Work, Power, and Machines
Work, Power, and Machines Physical Science – Unit 7 Chapter 9 Work Home Previous Next Help • What is work? – Work is the quantity of energy transferred by a force when it is applied to a body and causes that body to move in the direction of the force. • Examples: – Weightlifter raises a barbell over his/her head – Using a hammer – Running up a ramp Work Home Work in simple terms: Next • Transfer of energy that occurs when a force makes an object move Help • The object must move for work to be done Previous • The motion of the object must be in the same direction as the applied force Work Home Previous Next Help • The formula for work: – Work = force x distance –W=Fxd • Measured in Joules (J) – Because work is calculated as force times distance, it is measured in units of newtons times meters (N●m) – 1 N●m = 1 J = 1 kg●m2/s2 • They are all equal and interchangeable! James Joule - English scientist and inventor 1818-1889 Work Home Previous Next Help • 1 J of work is done when 1N of force is applied over a distance of 1 m. • kJ = kilojoules = thousands of joules • MJ = Megajoules = millions of joules Practice problem Home Previous Next A father lifts his daughter repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? Help W=Fxd Practice problem Home Previous Next A father lifts his daughter repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? Help W=Fxd W = 190 N x 2.0 m = 380 N●m = 380 J Practice problems Home Previous Next Help A mover is moving about 200 boxes a day. How much work is he doing with each box, assuming he lifts each 10 m with a force of 250 N. Practice problems Home Previous Next A mover is moves about 200 boxes a day. How much work is he doing with each box, assuming he lifts each 10 m with a force of 250 N. Help W=Fxd = 250 N x 10 m = 2,500 N●m = 2,500 J Practice problems Home Previous A box with a mass of 3.2 kg is pushed 0.667 m across a floor with an acceleration of 3.2 m/s2. How much work is done on the box? Next Help What do you need to calculate first????? Practice problems Home Previous A box with a mass of 3.2 kg is pushed 0.667 m across a floor with an acceleration of 3.2 m/s2. How much work is done on the box? Next Help F = ma = 3.2 kg x 3.2 m/s2 = 10.2 kg● m/s2 = 10.2 N Practice problems Home Previous Next Help A box with a mass of 3.2 kg is pushed 0.667 m across a floor with an acceleration of 3.2 m/s2. How much work is done on the box? F = ma = 3.2 kg x 3.2 m/s2 = 10.2 kg● m/s2 = 10.2 N W=Fxd = 10.2 N x 0.667 m = 6.80 N●m = 6.80 J Home Previous Next Help • Practice problems Get a calculator Home • Page 54 asks for distance…… Previous Next Help W= F x D Home • #1 Previous Next Help .6 m Home • #2 Previous Next Help .6m Home • #3 Previous Next Help 2.6m Home • #4 Previous Next Help 2.398m Home • P55 asks for force Previous Next Help W= F x D Home • #5 Previous Next Help 2 800 000N Home • #6 Previous Next Help 27N Home • #7 Previous Next Help 900 000N Home • #8 Previous Next Help 95 454N Home • P56 asks for W………thank goodness Previous Next Help W= F x D Home • #9 Previous Next Help 237 825J Home • #10 Previous Next Help 3.2 x 106 J Home • #11 Previous Next Help 5 625 000 J Home • #12 Previous Next Help 2 127 840J Home How about a harder one…. Previous Next Help #18 Home • #18 Previous Next • Calculate force first, then work Help 276 115J Power Home Previous Next Help • Power is a quantity that measures the rate at which work is done – It is the relationship between work and time – If two objects do the same amount of work, but one does it in less time. The faster one has more power. • Rate at which work is done or how much work is done in a certain amount of time Power Home Previous • Formula for power: Power = work time Next Help P = W/t • SI units for power – watts (W) • 1 kW – Kilowatt = 1000 watts • 1 MW – Megawatt= 1 million watts Power Home Previous Next Help • A watt is the amount of power required to do 1 J of work in 1 s. (Reference – the power you need to lift an apple over your head in 1 s) • Named for James Watt who developed the steam engine in the 18th century. Practice problems Home Previous A weight lifter does 686 J of work on a weight that he lifts in 3.1 seconds. What is the power with which he lifts the weight? Next Help P = W/t Practice problems Home Previous A weight lifter does 686 J of work on a weight that he lifts in 3.1 seconds. What is the power with which he lifts the weight? Next Help P = W = 686 J t 3.1 s 221 J/s = 221 W Practice problems Home Previous • How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night? Next Help P=W t Practice problems Home Previous • How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night? Next Help P=W t 1st convert 8 hr to seconds 8 hr (60 min/1hr)(60 sec/1min) = 28800 sec Practice problems Home Previous Next Help • How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night? P=W t 1st convert 8 hr to seconds 8 hr (60 min/1hr)(60 sec/1min) = 28800 sec 2nd calculate for energy W=Pxt = 60 W x 28800 sec = 1.7 x 107 J Home Previous Next Help • Practice problems Home • P58 asks for work Previous Next Help P= W/t Home • #1 Previous Next Help 412.5J Home • #2 Previous Next Help 1 710 000J Home • #3 Previous Next Help 7 500 000 J Home • #4 Previous Next Help 1.17 x 1010J Home Previous Next Help • P59 asks for time Home • #5 Previous Next Help 955.36 sec Home • #6 Previous Next Help 456.14sec Home • #7 Previous Next Help 1 500sec Home • #8 Previous Next Help 4.5sec Home Previous Next Help • P60 asks for power Home • #9 Previous Next Help 5 x 108 watts Home • #10 Previous Next Help 2.75 x 1010 Home Previous Next Help • Do 11 & 12 Home • #11 Previous Next Help 300sec Home • #12 Previous Next Help 6 162 000J Machines and Mechanical Advantage Home Previous Next Help • Which is easier… lifting a car yourself or using a jack? • Which requires more work? • Using a jack may be easier but does not require less work. – It does allow you to apply less force at any given moment. What is a machine? Home Previous Next Help • A device that makes doing work easier… is a machine • Machines increase the applied force and/or change the distance/direction of the applied force to make the work easier • They can only use what you provide! Why use machines? Home Previous Next Help • If machines cannot make work, why use them? – Same amount of work can be done by applying a small force over a long distance as opposed to a large force over a small distance. Effort and Resistance Home Previous Next Help • Machines help move things that resist being moved • Force applied to the machine is effort force (aka: Input force) • Force applied by the machine is resistance force (aka: Load, output force) Mechanical Advantage Home Previous Next Help • Mechanical advantage is a quantity that measures how much a machine multiplies force or distance • Defined as the ratio between output force and input force Mechanical Advantage Home Previous Next • Mechanical advantage = output force input force Help • Mechanical advantage= input distance output distance Machines Home Previous Simple Machines Next Help Lever Pulley Wheel & Axle Inclined Plane Screw Wedge The Lever family Home Previous Next • Lever – a rigid bar that is free to pivot about a fixed point, or fulcrum – Force is transferred from one part of the arm to another. Help Resistance arm Effort arm Fulcrum Engraving from Mechanics Magazine, London, 1824 “Give me a place to stand and I will move the Earth.” – Archimedes Home Previous Next Help Lever Home Previous Next Help • First Class Lever – – – – Most common type Fulcrum in middle can increase force, distance, or neither changes direction of force Lever Home Previous • Second Class Lever – always increases force – Resistance/load in middle Next Help Lever Home Previous Next Help • Third Class Levers – always increases distance – Effort in middle Home Previous Next Help Pulley Home Previous Next • Pulley – grooved wheel with a rope or chain running along the groove – a “flexible first-class lever” or modified lever Help F Le Lr Pulley Home Previous • Ideal Mechanical Advantage (IMA) – equal to the number of supporting ropes Next Help IMA = 0 IMA = 1 IMA = 2 Pulley Home • Fixed Pulley Previous Next Help IMA = 1 does not increase force changes direction of force Pulley Home Previous Next Help • Movable Pulley IMA = 2 increases force doesn’t change direction Pulley Home Previous • Block & Tackle Next Help combination of fixed & movable pulleys increases force (IMA = 4) may or may not change direction Wheel and Axle Home Previous Next Help • Wheel and Axle – two wheels of different sizes that rotate together – a pair of “rotating levers” – When the wheel is turned so is the axle Wheel so Axle Wheel and Axle Home Previous Next • Wheel and Axle – Bigger the difference in size between the two wheels= greater MA Help Wheel Axle What is an inclined plane? Home Previous Next Help • A sloping surface, such as a ramp. • An inclined plane can be used to alter the effort and distance involved in doing work, such as lifting loads. • The trade-off is that an object must be moved a longer distance than if it was lifted straight up, but less force is needed. What is an inclined plane? Home Previous Next Help • MA=Length/Height Incline Plane Family Home Previous Next Help • A wedge is a modified incline plane – Example ax blade for splitting wood – It turns a downward force into two forces directed out to the sides Incline Plane Family Home Previous Next Help • A screw looks like a spiral incline plane. – It is actually an incline plane wrapped around a cylinder – Examples include a spiral staircase and jar lids Home Previous Next Help Practice problems Home Previous Next Help • A roofer needs to get a stack of shingles onto a roof. Pulling the shingles up manually used 1549 N of force. Using a system of pulleys requires 446 N. What is the mechanical advantage? Mechanical advantage = output force input force Practice problems Home Previous Next • A roofer needs to get a stack of shingles onto a roof. Pulling the shingles up manually used 1549 N of force. Using a system of pulleys requires 446 N. What is the mechanical advantage? Help Mechanical advantage = output force input force = 1549 N = 3.47 446 N Home Previous • Practice problems 1) Asks for output force (N) Next 2) Asks for input distance (cm) Help 3) Asks for output force (N) Home • #1 Previous Next Help 444.4N Home • #2 Previous Next Help 11cm Home • #3 Previous Next Help 3 675N Home Previous Next Help 4) Asks for output distance (cm) 5) Asks for input force (N) 6) Asks for output distance (m) Home • #4 Previous Next Help 3/0.85= 3.52 Home • #5 Previous Next Help 2220/.0893= 24 860N Home • #6 Previous Next Help 1.57/12.5=0.1256m Home Previous Next Help Solve for MA in #7 & #8 Home • #7 Previous Next Help 3.28 Home • #8 Previous Next Help 23.99 Compound Machines Home Previous Next Help • Compound machines are machines made of more than one simple machine – Example include a pair of scissors has 2 first class levers joined with a common fulcrum; each lever arm has a wedge that cuts into the paper Energy Home Previous Next Help • Energy is the ability to cause changes. – It is measured in Joules or kg●m/s2 – When work is done on an object, energy is given off Energy Home Previous Next Help 5 main forms of energy: 1. Mechanical – associated with motion 2. Heat – internal motion of atoms 3. Chemical – the energy required to bond atoms together 4. Electromagnetic – movement of electric charges 5. Nuclear – released when nuclei of atoms fuse or split Mechanical Energy Home Previous Next Help • Mechanical Energy is the sum of the kinetic and potential energy of a large-scale objects in a system – Nonmechanical energy is the energy that lies at the level of atoms and does not affect motion on a large scale Energy Home Previous • Those 5 forms of energy can be classified into one of two states: Next – Potential energy – stored energy Help – Kinetic energy – energy in motion Kinetic Energy Home Previous Next Help • The energy of motion • An object must have mass and be moving to possess kinetic energy – The greater the mass or velocity--- the greater the kinetic energy – Formula: KE = ½ mv2 m = mass v = velocity Kinetic Energy Home Previous Next Help • Atoms and molecules are in constant motion and therefore have kinetic energy – As they collide then the kinetic energy is transferred from one to another Practice problem Home Previous A sprinter runs at a forward velocity of 10.9 m/s. If the sprinter has a mass of 72.5 kg. What is their kinetic energy? Next Help KE = ½ mv2 Practice problem Home Previous A sprinter runs at a forward velocity of 10.9 m/s. If the sprinter has a mass of 72.5 kg. What is their kinetic energy? Next Help KE = ½ mv2 = ½ (72.5 kg) (10.9 m/s)2 = .5 x 72.5 x 118.81 = 4306.86 kg●m/s = 4306.86 J Potential Energy Home Previous Next Help • The stored energy that a body possesses because of its position. – Examples: chemical energy in fuel or food or an elevated book because it has the potential to fall. • Potential energy due to elevated potential is called gravitational potential energy (GPE). Potential energy Home Previous Next • Formula: • PE = mgh m = mass g = gravity (9.8 m/s2) Help h = height Practice problems Home Previous Next A pear is hanging from a pear tree. The pear is 3.5 m above the ground and has a mass of 0.14 kg. What is the pear’s gravitational potential energy? Help PE = mgh Practice problems Home Previous Next A pear is hanging from a pear tree. The pear is 3.5 m above the ground and has a mass of 0.14 kg. What is the pear’s gravitational potential energy? Help PE = mgh = .14 kg x 9.8 m/s2 x 3.5 m = 4.8 kg●m2/s2 = 4.8 J Conservation of Energy Home Previous Next Help • Energy is almost always converted into another form of energy • One most common conversion is changing from potential energy to kinetic energy or the reverse. Conservation of Energy Home Previous • The transfer of energy from one object to the next is a conversion of energy. Next Help The law of conservation of energy states that all energy can neither be created or destroyed; it is just converted into another form. Conservation of Energy Home Previous • Energy conversions occur without a loss or gain in energy Next Help • Therefore…. KE = PE Energy Transformations Home Previous Next Help Conservation of Energy Home Previous Next Help • Amount of energy the machines transfers to the object cannot be greater than energy you put in • Some energy is change to heat by friction • An ideal machine would have no friction so energy in = energy out Efficiency Home Previous Next Help • Efficiency is a measure of how much work put into a machine is changed to useful work output by the machine • Not all work done by a machine is useful therefore we look at the efficiency of the machine Efficiency Home Previous • Formula for Efficiency • (Work output / Work input) X 100 – Efficiency = useful work output x 100 work input Next Help • Efficiency is always less than 100% because no machine has zero friction or 100% efficiency • Lubricants can make a machine more efficient by reducing friction – Oil – Grease Practice Problem Home Previous What is the efficiency of a machine if 55.3 J of work are done on the machine, but only 14.3 J of work are done by the machine? Next Help Efficiency = useful work output work input Practice Problem Home Previous What is the efficiency of a machine if 55.3 J of work are done on the machine, but only 14.3 J of work are done by the machine? Next Help Efficiency = useful work output work input = 14.3 J x 100 = 25.9 % 55.3 J Perpetual Motion Machines Home Previous Next Help • Perpetual motions machines are machines designed to keep going forever without any input of energy • It is not possible because we have not been able to have a machine with a complete absence of friction! Home • Newman’s machine Previous Next Help Joseph Newman, claimed it would produce mechanical power exceeding the electrical power being supplied to it Home Previous Next Help • Take out your homework Power, Work and Force I Home Previous Next Help 1. 6.48W 7. 9.45W 2. 6 692J 8. 1.8kg 3. 5.76S 9. 61.25W 4. 112.93 kg 10. 11 340J 11. 4344.6W 5. 6. 7.11W 16.4S Work and Power I Home Previous Next Help 1. 20J 10. 60W 2. 5 900J 11. 588W 3. 14 000J 12. 5000W 4. 50m 13. 115N in 15m(1725J 14. 20kg lift=1960J 5. 6. 5.10m 50W 15. 80% 7. 13W 16. 500J 8. 18 000J 17. over 490J 9. 100J 18. What do you think? 19. 25%
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