Ans: One joule is the work done when a constant force of 1 N moves an object through 1 m in the direction of that force.

Ans: During lifting, the supporting force and displacement are both upward, so the worker does positive work on the toolbox. While holding it motionless, displacement is zero, so this force does no mechanical work on the toolbox, even though the worker may feel tired.

Ans: Kinetic energy is energy due to motion; for example, a moving bus has kinetic energy given by KE = (1/2)mv². Potential energy is stored due to deformation or relative position; for example, a compressed spring stores potential energy. Both are measured in joules.

Ans: At constant speed, the upward force equals the weight: F = mg = 40 × 10 = 400 N. Work done by the hoist = Fh = 400 × 3 = 1200 J. Average power = W/t = 1200/6 = 200 W.

Ans: A fixed pulley changes the direction of effort but does not reduce its magnitude; its mechanical advantage is 1. An inclined plane reduces the effort by increasing the distance travelled; its mechanical advantage is L/h. For both ideal machines, input work equals the useful work mgh needed to raise the load. They provide convenience or a force advantage, not an energy saving.

Ans: Let the object reach speed v after moving through displacement s with constant acceleration a. From v² − u² = 2as and u = 0, s = v²/(2a). Newton's second law gives the net force F_net = ma. Therefore, net work W_net = F_net s = ma × v²/(2a) = (1/2)mv². By the work–energy theorem, net work equals the gain in kinetic energy. Since initial kinetic energy is zero, the final kinetic energy is KE = (1/2)mv². Its SI unit is joule (J).