Work measures energy transferred by a force acting through a displacement. For a constant force, W = Fs cos(theta), where theta is the angle between force and displacement. Work is positive when the force acts along displacement, negative when it acts against displacement, and zero when they are perpendicular. It is also zero if displacement is zero. The SI unit is joule (J): 1 J = 1 N m. Always identify which force does the work. During steady lifting, the upward applied force does positive work and gravity does negative work, although the balanced forces give zero net work.

Energy is the capacity to do work, measured in joules (J). Kinetic energy is energy associated with motion: KE = (1/2)mv², where m is mass in kilograms and v is speed in metres per second. It has no direction and cannot be negative. At unchanged speed, doubling mass doubles kinetic energy; at unchanged mass, tripling speed increases it nine times. The net work done by all forces on an object equals its change in kinetic energy: W_net = KE_final − KE_initial. Therefore, positive net work increases speed, while negative net work reduces speed.

Potential energy is stored because of an object's deformation or the relative positions of interacting objects. A compressed spring stores elastic potential energy even while motionless. Raising an object increases the gravitational potential energy of the object–Earth system. Near Earth's surface, where g is approximately constant, PE = mgh relative to a chosen zero-height level. Here m is measured in kilograms, g in metres per second squared, h in metres, and PE in joules. The gain depends on vertical height, not the route taken. Moving horizontally at the same height does not change gravitational potential energy.

Mechanical energy is the sum of kinetic and potential energies. During free fall with negligible air resistance, potential energy becomes kinetic energy while their sum stays constant. Friction can transform mechanical energy into thermal energy; energy is not destroyed. Other transformations include chemical energy from food becoming mechanical energy in muscles. Power describes how quickly work is done or energy is transferred. Average power P = W/t, with work in joules and time in seconds. Its SI unit is watt (W), equal to one joule per second. Completing the same task faster requires greater average power, not necessarily more work.

Simple machines change the magnitude or direction of effort, the applied force used to overcome a load. Mechanical advantage, MA = load/effort, has no unit. An ideal fixed pulley changes effort direction and has MA = 1. A frictionless inclined plane of length L and height h has MA = L/h: a longer ramp requires less effort over more distance. A lever is a rigid bar turning about a fulcrum. For perpendicular forces in balance, effort × effort arm = load × load arm, so MA = effort arm/load arm. Ideal machines conserve work; real machines require additional input because of friction.