Kinetic and Potential Energy

• In closed systems, momentum is always conserved in a collision
• In general, kinetic energy is not conserved in collisions, leading to inelastic collisions
• In perfectly elastic collisions, kinetic energy is transferred between objects and no energy is transformed into heat, sound or deformation
• Perfectly elastic collisions


  Kinetic energy is transferred between objects and no energy is transformed into heat, sound or deformation
• Perfectly elastic collisions do not exist in everyday situations, but they do exist in the interactions between atoms and subatomic particles
• Total energy is conserved in a closed system
• Perfectly inelastic collisions are those in which the colliding bodies stick together after impact with no kinetic energy
• Kinetic energy in collisions: E₁=-mv²
• Collisions such as a bouncing basketball, a gymnast bouncing on a trampoline, or a tennis ball being hit are moderately elastic, retaining about half the kinetic energy of the system
• In perfectly inelastic collisions, most or all of the initial kinetic energy of the system is transformed into other forms of energy
• Gravitational potential energy is directly proportional to the mass of the object, its height above the reference point, and the strength of the gravitational field
• Gravitational potential energy of an object, E, is the energy stored in its position in a gravitational field above a reference point
• Work needs to be done against the force of gravity to lift an object of mass through a distance
• Close to the surface of the Earth, the gravitational force is -9.8N/kg and the work done is equal to the potential energy gained
• Calculating changes in gravitational potential energy from a force graph

  The gravitational potential energy can be calculated using a graph if the force acting on an object varies. The graph shows the relationship between force and distance
• If work is being done by a body

  It could lose kinetic energy as it slows down
• If work is being done by the gravitational field

  The field loses gravitational potential energy as the object falls
• Both work and energy are scalar quantities and have only magnitude
• If work is done on the body by an external force

  The body would gain kinetic energy as it speeds up
• If work is done on the body by an external force

  The gravitational field would gain gravitational potential energy as the object rises
• If a weightlifter lifts the bar at constant speed

  The bar does not gain kinetic energy, but the gravitational field gains gravitational potential energy
• In drawing back an arrow, an archer does work on the bow

  Elastic potential energy is transformed to the kinetic energy of the arrow when the string does work on the arrow as it is released