Download Presentation
## Work, Power, and Machines

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -

**Work, Power, and Machines**Physical Science – Unit 7 Chapter 9**Work**• 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**Work in simple terms: • Transfer of energy that occurs when a force makes an object move • The object must move for work to be done • The motion of the object must be in the same direction as the applied force**Work**• The formula for work: • Work = force x distance • W = F x d • 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**• 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**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? W = F x d**Practice problem**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? W = F x d W = 190 N x 2.0 m = 380 N●m = 380 J**Practice problems**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**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. W = F x d = 250 N x 10 m = 2,500 N●m = 2,500 J**Practice problems**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? What do you need to calculate first?????**Practice problems**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**Practice problems**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 = F x d = 10.2 N x 0.667 m = 6.80 N●m = 6.80 J**Power**• 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**• Formula for power: Power = work time P = W/t • SI units for power – watts (W) • 1 kW – Kilowatt = 1000 watts • 1 MW – Megawatt= 1 million watts**Power**• 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**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? P = W/t**Practice problems**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? P = W = 686 J t 3.1 s 221 J/s = 221 W**Practice problems**• How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night? P = W t**Practice problems**• 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**Practice problems**• 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 = P x t = 60 W x 28800 sec = 1.7 x 107 J**Machines and Mechanical Advantage**• 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?**• A device that makes doing work easier… is a machine • Machines increase the applied force and/or change the direction of the applied force to make the work easier • They can only use what you provide!**Why use machines?**• 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**• 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)**Mechanical Advantage**• 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**• Formula: • Mechanical advantage = output force input force Or mechanical advantage= input distance output distance**Practice problems**• 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**• 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 = 1549 N = 3.47 446 N**Machines**Simple Machines • Lever • Pulley • Wheel & Axle • Inclined Plane • Screw • Wedge**The Lever family**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. Resistance arm Effort arm Fulcrum Engraving from Mechanics Magazine, London, 1824 “Give me a place to stand and I will move the Earth.” – Archimedes**Lever**First Class Lever Most common type can increase force, distance, or neither changes direction of force**Lever**Second Class Lever always increases force**Lever**Third Class Levers always increases distance**Pulley**Pulley grooved wheel with a rope or chain running along the groove a “flexible first-class lever” or modified lever F Le Lr**Pulley**Ideal Mechanical Advantage (IMA) equal to the number of supporting ropes IMA = 2 IMA = 0 IMA = 1**Pulley**Fixed Pulley • IMA = 1 • does not increase force • changes direction of force**Pulley**Movable Pulley • IMA = 2 • increases force • doesn’t change direction**Pulley**Block & Tackle • combination of fixed & movable pulleys • increases force (IMA = 4) • may or may not change direction**Wheel and Axle**Wheel and Axle two wheels of different sizes that rotate together a pair of “rotating levers” When the wheel is turned so so is the axle Wheel Axle**Wheel and Axle**Wheel and Axle Bigger the difference in size between the two wheels= greater MA Wheel Axle**What is an inclined plane?**• 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?**• MA=Length/Height**Incline Plane Family**• 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**• 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**Compound Machines**• 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