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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) â€‹

    • 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.​

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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

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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

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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:

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