Work and Machines What Is Work? How Machines Do Work Simple Machines - REVIEW Table of Contents

Work and Machines - What Is Work? Book M – Ch 4 Sec 1 pgs ( ) The Meaning of Work Work is done on an object when the object moves in the same direction in which the force is exerted.

Work and Machines Book M – Ch 4 Sec 1 pgs ( ) Calculating Work The amount of work done on an object can be determined by multiplying force times distance. Work = Force x Distance The SI unit for work is the newton x meter (N·m). This unit is also called a joule (J). To lift a box off of the floor onto a table you must exert a force of 50 newtons(N). The distance the box needs to be lifted is 0.5 meters(m). Calculate how much work it takes to lift the box. Show Work: Work = Force x Distance Work = 50 N x 0.5 m Work = 25 J - What Is Work?

Work and Machines Book M – Ch 4 Sec 1 pgs ( ) Power - What Is Work? Power is the rate at which work is done. Since work is equal to force times distance, the power equation can be rewritten as follows: The SI unit for power is Joule per second (J/s), also know as watt (W).

Work and Machines Calculating Power A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? Read and Understand What information have you been given? Force of the tow truck (F) = 11,000 N Distance (d) = 5.0 m Time (t) = 25 s - What Is Work?

Work and Machines Calculating Power A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? Plan and Solve What quantity are you trying to calculate? The Power (P) the tow truck uses to pull the car What formula contains the given quantities and the unknown quantity? Power = (Force X Distance)/Time Perform the calculation. Power = (11,000 N X 5.0 m)/25 s Power = (55,000 Nm)/25 s or 55,000 J/25 s Power = 2,200 J/s = 2,200 W - What Is Work?

Work and Machines Calculating Power A tow truck exerts a force of 11,000 N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? Look Back and Check Does your answer make sense? The answer tells you that the tow truck used 2,200 W to pull the car. This value is about the same power that three horses would exert, so the answer is reasonable. - What Is Work?

Work and Machines Calculating Power Practice Problem A motor exerts a force of 12,000 N to lift an elevator 8.0 m in 6.0 seconds. What is the power produced by the motor? 16,000 W or 16 kW - What Is Work?

Work and Machines Calculating Power Practice Problem A crane lifts an 8,000-N beam 75 m to the top of a building in 30 seconds. What is the crane’s power? 20,000 W or 20 kW - What Is Work?

Work and Machines Links on Work Click the SciLinks button for links on work. - What Is Work?

Work and Machines Book M – Ch 4 Sec 2 pgs What is a Machine? M achine - a device that allows you to do work in a way that is easier or more effective. Makes work easier by changing at least one of three factors 1. amount of force you exert 2. distance over which you exert your force 3. direction in which you exert your force

Work and Machines Book M – Ch 4 Sec 2 (pgs ) Input vs. Output Input force – the force exerted on the machine Moves the machine a certain distance Output force - the force the machine exerts on an object Does work by moving the object a certain distance Input Work = input force x input distance Output Work = output force x output distance When using a machine the input work will always equal the output work. In order to get a greater output force using a smaller input force you have to increase the input distance.

Work and Machines - How Machines Do Work Book M – Ch 4 Sec 2 (pgs ) Ex. The amount of input work done by the gardener equals the amount of output work done by the shovel.

Work and Machines Book M – Ch 4 Sec 2 (pgs ) A machine’s mechanical advantage is the number of times a machine increases a force exerted on it. If output force is greater than input force then the mechanical advantage is greater than 1 Ex. Mechanical can opener: Human input force = 10 Newtons Can opener output force = 30 Newtons MA = 3 The can opener applies 3 times more force on the can than the human does. Let’s Compare!

Work and Machines Mechanical Advantage The input force and output force for three different ramps are shown in the graph. - How Machines Do Work

Work and Machines Mechanical Advantage Input force Reading Graphs: What variable is plotted on the horizontal axis? - How Machines Do Work

Work and Machines Mechanical Advantage 400 N Interpreting Data: If an 80-N input force is exerted on Ramp 2, what is the output force? - How Machines Do Work

Work and Machines Mechanical Advantage Ramp 1: 10; Ramp 2: 5; Ramp 3: 2 Interpreting Data: Find the slope of the line for each ramp. - How Machines Do Work

Work and Machines Mechanical Advantage The slope of each ramp’s graph equals the change in output force divided by the change in input force. This is the formula for mechanical advantage. Ramp 1 has the greatest mechanical advantage. Drawing Conclusions: Why does the slope represent each ramp’s mechanical advantage? Which ramp has the greatest mechanical advantage? - How Machines Do Work

Work and Machines Book M – Ch 4 Sec 2 (pgs ) Efficiency of Machines Remember….Work = Force x Distance Ideally work input will be equal to work output. However, that is not usually the case. In reality, every machine faces the challenge of friction (opposing force) Efficiency compares the output work to the input work and is expressed in a percentage (%). The higher the percent the higher the efficiency. Ex. Rusty scissors would be less efficient than non-rusted scissors. Let’s Practice!

Work and Machines Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Read and Understand What information have you been given? Input Work (W input ) = 250,000 J Output Work (W output ) = 200,000 J - How Machines Do Work

Work and Machines Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Plan and Solve What quantity are you trying to calculate? The efficiency of the lawn mower = __ What formula contains the given quantities and the unknown quantity? Efficiency = Output work/Input work X 100% Perform the calculation. Efficiency = 200,000 J/250,000 J X 100% Efficiency = 0.8 X 100% = 80% The efficiency of the lawn mower is 80 percent. - How Machines Do Work

Work and Machines Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Look Back and Check Does your answer make sense? An efficiency of 80 percent means that 80 out of every 100 J of work went into cutting the lawn. This answer makes sense because most of the input work is converted to output work. - How Machines Do Work

Work and Machines Calculating Efficiency Practice Problem You do 20 J of work while using a hammer. The hammer does 18 J of work on a nail. What is the efficiency of the hammer? 90% - How Machines Do Work

Work and Machines Calculating Efficiency Practice Problem Suppose you left your lawn mower outdoors all winter. Now it’s rusty. Of your 250,000 J of work, only 100,000 J go to cutting the lawn. What is the efficiency of the lawn mower now? 40% - How Machines Do Work

Work and Machines Links on Mechanical Efficiency Click the SciLinks button for links on mechanical efficiency. - How Machines Do Work

Work and Machines Book M – Ch 4 Sec 3. (pgs ) There are six basic kinds of simple machines: the inclined plane, the wedge, the screw, the lever, the wheel and axle, and the pulley. Inclined Plane – flat sloped surface Allows you to exert a force over a longer distance

Work and Machines Book M – Ch 4 Sec 3. (pgs )

Work and Machines Book M – Ch 4 Sec 3. (pgs )

Work and Machines Book M – Ch 4 Sec 3. (pgs ) Lever - is a rigid bar that is free to pivot, or rotate, on a fixed point. Fulcrum - the fixed point that a lever pivots around It increases your input force and it changes the direction of your input force Levers are classified according to the location of the fulcrum relative to the input and output forces First, Second and Third Class

Work and Machines Book M – Ch 4 Sec 3. (pgs ) Wheel and axle - is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis. The object with the larger radius is called the wheel and the object with the smaller radius is called the axle. Ex: Door knob, Steering wheel, screw driver You apply an input force to turn the handle, or wheel. Because the wheel is larger than the shaft, or axle, the axle rotates and exerts a large output force

Work and Machines Book M – Ch 4 Sec 3. (pgs ) Pulley - is a simple machine made of a grooved wheel with a rope or cable wrapped around it. Pulling on one end of rope – input force Object moving – output force Can decrease the amount of input force needed to lift the object, and it can change the direction of your input force. Two types: Fixed Pulleys and Movable Pulleys By combining fixed and movable pulleys, you can make a block and tackle pulley system. The ideal mechanical advantage of a pulley is equal to the number of sections of rope that support the object

Work and Machines Book M – Ch 4 Sec 3. (pgs ) Simple machines at work in you: Levers - Most of the machines in your body are levers that consist of bones and muscles. Input – muscle; Fulcrum – joint; Output – work done (lifting hand) Wedges - Your incisors (front teeth) are shaped like wedges to enable you to bite off pieces of food. Compound machine – is a machine that utilizes two or more simple machines. Ideal Mechanical Advantage = IMA of simple machine x IMA of simple machine