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Work and Simple Machines Work and Simple Machines 1 Work: using a force Work to move an object a distance The force and the motion of the object have to be in the same direction. If the object does not move then no work is done. Work also depends on direction. The force has to be in the same direction as the motion or no work is done on the object. Lifting the Books Carrying the Books Force Force & Motion The same & Motion perpendicular Work is Not Done Work is done Examples: 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 kg grocery bag to her car? No 4 Work Equation Work = force x distance W = F x d W F d Unit: Joule (J) Or newton-meter (N·m) Work Practice 1. A crane uses an average force of 5200 N to lift a girder 25 m (W=Fd) 5200N x 25m= 130,000 N x m 130,000 J 2. The brakes on a bicycle apply 125 N of frictional force to the wheels as it travels 14.0 m No work because friction acts in opposite direction of motion Individual Work Practice (not in notes) 3. While rowing, John exerts a force of 165 N per stroke while pulling the oar 0.8 m. How much work is done in 30 strokes? 4. A 900-N mountain climber scales a 100-m cliff Power The rate at which work is done Remember that a rate = something that occurs over time Power Power = Work Time W F x d P t P t Unit: Watt (W) 1 W = J/s Large quantities measured in kilowatts Power Example It takes 100 J of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator? W=100 J t= 20s P=W/T P=100 J/ 20s P=5 J/s or 5 Watts Practice cont.. Anna weighs 565 N and goes up 3.25 m vertically by stairs. What is power if her time is 12.6 s? P=w/t W=Fd P=Fd/t 565N x 3.25m /12.6s 145.73 Watts Individual Power Practice (not in notes) 1. John does 3960 J of work on the oars in 60 s 2. A mechanic does 5350 J of work to lift a car 0.5 m in 50 s 3. Suppose you ride in a sleigh being pulled by horses at 16 km/h. Another sleigh being pulled at 10 km/h travels the same distance you do. Which horses are more powerful? How is speed related to power? Check for Understanding 1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power? Explain your answers.13 Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. Yet, Ben is the most powerful since he does the same work in less time. Power and time are inversely proportional. 14 How are the formulas for Force, Work, & Power related? 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force = Mass x Acceleration Work = Force x Distance Power = Work/Time 15 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Force=10kg x 2m/s/s Force=20 N Work=Force x Distance Work = 20N x 10m Work = 200 Joules Power = Work/Time Power = 200J/5s Power = 40 watts 16 Machines Machine – a device that makes doing work easier by… transferring a force from one place to another, changing the direction of a force,(pulley) increasing the magnitude of a force,(Car Jack) increasing the distance of which the force is applied ( ramp) Work Input Work input – The work done by you on the machine Win (Force in x distance in) comes from the force applied to the machine (effort force) Effort force – FE (Force in) The force applied to the machine (usually by you) Work output Work output – work done by the machine Wout (Force out x distance out) Resistance force – The force applied by the machine to overcome resistance FR (Force out) Resistance force Multiply forces by changing the direction of the input force multiply distances by changing the distance over which the force is applied Mechanical Advantage Mechanical Advantage (MA) – quantity that measures how much a machine multiplies force or distance output force (F ) input distance R OR MA = input force (FE) output distance Mechanical Advantage MA greater than one multiplies the input force MA less than one increases the distance and speed MA Example Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high MA Practice 1. A ramp is 6.0 m long and 1.5 m high 2. An automobile jack lifts a 9900 N car with an input force of 150 N 3. A sailor pulls down on a sail weighing 140 N with a force of 140 N Ideal machine Win = Wout 100% energy transfer. There is no such thing as an ideal machine – you always lose some energy (through friction, air resistance, etc.) How do MA & IMA differ? MA- Mechanical Advantage of a machine that has friction. IMA- Mechanical Advantage of an Ideal machine (no friction). Efficiency – a measure of how much of the work put into a machine is changed into useful output work by the machine. (less heat from friction) Efficiency = (Wout / Win ) x 100% Win is always greater than Wout Practice Kelsey applies 50J of work to a machine. The machine puts out 40J of work. What is the efficiency of a machine? GIVEN: WORK: Wout = 40J e= Wout / Win x 100 Win = 50J 40J / 50J= 0.80 0.80 x 100 = 80% Simple Machines Simple Machines Simple machine – one of the six basic types of machines Lever family: lever, pulley, wheel and axle Inclined plane family: inclined plane, wedge, and screw Levers Levers – rigid arm that turns around a fixed point (fulcrum) 1st class – fulcrum in middle crowbars, scissors, pliers, tin snips and seesaws. 2nd class – output force in middle nut crackers, wheel barrows, doors, and bottle openers. 3rd class – input force in middle tweezers, hammers, and shovels First Class Lever can increase force, distance, or neither changes direction of force Second Class Lever always increases force Third Class Lever always increases distance Lever – Ideal Mechanical Advantage (IMA) frictionless machine D Effort Distance IMA e D Resistance r Distance De must be greater than Dr in order to multiply the force. Pulley Pulley – rope, belt, or chain wrapped around a wheel Used to lift things (flag on flagpole) Block and tackle – many pulleys put together Single fixed has MA of 1, movable has MA of 2, multiple increases MA What is a pulley? 1. Consists of a wheel and a rope (chain) 2. A fixed pulley is actually a modified first class lever. It is fixed because part of it is attached and doesn't move. Ex. Elevators, dumb waiters, flagpoles 3. Moveable pulleys multiply force The Block and Tackle System of pulleys consisting of fixed and movable pulleys. Basically the weight of the object is spread over multiple ropes, meaning less force needs to be applied to raise it. Essentially the rope is longer, and you must pull it a longer distance to raise the object. Work = force x distance Multiple Pulleys Increase Mechanical Advantage By spreading the 4 Pulleys force over a greater distance (using more than one pulley) you will reduce the effort required to raise an object. In each case, the length of the rope gets longer, and the effort required to lift the block gets smaller 1 pulley = 100 N 2 pulleys = 50 N 4 pulleys = 25 N Check for Understanding 1. Which design will give you the greatest mechanical advantage? 2. If F1 = 100 N, what is F2? 3. Identify the movable and fixed pulleys in each design. Pulley Ideal Mechanical Advantage (IMA) equal to the number of ropes supporting a load IMA = 0 IMA = 1 IMA = 2 Fixed Pulley IMA = 1 does not increase force changes direction of force Movable Pulley IMA = 2 increases force doesn’t change direction Block and Tackle combination of fixed & movable pulleys increases force (IMA = 4) may or may not change direction Wheel and Axle Made up of 2 circular objects of different sizes – the wheel (larger object) turns about the axle (smaller object) Lever or pulley connected to a shaft (axle) Steering wheel, screwdriver Wheel and Axle Wheel Axle Wheel and Axle - IMA effort force is usually applied to wheel axle moves less distance but with greater force r effort radius IMA e rr resistance radius Inclined Plane Flat, slanted surface Ramp Spreads work out over a larger distance Inclined Plane DE IMA h l DR Although it takes less force for car A to get to the top of the ramp, all the cars do the same amount of work.
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