Work, Energy, and Power · ⁨งาน พลังงาน และกำลัง⁩

Learning Objective 3.1.A: Describe the translational kinetic energy of an object in terms of the object's mass and velocity.

• 3.1.A.1 An object's translational kinetic energy is given by the equation
  • Equation: $K = \dfrac{1}{2}mv^2$
• 3.1.A.2 Translational kinetic energy is a scalar quantity.
• 3.1.A.3 Different observers may measure different values of the translational kinetic energy of an object, depending on the observer's frame of reference.

Learning Objective 3.2.A: Describe the work done on an object or system by a given force or collection of forces.

• 3.2.A.1 Work is the amount of energy transferred into or out of a system by a force exerted on that system over a distance.
  • 3.2.A.1.i The work done by a conservative force exerted on a system is path-independent and only depends on the initial and final configurations of that system.
  • 3.2.A.1.ii The work done by a conservative force on a system—or the change in the potential energy of the system—will be zero if the system returns to its initial configuration.
  • 3.2.A.1.iii Potential energies are associated only with conservative forces.
  • 3.2.A.1.iv The work done by a nonconservative force is path-dependent.
  • 3.2.A.1.v Examples of nonconservative forces are friction and air resistance.
• 3.2.A.2 Work is a scalar quantity that may be positive, negative, or zero.
• 3.2.A.3 The amount of work done on a system by a constant force is related to the components of that force and the displacement of the point at which that force is exerted.
  • 3.2.A.3.i Only the component of the force exerted on a system that is parallel to the displacement of the point of application of the force will change the system's total energy.
    • Equation: $W = F_{\parallel}d = Fd\cos\theta$
  • 3.2.A.3.ii The component of the force exerted on a system perpendicular to the direction of the displacement of the system's center of mass can change the direction of the system's motion without changing the system's kinetic energy.
• 3.2.A.4 The work-energy theorem states that the change in an object's kinetic energy is equal to the sum of the work (net work) being done by all forces exerted on the object.
  • Equation: $\Delta K = \sum_{i} W_i = \sum_{i} F_{\parallel,i}\,d$
  • 3.2.A.4.i An external force may change the configuration of a system. The component of the external force parallel to the displacement times the displacement of the point of application of the force gives the change in kinetic energy of the system.
  • 3.2.A.4.ii If the system's center of mass and the point of application of the force move the same distance when a force is exerted on a system, then the system may be modeled as an object, and only the system's kinetic energy can change.
  • 3.2.A.4.iii The energy dissipated by friction is typically equated to the force of friction times the length of the path over which the force is exerted
    • Equation: $\Delta E_{\text{mech}} = F_f\,d\cos\theta$
• 3.2.A.5 Work is equal to the area under the curve of a graph of $F_{\parallel}$ as a function of displacement.

Boundary statement: AP Physics 1 only expects students to analyze the transfer of mechanical energy (as defined in Unit 3, Topic 4: Conservation of Energy), although students should be aware that mechanical energy may be dissipated in the form of thermal energy or sound. In AP Physics 2, students will also study how thermal energy can be transferred between systems through heating or cooling.

Learning Objective 3.3.A: Describe the potential energy of a system.

• 3.3.A.1 A system composed of two or more objects has potential energy if the objects within that system only interact with each other through conservative forces.
• 3.3.A.2 Potential energy is a scalar quantity associated with the position of objects within a system.
• 3.3.A.3 The definition of zero potential energy for a given system is a decision made by the observer considering the situation to simplify or otherwise assist in analysis.
• 3.3.A.4 The potential energy of common physical systems can be described using the physical properties of that system.
  • 3.3.A.4.i The elastic potential energy of an ideal spring is given by the following equation, where $\Delta x$ is the distance the spring has been stretched or compressed from its equilibrium length.
    • Equation: $U_s = \dfrac{1}{2}k(\Delta x)^2$
  • 3.3.A.4.ii The general form for the gravitational potential energy of a system consisting of two approximately spherical distributions of mass (e.g., moons, planets or stars) is given by the equation
    • Equation: $U_g = -G\dfrac{m_1 m_2}{r}$
  • 3.3.A.4.iii Because the gravitational field near the surface of a planet is nearly constant, the change in gravitational potential energy in a system consisting of an object with mass $m$ and a planet with gravitational field of magnitude $g$ when the object is near the surface of the planet may be approximated by the equation
    • Equation: $\Delta U_g = mg\Delta y$
• 3.3.A.5 The total potential energy of a system containing more than two objects is the sum of the potential energy of each pair of objects within the system.

Learning Objective 3.4.A: Describe the energies present in a system.

• 3.4.A.1 A system composed of only a single object can only have kinetic energy.
• 3.4.A.2 A system that contains objects that interact via conservative forces or that can change its shape reversibly may have both kinetic and potential energies.

Learning Objective 3.4.B: Describe the behavior of a system using conservation of mechanical energy principles.

• 3.4.B.1 Mechanical energy is the sum of a system's kinetic and potential energies.
• 3.4.B.2 Any change to a type of energy within a system must be balanced by an equivalent change of other types of energies within the system or by a transfer of energy between the system and its surroundings.
• 3.4.B.3 A system may be selected so that the total energy of that system is constant.
• 3.4.B.4 If the total energy of a system changes, that change will be equivalent to the energy transferred into or out of the system.

Learning Objective 3.4.C: Describe how the selection of a system determines whether the energy of that system changes.

• 3.4.C.1 Energy is conserved in all interactions.
• 3.4.C.2 If the work done on a selected system is zero and there are no nonconservative interactions within the system, the total mechanical energy of the system is constant.
• 3.4.C.3 If the work done on a selected system is nonzero, energy is transferred between the system and the environment.

Boundary statement: AP Physics 1 expects students to know that mechanical energy can be dissipated as thermal energy or sound by nonconservative forces.

Learning Objective 3.5.A: Describe the transfer of energy into, out of, or within a system in terms of power.

• 3.5.A.1 Power is the rate at which energy changes with respect to time, either by transfer into or out of a system or by conversion from one type to another within a system.
• 3.5.A.2 Average power is the amount of energy being transferred or converted, divided by the time it took for that transfer or conversion to occur.
  • Equation: $P_{\text{avg}} = \dfrac{\Delta E}{\Delta t}$
• 3.5.A.3 Because work is the change in energy of an object or system due to a force, average power is the total work done, divided by the time during which that work was done.
  • Equation: $P_{\text{avg}} = \dfrac{W}{\Delta t}$
• 3.5.A.4 The instantaneous power delivered to an object by the component of a constant force parallel to the object's velocity can be described with the derived equation.
  • Equation: $P_{\text{inst}} = F_{\parallel}v = Fv\cos\theta$