energy and work done

• Energy


  The capacity to do work
• The SI unit of energy is the joule (J)
• Types of energy stores

  • Kinetic energy
  • Gravitational potential energy
  • Elastic potential energy
  • Chemical potential energy
  • Internal (thermal) energy
• Kinetic Energy

  The energy possessed by an object due to its motion
• Kinetic energy depends on

  • The speed of the object
  • The mass of the object
• Gravitational Potential Energy

  The energy stored in an object due to its height from the ground or a reference point
• Gravitational potential energy is directly proportional to


  • The mass of the object
  • The vertical height of the object above the reference point
  • The gravitational field strength (g) acting on the object
• Elastic Potential Energy


  The energy stored in an elastic material when it is stretched or compressed
• Elastic potential energy depends on


  • The amount of extension or compression
• Chemical Potential Energy


  The energy stored in the chemical bonds between atoms or molecules
• Internal Energy (Thermal energy)

  The total kinetic energy due to random motion of the particles and the total potential energy between the particles in the system
• An isolated system does not have a net external force and does not exchange matter or energy with its surroundings
• In reality, there is no way to completely isolate a system and energy is never completely conserved within a machine
• With the presence of air resistance, kinetic energy is converted into gravitational potential energy and thermal energy on the way up
• On the way down, gravitational potential energy is converted into kinetic energy and thermal energy
• The mechanical energy decreases during the motion, but the total energy remains the same throughout the motion
• Any sound energy produced will be lost as thermal energy to the surroundings eventually
• Work


  Work is done on an object when there is a force acting on the object, the object moves, and the movement is in the direction of the force
• No work is done when there is no force
• Work done


  Defined as the product of a force and the distance moved in the direction of the force
• Work


  Work done by a constant force, in joules (J)
• SI Unit for Work done is joule (J)
• Energy is needed to do work
• When work is done on an object

  Energy is transferred
• Gravitational potential energy


  Energy transferred to an object when work is done against gravity
• Kinetic energy


  Energy transferred to an object when work is done to increase its speed
• Calculating work done

  Work done = Force × Distance moved