A man lifts a 50-lb box vertically 4 feet. How much work does he do?

Work = Force × Distance (when force and distance are in the same direction). Here: 50 lbs × 4 ft = 200 ft-lbs of work. Units: in US customary, ft-lbs; in SI, Joules (1 J = 1 N·m); 1 ft-lb ≈ 1.356 J. Work principles: (1) Only force IN THE DIRECTION OF MOTION does work. Carrying a box horizontally at constant height: gravity acts vertically; horizontal motion is perpendicular to gravity; no work is done against gravity (though work is done against friction); (2) Lifting AGAINST gravity: work = weight × height; (3) Sliding against friction: work = friction force × distance; (4) If force varies, work = area under force-distance graph. Energy: kinetic (motion) = ½mv²; gravitational potential = mgh; both measured in same units as work. Conservation of energy: total energy in a closed system is constant; transforms between forms. Power: rate of doing work = Work / Time. Units: Watts (J/s) or horsepower (1 hp = 550 ft-lbs/sec = 746 W). Example: lifting 50 lbs 4 ft in 2 seconds = 200 ft-lbs / 2 sec = 100 ft-lbs/sec = 100/550 hp ≈ 0.18 hp. Higher power = same work faster, OR more work in same time. Mechanical advantage doesn't reduce work — it spreads force over distance: lifting 100 lbs 1 ft (100 ft-lbs work) using a 2:1 lever requires 50 lbs over 2 ft (50 × 2 = 100 ft-lbs) — same work, less force needed.