Energy, Work & Power Sec. 3.5, 3.7, 3.9

Is he doing any WORK? He’s applying a FORCE, but is anything moving as a result of his applied force? FN FA Balance forces, no change in motion

Energy Doing work require ENERGY! Energy is the ability to do work Work is a transfer of energy

Work Work = force x displacement W = f x d In every case in which work is done, work requires 2 things: A force is applied to an object The object moves a distance in the direction of the force W f d

Example When a weightlifter lifts a barbell, he does work on the barbell When he is holding the barbell over his head, he does no work (on the barbell)

Work or NO work A lifeguard sitting on her stand _____ Mowing the lawn. _____ Pushing on a wall _____ Carrying a box 10 meters. _____ Writing out your homework _____ Watching television _____ Eating a burger. _____ Running in place. _____

Units for Work The unit of work combines the unit of FORCE Newtons With the unit of DISPLACEMENT / DISTANCE Meters It’s called a Newton-meter But…Since work requires ENERGY, then use the units for energy: Joule (J) 1 N of force to lift an 1 m = ____ Joule of work

Work = F x d How much WORK is done by applying 15 N of force to lift a chair 2 m above your head?

Example of Work How much work is done to a 10 kg mass that is lifted 2 m? Force weight = m x g Work w = f x d

Who’s doing more work? They are both doing the SAME work: Applying a FORCE on an object (dirt) to move it a distance.

They burn the same fuel… Which has more POWER? Lawnmower Engine They burn the same fuel… Jet Engine 30 minutes of work …but at different rates 0.5 s of work

Power Power is the RATE at which WORK is done Power = work done time or P = force x distance w p t

Hair dryer vs. Air drying your hair What is the WORK that needs to be done?

Units for Power The unit for work is the Joule of energy The unit for time is seconds So...The unit for power is Joules/second (J/s) Also called the Watt. After James Watt

1 J/s = 1 Watt 1000 W = kilowatt 1 million Watts = 1 mega- watt

practice A parent is pushing a child on a bicycle down the street. If 125 J of work is done on a child’s bicycle for 120 seconds, what was the power output of the parent? G U E S   Power

practice Anna walks upstairs on her way to class. She weighs 500 N and the stairs go up 3.25 m. What is her power output if she climbs the stairs in 12.6 seconds? G U E S   Work Power

Hair dryer 1875 what?

more powerful dryer hair

Power Which are you more tired… Walking a box up a flight of stairs or running a box up a flight of stairs? You are doing the same amount of work, so why are you more tired? POWER Power is the measure of how fast work is done

Work and Power Lab Prediction: Purpose: How much work is done, how much power is used to climb a flight of stairs. Prediction:

Work = f x d Power = work time Weight = m x g (N) Distance = 1st floor to 2nd floor (meters)

Work = f x d Power = work time Weight = m x g (N) Distance = 1st floor to 2nd floor (meters)

Conclusion Questions Who did more work: walker or runner? Explain what they did: Who used more power? Why?

Graph #1 Work done

Power graph Joules

Things to think about…. If you push an object twice as far while applying the same force, you do Four times as much work The same amount of work Twice as much work ½ as much work A job is done slowly, while an identical job is done quickly. Both jobs require the same amount of work, but different amounts of Energy Power Work None of these

Things to think about…. Which requires more work: Lifting a 10 kg sack a height of 2 m, or lifting a 5 kg sack a height of 4 m? The 10 kg sack The 5 kg sack They require the same amount of work Whereas impulse is the time that a force acts, work involves the Time and distance that a force acts Distance that a force acts Acceleration that a force produces

When James Watt invented his steam engine, he was asked how many horses the engine could replace. To find out, Watt built a rope and pulley device which he could measure the time it took a horse to lift various weights. Watt defined horsepower as the power a horse can produce steadily for a period of time. One horsepower is equal to 746 Watts.

Horsepower A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute.

Just for the fun of it, figure out how many horsepower the runner in your group has…to do this is simple. Divide the power in watts that you figured in your data chart by 746. That’s how much horsepower you have! HP = power in watts 746

Most hp cars? #1. 2009 Bugatti Veyron (1001 hp) #2. 2010 Lamborghini Murcielago (632 hp) #3. 2009 Mercedes-Benz SLR McLaren (617 hp) #4. 2009 Ferrari 599 GTB Fiorano (612 hp) 2009 Ford Escort (110 hp

Now you play! Ramps Game! http://www.gamegecko.com/ramps.php Parachute Game! www.bassfiles.net/parachute.swf

Poster Activity Choose: Poster, Comic strip, short story… Must include all of the following: Momentum Impulse Force, time Energy (KE and PE) Work Power