Chapter 5 Work and Machines Page 126-131

Review Motion Force 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 128:1,2,3 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:

Given: Asked: Formula: W=Fd Substitute: Answer: Page 128 Problem 1 A couch is pushed with a force of 75N and moves a distance of 5m across the floor. How much work is done in moving the couch? Given: Asked: Formula: W=Fd Substitute: Answer:

Given: Asked: Formula: W=Fd Substitute: Answer: Problem 2 A lawn mower is pushed with a force of 80N. If 12,00J of work is done in mowing the lawn, what is the total distance the lawn mower was pushed? Given: Asked: Formula: W=Fd Substitute: Answer:

Given: Asked: Formula: W=Fd Substitute: Answer: Problem 3 The brakes on a car do 240,000J of work in stopping a car. If the car travels a distance of 50m while the brakes are being applied, what is the force the brakes exert on the car? Given: Asked: Formula: W=Fd Substitute: Answer:

Given: Asked: Formula: W=Fd Substitute: Answer: Problem 4 The force needed to lift an object is equal in size to the gravitational force on the object. How much work is done in lifting an object with a mass of 5.0 kg a vertical distance of 2.0 m? Given: Asked: Formula: W=Fd Substitute: Answer:

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?

Problem 1 Page 130 In lifting a baby from a crib, 50 J of work are done. How much power is needed if the baby is lifted in 2 s? Given: Asked: Units? Formula: P= W/t Substitute: Answer

Problem 2 Page 130 If a runner’s power is 130 W as she runs, how much work is done by the runner in 10 minutes? Given: Asked: Units? Formula: P= W/t Substitute: Answer

Problem 3 Page 130 The power produced by an electric motor is 500 W Problem 3 Page 130 The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work? Given: Asked: Units? Formula: P= W/t Substitute: Answer

Energy and Work

Power Practice Problems P130: 1,2,3 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 Substitute Answer

Section Review P131: 5,6,7 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 is applied to an object.- Car jack (p 132) Multiplies force A machine can make work easier by increasing the distance which reduces the force needed. Ramp (p133) 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 so force will be less

Work, Distance and Force Machines don’t decrease the work (real machines increase the work), but they can decrease the force required. Ideal machine (p135) Workin= Workout Win = Fin *5 m , Wout= Win Wout= 300J but d=5m so F= 60N Wout= 100 N * 3m = 300J 5 m 3 m 100 N 4 m

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 distance – 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 (P135) Workin= Workout Forcein X distancein= ForceoutX distanceout Fin X 5cm = 1500N X 1 cm Fin= ? distance units cancel out if the same unit

Ideal Mechanical Advantage Workin= Workout Fin X din= FoutX dout F out Fin d in dout = F out Fin d in dout IMA = =

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 Workin x 100% < 100% efficiency =

Assignment Chapter 4 Practice Problems Practice Problems P 128:1,2,3 Power Practice Problems P130: 1,2,3 Section Review P131: 5,6,7 Page 137 – Applying math 5-6-7

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 Second-class Lever Input Force Output Distance Output Force Input Distance Second-class Lever Output Distance Output Force Input Distance Input Force Output Distance Third-class Lever Output Force Input Distance Input Force

Ideal Mechanical Advantage of a Lever Force output Force input IMA = Force output x Length output=Force input X Length input L input L output IMA =

Pulley Types (P 141-142) Fixed Pulley Movable Pulley Input Force 4N Output 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 Force input IMA = IMA = Number of Strings Lifting the Load

Wheel and Axle (P143) ra rw Radius of wheel IMA = Radius of axel Input Force rw Output Force Radius of wheel Radius of axel IMA =

Inclined Plane 100 N Length of Slope IMA = Height of Slope F= 100N, height = 3m W= 100 N * 3m = 300J W= 300J but d=5m so F= 60N 5 m 3 m 100 N Length of Slope Height of Slope IMA =

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

Section 3 Review Page 146 Applying Math 5-6-7

Assignment Practice Problems P 128:1,2,3 Power Practice Problems P130: 1,2,3 Section Review P131: 5,6,7 Page 137 – Applying math 5-6-7 Page 146 Applying Math 5-6-7 Practice Problem Page- Due Monday Note Taking Worksheet- due Monday Lab on Tuesday Chapter 5 Review Page 152-153 1-20, 24-28 Due Wednesday Test on Chapter 5 Friday

Formulae F out Fin d in dout Workout Workin x 100% < 100% F = m*a , FG= 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 (kW) F out Fin d in dout IMA = = Workout Workin x 100% < 100% efficiency =

IMA of Simple Machines Workout Workin x 100% < 100% efficiency = Force output Force input All Simple Machines MA = > 1 Lever L input L output IMA = Length of Slope Height of Slope IMA = Inclined Plane Radius of wheel Radius of axel Wheel and Axle IMA = Pulley IMA = Number of Strings Lifting the Load Workout Workin x 100% < 100% efficiency =

Chapter 5 Review Lever Input Arm Output Arm IMA A 25 75 B 53 42 C 36 D 32 99 E 10 30

Problems Page 153 Given: Asked: Units Formula: Substitute: Answer

Friday November 7 Get out your calculators and Reference Sheets Put away your books and notes. Answer all the questions on the test sheets Don’t overlook problems on the back of the test. When you have finished, place test in the tray. Place all homework in the tray After the test sit quietly and begin reading chapter 6 on page 158.