^Examples of zero,  +ve & -ve work

Zero work means either no displacement of system no net change in the KE of the system. Few examples of zero work are

1. If there is no motion, no work has been done no matter how much force is applied.
2. A static person e.g.  a gate keeper does no work, as his displacement is zero.
3. Work done by normal forces (any force that changes only direction) is always zero e.g. work  done by tension in the sting simple pendulum, by your weight while walking on a horizontal surface, by normal reaction on a body.
4. Work done by a gas during its free expansion.
5. Work done by a magnetic field in moving a charge in a circle.
6. Gravity does positive work on an descending mass, negative work on an ascending mass & zero work for both for horizontal motion & cyclic path.
7. Consider the case of a weight lifter. He does,
8. (a) positive work to lift as he lifts weights the weights up (to pull weights up along the displacement against the gravity)
9. (b) no work, to hold the weights at their position (as no displacement means, no work )
10. (c) negative work to bring weights down at         constant speed (as he has to pull opposite to displacement to maintain speed)