Chapter 5 - Work, Energy and Power

Chapter 5 - Work, Energy and Power

1. Work done and energy

   • Work done (Units Nm/J) = Force X distance moved in the direction of the force
     • Work done = Energy transferred (Energy is the capacity to do work)
     • W=Fx
   • When force is applied at an angle, the following formula can be used

2. Conservation of energy

   • Energy is the capacity for doing work
   • "Potential" energy can be stored, and converted into other forms. This includes G.P.E, elastic and electric. Non potential energy can only be transferred
   • Principle of conservation of energy - The total energy of a closed system remains constant, energy can never be created or destroyed, only transferred from one form to another

3. K.E and G.P.E

   • K.E is directly proportional to the mass of the object, and directly proportional to the speed squared
   • Gravitational Potential energy - The capacity for doing work as a result of an objects position in a gravitational field
     • This formula can only be used for a UNIFORM gravitational field (i.e close to Earth's surface) where g can be assumed to remain constant
       • It is gained with height
       • Using the idea of W=Fd, the force is Weight (mg), and the distance is the vertical height travelled (h), meaning you get G.P.E = mgh
     • G.P.E can be converted to K.E, e.g with a waterfall or rollercoaster. By putting them equal to each other, the final velocity v does not depend on the mass of the object. It's only true if there are no resistive forces

4. Power and efficiency

   • Power is the rate of work done (Or rate of energy transfer). Units are Js^-1, or Watts
   • Sometimes, a constant force has to be exerted to maintain speed, e.g a car on a road.
     • For the car, the net force is zero, since the power of the engine is equal to the rate of work done against frictional forces
       • This equation can be used for a variety of situations, like above
   • All energy in a closed system is conserved, but it does not mean all energy is converted into useful output energy

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