WORK, ENERGY & POWER

WORK

Work - the energy transfer that occurs when an object is moved over a distance in the direction of an external force acting on it
- vector quantity
- SI unit is Joule (J); 1J is equal to 1Nm (Newton-meter)
There are three key factors required for work to occur:
- Force
- Displacement
- Cause
In order for a force to qualify as having done work on an object, the object must be displaced.
Work can be quantified with F*d*cos(θ)
- F - force (Newtons)
- d - displacement in the direction of force (meters)
- θ - angle between the force and displacement vector (degrees)
Work-energy theorem - work done against resistance is equal to change in energy
- This theorem allows work and energy to be interchangeable in mathematical contexts.

ENERGY

Energy - the capacity of a physical system to do work
- scalar quantity
- SI unit is Joules (J)
Law of conservation of energy - energy cannot be created or destroyed, but it can transfer forms in an isolated systems
There are two main types of energy, which can take many forms.
- Potential energy - the energy stored in an object while it is not in motion; this energy is due to the object's position.
  - Gravitational potential energy - the potential energy an object carries due to its height and/or its capacity to fall in the direction of a gravitational force (e.g., a rock sitting at the edge of a cliff carries gravitational potential energy).
    - Gravitational potential energy can be quantified with PE = mgh
      - m - mass (grams)
      - g - gravitational acceleration (ms^-2)
      - h - height from ground (meters)
  - Elastic potential energy - the potential energy stored in elastic material due to tension (e.g., the pulled string of a bow carries elastic potential energy before the arrow is released).
  - Electric potential energy
  - Magnetic potential energy
  - Nuclear potential energy
  - Chemical potential energy
- Kinetic energy - the energy an object has because of its motion; the work needed to accelerate an object from rest to a certain velocity
Mechanical energy - the sum of an object's kinetic energy and gravitational potential energy
Internal energy - the energy of the particles inside matter

POWER

Power - a measurement of work done/energy per unit time
- quantified with P = W/t = E/t
  - P - power (watts)
  - W - work done (joules)
  - t - time (seconds)
  - E - energy (joules)
- SI Unit is the watt, which is equivalent to Joule/time
Power can also be redefined as the product of the force applied to an object and its (resulting) velocity
- this definition is quantified with P = Fv
  - F - force (Newtons)
  - v - velocity (meters per second)
Power rating - how much energy is used per a unit of time

CONSERVATION OF ENERGY & EFFICIENCY

Law of conservation of energy - energy cannot be created or destroyed, but it can transfer forms in an isolated systems
Many systems require energy to be transformed (e.g., A kettle transforms electrical energy into thermal energy; a ball falling off a cliff transforms gravitational potential energy to kinetic energy).
- Typically, when energy is transformed in systems, a significant fraction is lost via transformation into non-useful energy.
- This useless energy is most often lost in the form of thermal or sound energy; e.g., a lamp’s purpose is to emit light, but thermal energy is often also emitted, meaning useful potential energy is lost.
The efficiency of a system is measured by the following: