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Work, Energy & Power What’s work? • A scientist delivers a speech to an audience of his peers. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car? 5 What’s work? • A scientist delivers a speech to an audience of his peers. – No • A body builder lifts 350 pounds above his head. – Yes • A mother carries her baby from room to room. – No • A father pushes a baby in a carriage. – Yes • A woman carries a 20 km grocery bag to her 6 Formula for work Work = Force x Distance • Force=Newtons (N) • Distance=meters (m) • Work=Newton-meters (N*m) – 1 N*m=1 Joule (J) • Work=Joules (J) 7 Work=Force*Distance • Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? • 200 joules (W = 20N x 10m) 8 Power the rate at which work is done • Power = Work/Time • Power=(force x distance)/time • Units: – Work=Joules (J)=Newtons*meters (N*m) – Power=Watts = Joules/second 10 Who Needs It?? ENERGY: POTENTIAL & • Potential energy • Kinetic energy – Stored up energy or energy held in readiness – Energy of motion – KE= ½ mv2 – PE=mgh • • • • PE=Joules (J) m=mass (kg) g=free fall acceleration h=height (m) KINETIC • KE=Joules (J) • m=mass (kg) • v=velocity (m/s) (9.8 m/s2) ENERGY: The ability to do work & cause change • POTENTIAL ENERGY – Stored-up energy, or energy held in readiness – EXAMPLES: • Elastic Potential Energy – Like slingshot or bungee cord • Gravitational Potential Energy – Like diver on diving board or light bulb in off position • KINETIC ENERGY – Energy of motion – EXAMPLES • moving bicycle • moving auto • high jumper jumping • runner POTENTIAL ENERGY Energy stored or held in readiness Most potential What is the relationship between Kinetic & Potential Energy? http://www.classzone.com/books/ml_science_share/vis_sim/ mem05_pg69_potential/mem05_pg69_potential.html THE TRADE OFF Between Potential & Kinetic Energy KINETIC ENERGY Energy of motion TYPES OF ENERGY • MECHANICAL – Car, plane, wagon • THERMAL – Heat from moving molecules – Inside all matter; furnace • ELECTROMAGNETIC – Visible light, microwaves, radio waves • CHEMICAL – Batteries, matches, body cells • ELECTRICAL – Lightning, power lines • NUCLEAR – Nuclear power plants, atomic bomb, sun & stars FORCES & MOTION • FORCE – A Push or a Pull • MOTION – A Change of Position or Location FRICTION AS A FORCE • Strength of friction depends on: – Types of surfaces involved – How hard surfaces push together TYPES OF FRICTION • SLIDING FRICTION – 2 solid surfaces – Strength depends on roughness of surfaces • ROLLING FRICTION – One solid rolls over the other – Much easier to move object • FLUID FRICTION – Solid moving through a liquid or gas – Less than sliding friction GRAVITY AS A FORCE • FREE FALL – Object accelerates as it falls due to gravity – Object accelerates at rate of 9.8 m/s 2 • Every second velocity increases by 9.8 meters per second • @ 2 seconds velocity is 9.8m/s + 9.8m/s = 19.6 m/s • @ 3 seconds velocity is 9.8m/s + 9.8m/s + 9.8m/s = 29.4 m/s GRAVITY AS A FORCE • AIR RESISTANCE – Type of fluid friction that opposes motion • WEIGHT – Measures force of gravity on object – Weight = Mass X Acceleration due to Gravity WHAT MAKES OBJECTS START MOVING . . . STOP MOVING . . . SLOW DOWN . . . SPEED UP . . . or CHANGE DIRECTION? NEWTON’S FIRST LAW • An object at rest remains at rest and an object in motion remains in motion unless acted upon by an outside force. NEWTON’S SECOND LAW • THE FORCE EXERTED BY AN OBJECT IS RELATED TO ITS MASS & ITS ACCELERATION • AN OBJECTS ACCELERATION IS RELATED TO ITS MASS & THE FORCE REQUIRED TO MOVE IT • FORCE = MASS X ACCELERATION WHAT AFFECTS WHAT? FORCE = MASS X ACCELERATION FORCE ACCELERATION MASS MASS ACCELERATION FORCE FORCE ACCELERATION NEWTON’S THIRD LAW • For every action, there is an equal & opposite reaction THE END