^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
- If there is no motion, no work has been done no matter how much force is applied.
- A static person e.g. a gate keeper does no work, as his displacement is zero.
- 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.
- Work done by a gas during its free expansion.
- Work done by a magnetic field in moving a charge in a circle.
- Gravity does positive work on an descending mass, negative work on an ascending mass & zero work for both for horizontal motion & cyclic path.
- Consider the case of a weight lifter. He does,
- (a) positive work to lift as he lifts weights the weights up (to pull weights up along the displacement against the gravity)
- (b) no work, to hold the weights at their position (as no displacement means, no work )
- (c) negative work to bring weights down at constant speed (as he has to pull opposite to displacement to maintain speed)