Concise Physics Handbook

Work is done when a force moves a body. If the force is applied at an angle, its projection onto the direction of displacement is considered. Maximum work is done when the angle alpha is 0 degrees, and zero work is done at 90 degrees.

Power shows how fast work is done. The greater the power, the faster the action is performed. The unit of measurement is watt (W).

Kinetic energy is the energy of motion. It depends on the mass of the body and the square of its velocity: if the velocity doubles, the energy increases by 4 times.

Potential energy arises when a body is lifted above a reference level. The higher the body and the heavier it is, the greater its stored energy.

Total mechanical energy is the sum of the energy of motion (kinetic) and position (potential). In ideal conditions (without friction), this sum remains constant: if one decreases, the other increases.

Energy does not disappear or appear from nothing — it transforms from one form to another. In a closed system, its total sum is conserved. When external forces act, their work $A_{\text{external}}$ is added to the calculation.

If there is no friction, resistance, or other losses, the total energy is conserved: whatever was at the beginning remains at the end. An example is the motion of a pendulum in a vacuum.

Efficiency shows what part of the expended work was useful. The rest are losses (e.g., due to heating, friction). The closer the efficiency is to 1, the more efficient the system.

The energy form of efficiency: shows what fraction of all input energy was converted into the desired output. Used, for example, in heat engines or electrical appliances.