moment of force (torque) i.e. the turning effect of a
body(here,screw jack)= force applied X perpendicular distance of
force from the axis of rotation.
so in order to produce maximum turning effect of the screw jack
by a force, the force is applied at a point on the body for which
the distance from the axis is maximum so that the given force
provides maximum torque to turn the screw jack.
Thus,the screw jack is provided with a long arm.