Final answer:
A motionless weight lifter is not doing work because in physics, work requires a displacement in the direction of the applied force. Since the weight lifter holding weights steady does not cause any displacement, there is no work being done, even though the muscles are exerting force.
Step-by-step explanation:
Explaining why a motionless weight lifter is NOT doing work involves understanding the definition of work in physics. Work is defined as the process of a force causing a displacement. The formula for work (W) is W = Force (F) multiplied by displacement (d) and cosine of the angle (θ) between the force and displacement direction, expressed as W = F * d * cos(θ). For work to occur, there must be a displacement in the direction of the force applied. In the case of a motionless weight lifter, the weight lifter applying a force to hold weights steady does not cause any displacement of the weights. As there is no displacement (d = 0), the work done according to the formula is also zero (W = 0). This is true even though the weight lifter is exerting force and might feel fatigued. The fatigue is due to the muscles exerting force internally, which is not considered work on the weight-Earth system.