Give two illustrations for each of following.
(a) Positive work
(b) Negative work
(c) Zero work.

Positive work:
(i) Work done by gravity on a free falling body is positive.
(ii) Work done by applied force when taken vertically up against gravity is positive.

Negative work:
(i) Work done by friction force on a moving body is negative.
(ii) The work done by gravity on a body moving up is negative.

Zero work:
(i)Work done by electrostatic force of nucleus on electron revolving in a circular orbit is zero.
(ii) Work done by tension force in string on a stone whirled in a circle is zero.