VIEWS: 24 PAGES: 34 POSTED ON: 7/30/2011
Chapter 5 Work and Machines Page 126-131 Review • Motion – Distance, time, speed (velocity), acceleration – Speed = rate of change of distance – Acceleration = rate of change of velocity • Force – Force, mass, acceleration – A force is required to change the velocity of an object. • F = ma – Weight is the gravitational attraction on an object • W = mg (g= 9.8 m/s2 the acceleration of gravity) Review of Energy • Energy is the ability to do work. • Different forms of energy • Different types of mechanical energy – KE = ½ mv2, PE (GPE)= mgh • Energy is always conserved • Energy may be conserved but it is often changed into a useless form (heat) by friction and air resistance. • But Einstein said, E = mc2 Work • Work makes something move • Work is the transfer of energy when a force makes an object move. • Two conditions – A force must make something move – The direction of motion must be in the direction of the force – Carrying books • Work is a transfer of energy • W= Fd (force in Newtons (kg m/s2, distance in m) Practice Problems P 128 (turn in Monday) • You push a refrigerator with a force of 100N. If you move the refrigerator a distance of 5m while you are pushing, how much work is done? • Given: • Asked: ?units • Formula: Power • Power is the rate of doing work. • How much work is done in 1 sec • P = W/t • W (joules) joules = kg m2/s2 • Unit for power is joules/second called a Watt • A Watt is kg m2/s3 • Since work is energy transferred • P= E/t Power Example • You push a box with a force of 100 N across the floor 5 meters it takes you 45 seconds. Your friend pushes a similar box also with a 100 N force also 5 meters across the floor but it only takes your friend 30 seconds. – How much work do you each do? – How much power do you each expend? Power Practice Problems P130 (Turn in Tuesday) • You do 900 J of work pushing a sofa. If it took 5s to move the sofa, how much power did you use? – Given – Asked? Units? – Formula Review (P131) • Work is a transfer of energy • W= Fd • (force in Newtons (kg m/s2, distance in m) • N*m or Joules • Power is the rate of doing work. • How much work is done in 1 sec • P = W/t • W (joules) joules = kg m2/s2 , t= seconds • Joules/second = Watt Machines (P132) • A machine is a device that makes work easier. • Simple machines (not motorized) • How machines make work easier – A machine can increase the force that can be applied to an object.- Car jack (p 132) – A machine can change the direction of the force- splitting wedge (p133) Work Done by Machines • Figure 7 Page 133 • Choice – lift the chair straight up the height of the truck. • W = Fg x height • Or- slide the chair up the ramp • Work will be the same • Distance will be greater but force will be less Factors of Work • Input force – the force applied to the machine • Output force – the force applied by the machine • Input distance – the distance the input force moves • Output force – the distance the output force moves. • work in = input force x input distance • work out = output force x output distance Pulling a Nail • work out = work in • Figure 10 page 135 • hammer claw moves 1 cm to pull a nail • Handle moves 5 cm • Output force of 1,500 N • Input force = ? Ideal Machines • Workin= Workout Forcein X distancein= ForceoutX distanceout • Fin X 5cm = 1500N X 1 cm • Fin= ? • distance units cancel out if the same unit Work, Distance and Force Machines don’t decrease the work, but they can decrease the force required. Ideal machine (p135) Workin= Workout Wout= 100 N * 3m = Win = Fin *5cm , Wout= Win 300J W= 300J but d=5m so F= 60N 5m 3m 100 N 4m Ideal Mechanical Advantage Workin= Workout Forcein X distancein= ForceoutX distanceout Fin X din= FoutX dout F out d in = Fin dout IMA = F out = d in Fin dout Real Machines • Work out< Workin • Some of the input work is always converted into heat by friction. • Efficiency tells us how much of the input work is converted into output work. Workout x 100% < 100% efficiency = Workin Practice • Page 137 – Applying math 5-6-7 • Wednesday – – AM Testing – PM Problems • Thursday – Field Trip • Friday – Lab • Monday – Practice Problems • Tuesday – Chapter review • Wednesday- Chapter 5 Test Simple Machines Section 3 Page 138-146 Types of Simple Machines • Simple machine does work with only one movement. • Lever • Pulley • Wheel and Axle • Inclined Plane • Compound Machines Lever • A bar that is free to pivot (turn) about a fixed Point • The fixed point is called the fulcrum • Input arm – distance from the input force to the fulcrum • Output arm – The distance from the output force to the fulcrum. • If output arm is shorter than the input arm then the output force is greater than the input force. Types of Levers (P138-139) First-class Lever Input Force Output Force Output Input Distance Distance Second-class Lever Output Output Force Distance Input Distance Input Force Output Third-class Lever Distance Output Force Input Distance Input Force Ideal Mechanical Advantage of a Lever Force output IMA = Force input Force output x Length output=Force input X Length input L input IMA = L output Pulley Types (P 141-142) Fixed Pulley Output Force 4N Input Force 4N 4N Movable Pulley 2N Input Force 2N Output Force 4N 4N Types of Pulleys (cont) Block and Tackle 1N 1N 1N Input Force 1N 1N Output Force 4N 4N Ideal Mechanical Advantage of a Pulley Force output IMA = Force input IMA = Number of Stings Lifting the Load Wheel and Axel (P143) Input Force rw ra Output Force Radius of wheel IMA = Radius of axel Inclined Plane W= 100 N * 3m = 300J W= 300J but d=5m so F= 60N F= 100N, height = 3m 5m 3m 100 N Length of Slope IMA = Height of Slope Types of Inclined Plane • Ramps, inclines, road grades • Screw • Wedge • IMA = length of slope/height of slope • Page 144 Compound Machines • Two or more simple machines that operate together. –Can opener –Automobile –Space shuttle Ideal Mechanical Advantage of Simple Machines Force output > 1 All Simple Machines IMA = Force input L input Lever IMA = L output Inclined Plane Length of Slope IMA = Height of Slope Wheel and Axle Radius of wheel IMA = Radius of axel Pulley System IMA = Number of Stings Lifting the Load Section Review • Page 146 Applying Math • 5-6-7 Formulae F = m*a , w= m*g unit kg m/s2 = Newton W= F*d units N*m = Joule P= W/t or P= E/t unit J/s = Watt Workout x 100% < 100% efficiency = Workin Assignment • Chapter 5 Review • Page 152-153 • 8,10,11,12,13,15,16,17,19,20,24-28 • Due on Tuesday • Test on Chapter 5 Wednesday – Work and Power – Simple Machines • IMA • Efficiency