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Work, Power and Energy, (study buddy) I. Work What is Work? (definition/description/equation) Symbol: W Unit: Joule or Newton-meter Work is done when a net Force is applied to an object, and it moves in the direction of the applied force Work = Force * Distance (must use fundamental units: meters, Newtons, etc) Work: (1) applied force, (2) causes motion, (3) in the same direction Energy is expended when work is done Work is directly proportional to both Force and distance moved Examples of Work and non-work: Note that Torque also = F•d. but the d’s are defined differently. In torque, the object may not be moving. II. Power What is Power? (definition/description/equation) Symbol: P Unit: Watt = J/s Power is the rate at which work is done Power = Work/time Examples of Power: III. Energy What is Energy? Energy is the ability to cause work to be done : makes matter move or change Two Types of Energy: o Kinetic Energy (definition/description/equation) Symbol: Uk or KE Unit:Joule Energy of Motion Kinetic Energy = ½ * m * v2 (note that KE cannot be negative) Change in Kinetic Energy = Uk final – Uk initial KE is directly proportional to the mass, but is directly proportional to the velocity squared o Potential Energy (definition/equation) Symbol: PE Unit:Joule Stored Energy. It has the potential to do work. The common potential energies used in physics are functions of position. Is (+) when object can do work. Is (-) when work must be done on object o Types of Potential Energy: Gravitational Potential Energy GPE, Ug Energy Stored due to height . Ug = m * g * h = mass * accel of gravity * height Elastic Potential Energy EPE, Ue Energy Stored due to stretching or compressing an elastic material, (usually a spring) Ue = ½ * k * x2 = ½ * spring constant * distance spring was stretched or compressed2 Hooke’s Law Direct relationship between the Force applied to a spring, and the amount it stretches or compresses F = k * x : The Force applied to a spring = spring constant * distance spring stretches or compresses. This relationship is often used to find the spring constant, (as force and distance can usually be easily measured) Other forms of Potential Energy: There are other forms of potential energy such as thermal, nuclear or chemical. IV. Law of Conservation of Energy Statement of the Law of Conservation of Energy: Energy cannot be created nor destroyed in any physical or chemical process However, energy can change forms, (from kinetic to potential, or vice versa) Consider a shot-put falling onto a grassy field A B C D Position A = ball being held at some height above the ground Position B = ball halfway to the ground Position C = right before the ball is stopped, but it has no height. Position D = ball has embedded itself into the ground If the total Energy at the beginning was Utotal , what can we say about the energy at each point in terms of Kinetic and Potential? Energy at point A: All energy is gravitational potential, (ball not moving yet) Utotal = Ug Energy at point B: Total energy is the sum of Gravitational + Kinetic Utotal = Ug + Uk Energy at point C: All energy has transformed into Kinetic Utotal = Uk Energy at point D: All energy has been dissipated into other forms, such as heat, compression of the soil, etc. Throughout this process, from A to D, the Utotal was constant. Conservation of Mechanical Energy: UTotal = Ukinetic + Upotential (Ignores friction or other losses) V. Work-Energy Theorem: The Net Work done by or on an object is equal to that object’s change in Kinetic Energy: W = ΔUk