Question

Why is a jack screw provided with a long arm?

Answer 1

Torque produced due a force $F$ acting on the arm at a distance $r$ is given by $\tau =r\times F$

Thus more is the distance of point of application of force, more is the torque produced in the jack screw and thus more easily it rotates.

Thus in order to produce more torque (or to rotate it easily), jack screw is provided with a long arm.