what is linear stepper motor and rotary stepper motor? any details or description for these?

A stepper motoris a motor that uses two or more electromagnetic pole assemblies with teeth on them such the teeth are are off set so you can get very precise movements base on which poles are energized.

There are two both linear and rotary steppers. Linear stepper motors are laid out in a linear configuration. Rotary steppers are similar to a conventional motor.

Which coupling is used for joing to shafts of different diameter.....?

There are a number of type of couplers which can be used to join shafts different diameters:

1.Spider Connectors - These connectors also allow for misalignment.

2. Straight connectors - With different hole diameters in each end.

Why is a "Cup Car" with a full tank of fuel faster than the same car with a half tank of fuel?

small adjustment can make a world of difference in performence pressure but with 2 identical car of same weight and same horsepower i would think that the power the power to weight ratio on the car with less fuel would be faster due to the pushing less weight