Chapter 6 Work and Energy Advanced Physics Chapter 6 Work and Energy
Work and Energy 6-1 Work done by a Constant Force 6-2 Work done by a Varying Force 6-3 Kinetic Energy, and the Work-Energy Principle 6-4 Potential Energy 6-5 Conservative and Nonconservative Forces 6-6 Mechanical Energy and Its Conservation 6-7 Problems Solving Using Conservation of Mechanical Energy 6-8 Other Forms of Energy 6-9 Energy Conservation with Dissipative Forces: Solving Problems 6-10 Power
6-1 Work done by a Constant Force Describes what is accomplished by the action of a force when it acts on an object as the object moves through a distance The transfer of energy by mechanical means The product of displacement times the component of the force parallel to the displacement Both work and energy are scalar quantities
6-1 Work done by a Constant Force W = Fd Or W = Fd cos where is the angle between the direction of the applied force and the direction of displacement
6-1 Work done by a Constant Force W = Fd cos Force Displacement
6-1 Work done by a Constant Force Units: joule (N•m) 1 joule = 0.7376 ft•lb
6-1 Work done by a Constant Force Negative work? What about friction? Work done on Moon by Earth? Work done by gravity depends only on height of hill not incline angle.
6-2 Work done by a Varying Force Work done by a variable force in moving an object between 2 points is equal to the area under the curve of a Force (parallel) vs. displacement graph between the two points. Why? Or we will have to do some calculus on it!
6-2 Work done by a Varying Force
6-3 Kinetic Energy, and the Work-Energy Principle The ability to do work (and work is?) Kinetic Energy Energy of motion; a moving object has the ability to do work Translational Kinetic Energy (TKE) Energy of an object moving with translational motion (?)
6-3 Kinetic Energy, and the Work-Energy Principle Translational Kinetic Energy (KE) KE = ½ mv2
6-3 Kinetic Energy, and the Work-Energy Principle The net work done on an object is equal to the change in its kinetic energy Wnet = Kef – Kei = KE TKE m and v2 But…what about potential energy????
6-4 Potential Energy Potential Energy Energy associated with forces that depend on the position or configuration of a body (or bodies) and the surroundings Gravitational Potential Energy Potential energy due to the position of an object relative to another object (gravity)
PEgrav = mgy 6-4 Potential Energy Gravitational Potential Energy Potential energy due to the position of an object relative to another object (gravity) PEgrav = mgy
W = -PE 6-4 Potential Energy Potential Energy In general the change in potential energy associated with a particular force is equal to the negative of the work done by the force if the object is moved from one point to another. W = -PE
Elastic PE = ½ kx2 6-4 Potential Energy Elastic Potential Energy Potential energy stored in an object that is released as kinetic energy when the object undergoes a change in form or shape For a spring: Elastic PE = ½ kx2 Where k is the spring constant
6-4 Potential Energy Fs = -kx Elastic Potential Energy For a spring: The force that the spring exerts when it is pushed or pulled is called the restoring force (Fs) and is related to the stiffness of the spring (spring constant-k) and the distance it is compressed or expanded Fs = -kx
6-4 Potential Energy Fs = -kx Elastic Potential Energy For a spring: This equation is called the spring equation or Hooke’s Law
6-5 Conservative and Nonconservative Forces Forces for which the work done by the force does not depend on the path taken, only upon the initial and final positions. Examples: Gravitational Elastic Electric
6-5 Conservative and Nonconservative Forces Forces for which the work done depends on the path taken Examples: Friction Air resistance Tension in a cord Motor or rocket propulsion Push or pull by a person
6-5 Conservative and Nonconservative Forces Work-Energy Principle (final) The work done by the nonconservative forces acting on a object is equal to the total change in kinetic and potential energy. Wnc = KE + PE
6-6 Mechanical Energy and Its Conservation Total Mechanical Energy (E) E = KE + PE
6-6 Mechanical Energy and Its Conservation Principle of Conservation of Mechanical Energy If only conservative forces are acting, the total mechanical energy of a system neither increase nor decreases in any process. It stays constant—it is conserved KE1 + PE1 = KE2 + PE2 KE = -PE
6-7 Problems Solving Using Conservation of Mechanical Energy E = KE + PE = 1/2mv2 + mgy KE = -PE 1/2mv21 + mgy1 = 1/2mv22 + mgy2 Sample problems: P.160 – 165
6-8 Other Forms of Energy Other Forms of Energy: According to atomic theory, all types of energy is a form of kinetic or potential energy. Electric energy Energy stored in particles due to their charge KE or PE? Nuclear energy Energy that holds the nucleus of an atom together
6-8 Other Forms of Energy Other Forms of Energy: Thermal energy Energy of moving (atomic/molecular) particles KE or PE? Chemical energy Energy stored in the bonds between atoms in a compound (ionic or covalent)
6-8 Law of Conservation of Energy The total energy is neither increased nor decreased in any process. Energy can be transformed from one form to another, and transferred from one body to another, but the total remains constant This is one of the most important principles in physics!
6-9 Energy Conservation with Dissipative Forces: Solving Problems Forces that reduce the total mechanical energy Examples: Friction Thermal energy
6-9 Energy Conservation with Dissipative Forces: Solving Problems Problem Solving (Conservation of Energy) Draw a diagram Label knows (before/after) and knowns (before/after) If no friction (nonconservative forces) then… KE1 + PE1 = KE2 + PE2 If there’s friction (nonconservative forces) then add into equation Solve for the unknown
6-10 Power Power The rate at which work is done The rate at which energy is transferred Units (what?) 1W = 1J/s 746 W = 1 hp
6-10 Power Power P = W/t = Fd/t P = F v (since v =d/t)