Probably one of the best applications we can find is that of gears in a car transmission.
A gear look like a circle and it has teeth all around it.
In sum, transmissions contain several combinations of large and small gears.
Say for instance a small gear (20 teeth) drives a large gear(40 teeth). The large gear will turn at half (20/40 =1/2) the speed of the small gear.
However, this situation increases the turning force (or torque) of the large gear.In general, the larger the gear the bigger the torque.Therefore, you can see that knowing the ratio of gears, can help us determine the speed and how much torque each gear will deploy.This concept is important when putting a transmission together. The situation we just describe may for instance set the proper gear ratio for moving a load.
However, at cruising speed, gears may have the same ratio so that the amount of torque that enter the transmission equal the amount of torque that goes out.Automotive engineers do lots of gear ratios calculation before they can put a transmission together.
This concludes our introduction.
By Francis and Christopher