Work, Energy & Power

5 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?

6 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 car? –No

7 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)

8 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 joules200 joules (W = 20N x 10m)

10 Power 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

Who Needs It??

ENERGY: POTENTIAL & KINETIC ENERGY: POTENTIAL & KINETIC Potential energy –Stored up energy or energy held in readiness –PE=mgh PE=Joules (J) m=mass (kg) g=free fall acceleration (9.8 m/s 2 ) h=height (m) Kinetic energy –Energy of motion –KE= ½ mv 2 KE=Joules (J) m=mass (kg) v=velocity (m/s)

ENERGY: ENERGY: The ability to do work & cause change KINETIC ENERGY –Energy of motion –EXAMPLES moving bicycle moving auto high jumper jumping runner 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

POTENTIAL ENERGY Most potential Energy stored or held in readiness

What is the relationship between Kinetic & Potential Energy? 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 2 seconds velocity is 9.8m/s + 9.8m/s = 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 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.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 ACCELERATIONTHE 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 ITAN OBJECTS ACCELERATION IS RELATED TO ITS MASS & THE FORCE REQUIRED TO MOVE IT FORCE = MASS X ACCELERATIONFORCE = MASS X ACCELERATION

WHAT AFFECTS WHAT? FORCE FORCE ACCELERATION ACCELERATION ACCELERATION FORCE MASS MASS FORCE ACCELERATION FORCE = MASS X ACCELERATION

NEWTON’S THIRD LAW For every action, thereFor every action, there is an equal & opposite reaction is an equal & opposite reaction

THE END