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Work, Power, & Machines Chapter 14 Mr. Kovacs Integrated Chemistry and Physics 1 What is work? • The product of the force applied to an object and the distance through which that force is applied. 2 What is work? • According to the physics definition, you are NOT doing work if you are just holding the weight above your head. • You are doing work only while you are lifting the weight above your head. • No movement : No work 3 For work to be done on an object, the object must ___________?____________. • move in the direction of the force. 4 Work Requires Motion • If the wall doesn't move, the prisoner does no work. • No movement : No work 5 Work Depends on Direction • 1) Work must have a force • 2) The force must be in the direction of the motion Force, F distance, d 6 Calculating Work • To do work on an object you have to push the object a certain distance in the direction that you are pushing • Work = force x distance = F x d • If I carry a box across the room I do not do work on it (the box) because the force is not in the direction of the motion. Was any work done? 7 Is work being done or not? Mowing the lawn • YES Weight-lifting • YES Carrying groceries • NO Moving furniture up • YES a flight of stairs Pushing against a • NO locked door Swinging a golf club • YES 8 Is work being done or not? • Climbing stairs? • Lifting a book? • Pushing a shopping cart? • Carrying a football? 9 Calculating Work All or part of the force must act in the direction of the movement. 10 Units of Work: The Joule • 1 newton-meter is a quantity known as a joule (J). • Named after British physicist James Prescott Joule. •(1818-1889) 11 What is the SI unit of work? Duh!!!!! • The joule! • Or 1 NM! 12 Using the Work Formula • Work = Force x Distance F = 500 pounds (2000 N) D = 8 feet (2.5 meters) • W = 2000 N x 2.5 m = 5000 N-m = 5000 J 13 Do you do more work when you finish a job quickly? •NO • Work does NOT involve time, only force and distance. 14 • How quickly work is done. • Amount of work done per unit time. • If two people mow two lawns of equal size and one does the job in half the time, who did more work? • Same work. Different power exerted. • POWER = WORK / TIME 15 What does power measure? • The rate of doing work!!!!!! • How fast the work is done! • Work/time 16 James Watt • A unit named after Scottish inventor James Watt. • Invented the steam engine. • P = Work/time – Joules/second – 1 watt = 1 J/s 17 Calculating Power: Page 415 1.0 m 18 You row a boat across a pond. You do 3600 J of work on the oars in 60 seconds. How much power did you use? • 3600 J /60 sec = 60 J/sec = 60 W 19 watts • Used to measure power of light bulbs and small appliances • An electric bill is measured in kW/hrs. • 1 kilowatt = 1000 W 20 Horsepower (hp) = about 746 watts • Traditionally associated with engines. (car,motorcycle,lawn-mower) • The term horsepower was developed to quantify power. A strong horse could move a 746 N object one meter in one second. 21 What is the SI unit of power? • Watt 22 How much power does a 100 watt light bulb use if it is turned on for 30 seconds? • One more duh! • 100 watts!!!!!!!!!!!!!!!!!!! 23 End of Section 1 24 Machines Do Work • A device that makes work easier. • A machine can change the size, the direction, or the distance over which a force acts. 25 Ramps are useful machines! • It makes it easier to move. Increasing Distance Reduces Force 26 Increasing Force A ramp can reduce the force WORK DONE WORK DONE big force little distance little force big distance 27 Forces involved: • Input Force • Output Force –FI –FO –Force –Force applied to applied by a machine a machine 28 Two forces, thus two types of work • Work Input • Work Output Work done on a Work done by a machine machine =Input force x the =Output force x the distance through distance through which that force acts which the resistance (input distance) moves (output distance) 29 Figure 7 page 419 30 Can you get more work out than you put in? •NO Work output can never be greater than work input. 31 End of Section 2 32 How Does Input Force Location Affect a Machine? A nutcracker is a machine used to make cracking nuts easier. As shown below, use a nutcracker to crack three nuts, each time squeezing the nutcracker’s handles at a different location. 33 Applying force at which handle location resulted in the nutcracker cracking the nuts the most easily? The nutcracker worked best when force was applied at location 1. How does the distance from the nutcracker’s pivot point to the point where the force is applied affect the nutcracker’s ability to crack nuts? The greater the distance between the pivot and the force, the better the nutcracker was at breaking nuts. 34 Mechanical Advantage (MA) • The number of times a machine multiplies the input force. 35 Actual Mechanical Advantage • ACTUAL • Involves friction. • Calculated the same for all machines • Actual Mechanical Advantage = Output force/Input force 36 Ideal Mechanical Advantage • IDEAL • Involves no friction. • Is calculated differently for different machines • Usually input distance/output distance – Actual mechanical advantage is always less than ideal mechanical advantage. 37 Calculating Mechanical Advantages: 38 Calculating Mechanical Advantages: • MA equal to one. (output force = input force) • Change the direction of the applied force only. 39 Calculating Mechanical Advantages: • Mechanical advantage less than one • An increase in the distance an object is moved (do) 40 Efficiency • Efficiency can never be greater than 100 %. Why? • Some work is always needed to overcome friction. • A percentage comparison of work output to work input. – work output (WO) / work input (WI) 41 End of Section 3 Thank you! 42 1. The Lever • A bar that is free to pivot, or move about a fixed point when an input force is applied. • Fulcrum = the pivot point of a lever. • There are three classes of levers based on the positioning of the input force, output force, and fulcrum. 43 First Class Levers • Fulcrum is located between the effort and resistance. • Makes work easier by multiplying the effort force AND changing direction. 44 First Class Levers • Work Out = Work In • Small force applied over large distance is the same as large force applied over a small distance. F d=F d 45 Second Class Levers • Resistance is found between the fulcrum and input force. • Makes work easier by multiplying the input force, but NOT changing direction. 46 Third Class Levers • Input force is located between the output force and the fulcrum. • Does NOT multiply the input force, only multiplies the distance. • Examples: 47 Mechanical advantage of levers. • Ideal = input arm length/output arm length • input arm = distance from input force to the fulcrum • output arm = distance from output force to the fulcrum 48 Mechanical advantage of levers. 49 2. The Wheel and Axle • A lever that rotates in a circle. • A combination of two wheels of different sizes. • Smaller wheel is termed the axle. • IMA = radius of wheel/radius of axle. 50 3. The Inclined Plane • A slanted surface used to raise an object. • Examples: ramps, stairs, ladders • IMA = length of ramp/height of ramp Can never be less than one. 51 4. The Wedge • An inclined plane that moves. • Examples: knife, axe, razor blade • Mechanical advantage is increased by sharpening it. 52 5. The Screw • An inclined plane wrapped around a cylinder. • The closer the threads, the greater the mechanical advantage • Examples: bolts, augers, drill bits 53 6. The Pulley • A chain, belt , or rope wrapped around a wheel. • Can either change the direction or the amount of effort force • Ex. Flag pole, blinds, stage curtain 54 The Pulley 55 Pulley types • FIXED • MOVABLE • Can only change • Can multiply an effort the direction of a force, but cannot force. change direction. • MA = 1 • MA > 1 56 Page 432 Figure 19 57 • A combination of two or more simple machines. • Cannot get more work out of a compound machine than is put in. 58 Assignment: • Pages 441-442 • 1-11, 13, 14, 15, 17, 19, 22, 26, 27, 28, 29 • WB Section 4 59 14.2 Work • 5. A woman lifts her 100-newton child up one meter and carries her for a distance of 50 meters to the child’s bedroom. How much work does the woman do? 100 N X 1 m = 100 N·m or 100 joules Note: No work is done on the child when she carries it. 60 14.2 Power 5. A horse moves a sleigh 1.00 kilometer by applying a horizontal 2,000-newton force on its harness for 45 minutes. What is the power of the horse? (Hint: Change Km’s to m’s and convert time to seconds.) 45 min = 2700 s 2000 n X 1000 m / 2700 s 740.74 watts 61 14.3 Mechanical Advantage 5. A machine with a mechanical advantage of 2.5 requires an input force of 120 newtons. What output force is produced by this machine? 2.5 = x / 120 n X = 2.5 x 120 n X = 300 newtons 62 • For work to be done on an object, the object has to ____________________. 63 • Any part of a force that does not act in the direction of an object’s motion does no ____________________ on an object. 64 • The SI unit of work is the __________. 65 • The rate at which work is done is called ____________________. 66 • The SI unit of power is the _________. 67 • A machine is a device that can multiply _____________. 68