Slideshow: Work and Energy Notes

Textbook: Chapter 6 in Mastering Physics (get online code for registration on about page of google classroom)


Work Energy Theorem

    • Forces that speed an object/system up add energy to the object/system according to W = ΔK= ΣFdcos(θ)
    • Only forces parallel to motion change the K, others cause a change in direction.
    • Qualitatively and quantitatively predict changes in K based on Fll vs displacement diagrams
    • Design experiment to find work done on a force distance graph

Systems and Objects

    • Single objects can only have K, but systems can have potential energy. We must include other objects in the system definition to discuss potential energy.
    • Consider K (kinetic energy), Ug (gravitational potential energy), Us (elastic/spring potential energy) and Internal energy (consider when friction removes energy from system) at different locations.
      • Include pendulums, falling objects and mass spring oscillators. Derive expressions for energy interactions.
      • Express energy in diagram and description
    • Closed systems do not exchange energy with outside objects. Open systems exchange energy with objects outside the system.
    • Define open and closed systems for common scenarios

Conservation of energy

    • Total Energy is always conserved in a closed system.
    • In open systems energy is not conserved within the system, but energy in the universe is still conserved.
    • Add W to the conservation equation to account for work done on a system


Common Misconceptions:

Misconception: Energy gets used up or runs out.

  • Correct principle: Energy is a conserved quantity that cannot be created or destroyed. It can be transformed from one form to another (like converting into potential energy or thermal energy), or it can be transferred to another objects (work done).

Misconception: Something not moving can't have any energy.

  • An object that is not moving will not have any kinetic energy, but it can have internal energy (thermal, chemical and nuclear potential energies) and depending upon how you define your system, it may have potential energy due to its position relative to another object.

Misconception: A force acting on an object does work even if the objects does not move.

  • Correct principle:

Misconception: Energy is destroyed in transformations from one type to another.

Misconception: Energy can be recycled.

Misconception: Gravitational potential energy is the only type of potential energy.

Misconception: When an object is released to fall, the gravitational potential energy immediately becomes all kinetic energy.

Misconception: Energy is not related to Newton's laws.

Misconception: Energy is a force.