Might be that i should compare you all just like my class-mates who were such boring , pathetic, book-worms!(even brainless too) -------------- eric Link to comment Share on other sites More sharing options...
insane_alien Posted June 2, 2008 Share Posted June 2, 2008 how can you expect an answer if nobody understands the question. how was my response absurd? i calculated the friction force with friction coefficients found in the literature. i then used newtonian kinetics to show that this force was indeed enough to accelerate the train. Link to comment Share on other sites More sharing options...
bellbottom25 Posted June 2, 2008 Author Share Posted June 2, 2008 What should i say physics is loved to be imagined! -------------- eric Link to comment Share on other sites More sharing options...
insane_alien Posted June 2, 2008 Share Posted June 2, 2008 so, what is the question? Link to comment Share on other sites More sharing options...
bellbottom25 Posted June 2, 2008 Author Share Posted June 2, 2008 Let me state it clearly, metal over metal in case of metal wheels of a locomotive over metal rails never produce friction! --------------- eric Link to comment Share on other sites More sharing options...
YT2095 Posted June 2, 2008 Share Posted June 2, 2008 Let me state it clearly, metal over metal in case of metal wheels of a locomotive over metal rails never produce friction! --------------- eric and Again... Says Who? Link to comment Share on other sites More sharing options...
bellbottom25 Posted June 2, 2008 Author Share Posted June 2, 2008 My name is kunal...u do know me very well! ---------------- eric Link to comment Share on other sites More sharing options...
insane_alien Posted June 2, 2008 Share Posted June 2, 2008 take 2 coins sit them on a book or something, one coin on top of the other. tilt the book slightly, if there was no friction then the top coin would slide off at the slightest tilt. however, i think you'll find that there is a significant range where the coin does not slide. this means there has to be friction there. Link to comment Share on other sites More sharing options...
swansont Posted June 2, 2008 Share Posted June 2, 2008 Let me state it clearly, metal over metal in case of metal wheels of a locomotive over metal rails never produce friction! The train, then, could never start moving because of the wheels — no friction — and you could push it and let it slide. The statement is just flat-out wrong. Link to comment Share on other sites More sharing options...
Edtharan Posted June 2, 2008 Share Posted June 2, 2008 Let me state it clearly, metal over metal in case of metal wheels of a locomotive over metal rails never produce friction! You are wrong. Metal on Metal does produce friction. There have been many examples in this thread that you can do yourself that prove that metal on metal does produce friction. Here is a really simple one. Get two chunks of metal and rub them together. To make sure that it is not just the mechanical effort of using your own arms that creates a difference, place one on the floor and the other on top. Then stack weights on them to increase the amount of force pushing them together. If there was no friction then no matter how hard you push them together it would not get any harder to rub them together. However. If metal on metal does produce friction, then the harder you push them together the harder it will be to rub them together. Link to comment Share on other sites More sharing options...
Phi for All Posted June 2, 2008 Share Posted June 2, 2008 Let me state it clearly, metal over metal in case of metal wheels of a locomotive over metal rails never produce friction!What would happen if you lubricated both the metal wheels and the metal rails? Would the train have more trouble starting and stopping? Yes? Isn't the lubrication reducing the friction that is already present between the two metal objects? Link to comment Share on other sites More sharing options...
swansont Posted June 2, 2008 Share Posted June 2, 2008 Similar metals will do more than have friction, too. Galling/cold-welding is lots of fun to deal with. Learned about that one the hard way — I ended up shearing the bolt off as I tried to tighten it, man-beast that I am. Link to comment Share on other sites More sharing options...
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