• HOME
  • YEAR 9
    • Assessment System
    • Inheritance
    • Plants >
      • Kitchen Garden Project
    • Acids and Alkalis
    • Energetic Reactions
    • Energy Resources
    • Mechanics
    • Environmental Science
  • IGCSE
    • Forces and Motion
    • Energy and Energy Resources
    • Solids, Liquids and Gases
    • Waves
    • Astrophysics
    • Electricity
    • Magnetism
    • Nuclear Physics
    • IGCSE revision
  • Physics I
    • Kinematics >
      • Variables and Units
      • Describing Motion
      • Equations of Motion
      • Problem Solving
      • Projectiles
      • Kinematics Animations
    • Dynamics >
      • Forces
      • Static Equilibrium
      • Newton's Laws
      • Friction
      • Advanced Problems
    • Gravitation and Orbits >
      • Circular Motion
      • Vertical Circles
      • Universal Gravitation
      • Orbits
      • Circular Motion Animation
    • Energy >
      • Work
      • Springs
      • PE and KE
      • Conservation of Energy
      • Work-Energy Theorem
    • Linear Momentum >
      • Impulse
      • Conservation of Momentum
      • Types of Collision
      • 2-D Collisions
    • Simple Harmonic Motion >
      • Mass on Spring
      • Pendulums
      • SHM Animations
    • Rotational Mechanics >
      • Torque
      • Rotational Kinematics
      • Rotational Dynamics
      • Angular Momentum
      • Rotational Energy
    • Mechanical Waves >
      • Waves on a String
      • Sound
    • AP-1 Revision
    • AP Physics C (Mechanics)
  • Physics II
    • Fluid Mechanics
    • Thermal Physics
    • Electrostatics
    • Magnetic Fields >
      • EM Induction
    • Interference and Diffraction
    • Optics
    • Modern Physics
    • AP 2 Revision
  • OCEANOGRAPHY
    • The World Ocean >
      • What is Oceanography
      • History
      • Lat and Long
      • Size and Origin
      • Plate Tectonics
    • Seawater >
      • A Salty Sea
      • Measuring Salinity
      • Thermal Properties
      • Density Profiles
      • Drinking Seawater
    • Circulation and Climate >
      • Global Heating
      • Coriolis Effect
      • Surface Currents
      • Vertical Motion
      • Thermohaline Circulation
      • El Nino
      • Carbon Cycle
    • Waves and Tides >
      • Wave Motion
      • Formation of Waves
      • Beaches
      • Tsunamis
      • Tides
    • Observation Systems >
      • Challenges
      • The CTD
      • Moorings
      • Sound Waves
      • Robotics
      • Satellites
    • Weather and Navigation >
      • Weather Systems
      • Weather Forecasting
      • Hurricanes
      • Navigation
      • Life at Sea
    • Oceans and Mankind >
      • Ocean Acidification
      • Pollution
      • Fish Stocks
      • Climate Change
      • Energy Resources
    • Atlantic Explorer Cruise
    • ROVs
  • COLLEGE
    • College Physics I
    • College Physics II
  • CONTACT
Island Physics

energy

Home >> AP Physics I >> Energy >> Work-Energy Theorem
Momentum >>

4.5 - The Work-Energy Theorem

Objectives:
  • Understand that work is required to change the kinetic energy of an object.
  • To be able to use the work-energy theorem to solve problems
The so-called work-energy theorem stems from the definition of kinetic energy.  The kinetic energy of an object is defined as the work required to stop it, or conversely, to accelerate it.  We usually just consider the limiting case of moving to stationary or vice versa.  However, it can be just that the object changes speed - so the work done is equal to the difference between the initial and final kinetic energies.  This is useful as it can provide an easier method of solving an accelerated object problem than using kinematics and dynamics.  Note: we must still arrive at the same answer though!
\[W = \Delta KE\]
​So, if an object is at the top of a rough slope, it will slide down losing its potential energy, but not all of it will be converted into kinetic energy, so will be transferred to heat via friction.  As the kinetic energy at the bottom will be less than if there was no friction, then the object will be slower.  The work done by friction can be calculated using \(work = force \times distance\).
\[GPE_{top}=KE_{bottom}+W_{friction}\]

​​If a moving object is pushed up a hill by an external force, then its final potential energy is the sum of its initial kinetic energy and the work done on raising it.  The 'extra' energy has been added by the external force.  If springs are included, then the elastic potential energy must be accounted for.
​

This is the final part of the 'accounting' of the energy before and after an event.  Overall, the total energy before must equal to that after.  If a force is applied to an object, then the work done by that force increases the net energy.  If the object does work on its surroundings then its net energy is reduced.  It is always worth pausing and considering whether work has been added or removed from the object.  This is known as the First Law of Thermodynamics and we will look at this concept in  more detail in Unit 14 - Thermal Physics (AP-2).

4.6 - Systems

One of the BIG differences with the new AP Physics 1 course is that it likes to ask about SYSTEMS.  For example, in the old days we would talk about the work done by raising an apple from the ground and it gains GPE.  We rarely consider the Earth as it essentially does not move.  But if we were to consider both the apple and the Earth simultaneously,  does the system gain, lose or have the same energy as before?  One of the ways to think about this is to consider if work has had to be done to or from outside the system.  I personally find this rather tricky! 



​Momentum >>

Other Resources

Picture
PhET Simulation: Energy Skate Park
CK-12: Kinetic Energy
Picture
PhET Simulation: Harder Skate Park
CK-12: Potential Energy
  • HOME
  • YEAR 9
    • Assessment System
    • Inheritance
    • Plants >
      • Kitchen Garden Project
    • Acids and Alkalis
    • Energetic Reactions
    • Energy Resources
    • Mechanics
    • Environmental Science
  • IGCSE
    • Forces and Motion
    • Energy and Energy Resources
    • Solids, Liquids and Gases
    • Waves
    • Astrophysics
    • Electricity
    • Magnetism
    • Nuclear Physics
    • IGCSE revision
  • Physics I
    • Kinematics >
      • Variables and Units
      • Describing Motion
      • Equations of Motion
      • Problem Solving
      • Projectiles
      • Kinematics Animations
    • Dynamics >
      • Forces
      • Static Equilibrium
      • Newton's Laws
      • Friction
      • Advanced Problems
    • Gravitation and Orbits >
      • Circular Motion
      • Vertical Circles
      • Universal Gravitation
      • Orbits
      • Circular Motion Animation
    • Energy >
      • Work
      • Springs
      • PE and KE
      • Conservation of Energy
      • Work-Energy Theorem
    • Linear Momentum >
      • Impulse
      • Conservation of Momentum
      • Types of Collision
      • 2-D Collisions
    • Simple Harmonic Motion >
      • Mass on Spring
      • Pendulums
      • SHM Animations
    • Rotational Mechanics >
      • Torque
      • Rotational Kinematics
      • Rotational Dynamics
      • Angular Momentum
      • Rotational Energy
    • Mechanical Waves >
      • Waves on a String
      • Sound
    • AP-1 Revision
    • AP Physics C (Mechanics)
  • Physics II
    • Fluid Mechanics
    • Thermal Physics
    • Electrostatics
    • Magnetic Fields >
      • EM Induction
    • Interference and Diffraction
    • Optics
    • Modern Physics
    • AP 2 Revision
  • OCEANOGRAPHY
    • The World Ocean >
      • What is Oceanography
      • History
      • Lat and Long
      • Size and Origin
      • Plate Tectonics
    • Seawater >
      • A Salty Sea
      • Measuring Salinity
      • Thermal Properties
      • Density Profiles
      • Drinking Seawater
    • Circulation and Climate >
      • Global Heating
      • Coriolis Effect
      • Surface Currents
      • Vertical Motion
      • Thermohaline Circulation
      • El Nino
      • Carbon Cycle
    • Waves and Tides >
      • Wave Motion
      • Formation of Waves
      • Beaches
      • Tsunamis
      • Tides
    • Observation Systems >
      • Challenges
      • The CTD
      • Moorings
      • Sound Waves
      • Robotics
      • Satellites
    • Weather and Navigation >
      • Weather Systems
      • Weather Forecasting
      • Hurricanes
      • Navigation
      • Life at Sea
    • Oceans and Mankind >
      • Ocean Acidification
      • Pollution
      • Fish Stocks
      • Climate Change
      • Energy Resources
    • Atlantic Explorer Cruise
    • ROVs
  • COLLEGE
    • College Physics I
    • College Physics II
  • CONTACT