WHY RAILWAY BRIDGES ARE MADE UP OF STEEL RATHER THAN CONCRETE ??

Trains are quite heavy and they generate high frequency vibrations when moving through the bridge.

Bridge can be made up of 2 materials i.e steel or concrete.Concrete is brittle in nature as a result small vibrations may break it.For example you might have seen jack hammer.How does it break concrete on which it is used? Does it generate very large force? No. It vibrates very quickly and splits up concrete into smaller chunks. This is because of brittle nature of concrete and low fatigue value.

But steel is not brittle, it is ductile. It can sustain such oscillations and has higher fatigue strength. As a result it can absorb high vibrations and sustain.

WHY TIRES ARE BLACK IN COLOUR ??

Although the natural colour of rubber is a milky white, the black colour we see is because of carbon black.Actually Carbon Black is not added as a coloring agent,it is a filler. 

Vulcanized rubber does not have very good wear resistance and hence a tire made of pure vulcanized rubber would wear out soon.

Carbon Black also increases the damping properties of rubber, thus transferring less shock loads to the automobile.

In addition to this Carbon Black is a good conductor of heat and would help conduct heat away from the surface with contact to the road and help maintain a healthy tire temperature.

An added bonus is that addition of carbon black increases the coefficient of friction with the road surface thus making it safer to drive.

RELATION BETWEEN RPM AND TORQUE EXPLAINED !!

Torque is the twisting or rotating force that the engine exerts on the crankshaft; it is expressed in N-m or Kg-m.

It is generally measured at a radius of one meter from the crankshaft's center.

Torque and Speed ( RPM) Are inversely proportional with each other

Here is the torque-speed curve.

As the RPM Of an engine increases, torque increases upto a certain value of RPM. And as the RPM goes on increasing, torque starts decreasing.

To understand this behaviour, you need to understand one more term which is called as “ Low-End torque”. Low -End Torque is the amount of torque produced at Low RPM range of an engine ( around 1000–2000 rpm) .This low end torque is very crucial when moving a vehicle from stand-still or driving in slow-speed conditions such as in traffic. If the greater amount of torque is generated at the lower end of engine rpm, it implies that the engine has more ‘low-end-torque’ or better pulling ability at slow speeds.

It also implies that vehicle can move very quickly from rest/stand still.

Now as the engine speed keeps increasing, torque starts to decrease because of the increase in friction between engine’s moving parts.

Engine torque is also related to the gearing. The lower the gear, greater is the pulling ability of an engine and hence greater the torque because lower gears are greater in diameter and we know that,

Torque=Force x Perpendicular Distance(Larger diameter of smaller gear).

So at lower RPM we get maximum torque as we are in smaller gear.Torque helps the wheels to turn that is why it is suggested to start vehicle in first gear as toque is maximum at that point of time.

If you know the Horsepower (HP) of an engine, then you can calculate the torque using the formula :-

Torque = (HP*5252)/RPM