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Notes on WORK What do you think of when you hear the word “work”? WORK happens when a FORCE moves an object through a distance W F d displacement (Newtons) W = F x d (meters) Work is measured in Newton meters or JOULES work is a scalar quantity: F and d have to be parallel to each other if F and d are perpendicular then that Force is not the cause of the work done F d Forces exerted at an angle: only the vector component parallel to the distance moved does work F Fx W = F cos θ d Since work is F x d and one Force we deal with is Fg (force of gravity) and Fg = mg work could = (mg)d Examples of Work Problems: 500N 8m 4m To get the 500N block to the top takes the same amount of WORK whether you lift it straight up or push it up the ramp. The FORCE to lift anything is its WEIGHT Fg = m g W = Force x distance lifting Work = (500N) (4m) = 2000Nm or 2000 Joules slide up ramp Work = F x distance up the ramp 2000J = F (8m) F = 250 N I doubled the distance so the Force is halved An Inclined Plane is a Simple Machine. Simple Machines allow us to do the same amount of work with less force (effort). Simple Machines include: inclined planes, levers, screws, wedge, pulley, wheel & axle Notes on POWER POWER = rate that work is done P = work time Joules = Watt sec a 100 Watt light bulb puts out 100 Joules of NRG per sec 1 horsepower = 746 Watts 1 kW = 1000 W ☺ always coming back to past lessons P = work time remember that work = F d also P = F d t and Example Power Problems: so P = F d t or P = F x velocity the Force might be Fg which = mg so P = mg d t Energy Notes Energy = ability to do work Forms of Energy: solar, electrical, mechanical, thermal, chemical, nuclear, hydroelec, light, sound, wind, potential, kinetic, electromagnetic, etc Chemistry – we focused on thermal, chemical, and nuclear Physics – 1st sem focus on mechanical, kinetic, potential 2nd sem study electrical, magnetic, thermal, sound, light Mechanical Energy: nrg which is possessed by an object due to its motion or its stored nrg mechanical nrg = kinetic + potential (as a car rolls down a hill it loses potential energy and gains kinetic energy) Kinetic Energy – nrg of a moving object KE = ½ x mass x velocity2 KE & mass are directly related (if m is doubled, KE doubles) KE & v2 are exponentially related (if v doubles, KE quadruples; if v triples, KE x 9) ex: In 1994 Leroy Burrell set a world record in the 100m dash by running it in 9.85 sec. What was his mass if his kinetic energy was 3400J ? (assume constant speed) Potential Energy - nrg of position, shape, or form position example: an object at the top of a hill or cliff or table that has the potential to fall from a height shape example: a spring has (stored energy) potential to snap back into shape form example: a rubber band, a snap bracelet, a bow to shoot an arrow gravitational potential energy = potential (stored) energy due to a location relative to a reference level. Assume reference is ground or floor unless otherwise stated. GPE = mass x acceler due to gravity x height above or below reference kg 9.8 m/s2 meters GPE = m g h elastic potential energy = potential energy of an “elastic” object that is stretched or compressed the spring or rubber band or bow string has to be able to go back to its original shape and size relaxed length stretched length lr ls elastic potential energy = ½ x spring constant x how far it stretched (stiffness) (ls - lr) N/m meters EPE = ½ k x2 EPE = 1 N m2 m Example Potential Energy Problems: = Nm or Joules Notes Conservation Of Energy Law of Conservation of Energy = energy cannot be created nor destroyed, only changed form In other words numerically, Total Energy will remain constant Mechanical Energy = sum of kinetic and potential ME = KE + GPE + EPE GPE = 75J KE = 0 Conservation of NRG Etop = Ebottom for cliff diver GPEt + KEt = GPEb + KEb GPE = 0 KE = 75 J Pendulum GPEmax KE=0 GPEmax KE=0 loses GPE gains KE halfway GPE = KE V= 0 KE = 0 GPEmax = 100J Roller Coaster starts high so we have lots of PE GPE = mgh losing GPE gaining KE b/c h is lower V increasing if GPE = 60J then KE = 40J GPE = 0 J KE = 100 J Example Conservation of Energy Problem GPE = 50J KE = 50J Work-Energy Theorem If you do Work on an object, you change its (kinetic and potential) Energy If I lift books from desk Did I do Work? Was there a force applied in direction of object moved ? Did I change the GPE of the book? KE? Work = ∆ E if work = change in Kinetic Energy F d = KEf - KEi F d = ½ m v2f - ½ m v2i if work = change in Gravitational Potential Energy F d = mghf - mghi F d = m g ∆h if work = change in Elastic Potential Energy F d = m k x 2f - m k x 2i