What is the unit of measurement that defines the space available in a rack? How tall are standard racks?

The unit of measurement that defines the space available in a rack are  the inches for the width and for the height the units of Rack (U), this unit is equivalent to 44.45 mm.  Standard racks are 19 inches wide and 42U high. A rack is a type of shelf where servers can be stacked on top of each other.

force is based upon mass and acceleration.the statement is not always true.

it can be explained from newton's second law of motion.

as per newton's second law of motion the rate of change of momentum is directly proportional to the applied force and takes place along the direction of force.

mathematically it can be written as -

                                              ⇒      

here   k is the proportionality constant whose value is 1.

                                                            [a is the acceleration=dv/dt]

hence we see that force not only depends on mass and acceleration but also on velocity and rate of change of mass. force f will depend on mass and acceleration only when mass is constant.

hence the statement that force is based upon mass and acceleration is false.