Work, Energy & Momentum Applied Science III Mr. Finau What exactly is a force? Force is a push or a pull; can cause an object to accelerate A construction worker drags a bag of concrete across the road; a mover slides a refrigerator up the ramp of his truck; and a carpenter lifts wooden beams to the top of a house. What are they all doing? Applying a force to an object Over a certain distance Work! If a Penndot employee lifts a bag of concrete onto his shoulder, is he doing work? Why? Yes, the worker applies a force to the bag to lift the bag a certain distance up to his shoulder If a kid pushes on a truck, does he do any work? Why? No, although the kid applies a force, the truck doesnt move. What exactly is Work? Work is the transfer of energy by applying a force over a distance Units Joules (J) What does Work depend on? Work depends on the amount of force applied to an object & total distance that is traveled Which does more worka man lifting an apple above his head or a man lifting a couch above his head? Why? The man lifting a couch does more work He must apply more force in order to lift a couch than to lift an apple Which does more worka man pushing a couch 5 feet or a man pushing a couch 10 ft? Why? How much more work? The man pushing a couch 10 ft does more work It takes more work to push a greater distance He does twice as much work because its twice the distance What is the relationship between Work and Force? What is the relationship between Work and Distance? Work and Force are Directly Proportional as force increases, work increases Work and Distance are Directly Proportional as distance increases, work increases And nowa lil video What would be a good definition and equation for work? What are the units for work? Work is the transfer of energy to an object by a force applied over a distance Work = Force X Distance Bigger Force means bigger Work Bigger Distance means bigger Work Units Joules (J) If I push against my desk, how much work do I do? I do NO work because my desk does not move any distance If I push a book with a constant 5 N of force for 2 meters, how much work do I do? Work = Force X Distance Force = 5 N pushing on the book Distance = 2 m Work = 5 X 2 = 10 J If I push down on a couch as my friend slides it to the left, am I doing work? No, a force perpendicular to the direction of motion does no work because it does NOT transfer energy to the object It does NOT change the objects motion weight moving What do you need in order to do work? A good night sleep? Plenty to eat? Maybe a little coffee in the morning? ENERGY! I throw a baseball at 50 mph. Did I give the ball energy? If Randy Johnson throws a 100 mph fastball, which baseball has more energy? Why? Yes, if it hit you, it would do work in the form of causing you pain The 100 mph baseball because it has more speed The Famous Randy Johnson Bird Pitch If Randy Johnson then threw a brick at 100 mph, which would have more energy? The baseball or the brick? Why? The brick has more energy because the brick has more mass What kind of energy does the baseball have? How could we define this type of energy? Kinetic Energy The energy due to an objects motion What does Kinetic Energy depend on? Depends on the amount of speed of the object Depends on the amount of mass of the object What are the relationships between Kinetic Energy and what it depends on? Kinetic Energy and Speed are Directly Proportional Kinetic Energy and Mass are Directly Proportional A very moving video A boulder sitting on a cliff is not moving. Does it have Kinetic Energy? Does it have energy? Does not have kinetic energy But it does have energy; it could potentially crush something at the bottom What is this kind of energy called? Why do we name it that? Potential Energy An object with Potential Energy has the potential to do work What would have more energy, a 100 kg boulder sitting at the top of a hill or a 5000 kg boulder sitting at the top of the same hill? Why? The 5000kg boulder has more energy It has much more mass What would have more energy, a 100 kg boulder sitting 50 m high or sitting 1000 m high? Why? The boulder at 1000m has more energy It is higher up, thus has more potential to do work Does an object in space have potential energy? Why? No, an object floating in space has no potential energy There is no gravity to pull the object down What does Potential Energy depend on and what is their relationship? Potential energy and Mass are directly proportional Potential energy and Height are directly proportional Potential energy and Gravity are directly proportional What are the equations for energy? Potential Energy = mass X gravity X height Kinetic Energy = mass X velocity 2 A potentially good video Using your equations from Potential Energy and Kinetic Energy, how much PE & KE does a 2 kg book resting.5 m high on a book shelf have? (You can estimate g = 10 m/s 2 ) KE = mv 2 = (2)(0) 2 = 0 J PE = mgh= (2)(10)(.5) = 10 J How much Kinetic Energy does a 4 kg ball traveling at 2 m/s have? KE = mv 2 = (4)(2) 2 = 8 J I carry a stack of books from the bottom of the stairwell to the top in 5 seconds. Dr. Hepinger can do the same job in 3 seconds. Who does more work? Remember what Work depends on? Work = Force X Distance We exerted the same force to carry the same books We traveled the same distance Both do the same amount of work Who exerts more power, Dr. Hepinger or Mr. Finau? Why? Dr. Hepinger exerts more power He completes the same amount of work in a shorter amount of time What does Power depend on? What is the equation for power? Amount of work done Time to complete work Equation: Units for Power are Watts If I push a stack of books with 4 N of force over 3 m for 2 seconds, how much power do I exert? P = W/t How do I find Work again? Work = Force X Distance = (4)(3) = 12 J Power = 12/2 = 6 Watts Quiz on Wed Study Notes on: Work Power Kinetic Energy Potential Energy Review Guide TOMORROW will help, but wont have every example on it; refer to notes to help study