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## Unit 7 – Work & Energy

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**Unit 7 – Work & Energy**Lesson 1: Work & Kinetic Energy & Power Lesson 2: Gravitational Potential Energy & Conservation of Mechanical Energy Lesson 3: Elastic Potential Energy (optional) Lesson 4: Mechanical Energy NOT Conserved**Lesson 1: Work & Kinetic Energy**Demos: skidding**Work**What is the variable for Work? What is the equation for Work when the Force is constant. (The equation that includes θ, but not ) What is the unit for Work, and based on the equation for Work, what units make this up?**Work**When is Work done (according to physics)? When is Work NOT done (W=0) even when a Force acts on an object?**Work**What would cause more Work to be done in a certain situation compared to another situation? (address all 3 variables) In other words, according to the equation, what would cause there to be an increase in the Work?**Work**When is Work positive? When is Work negative? Is Work a vector or a scalar?**Work**How do you find the total or net Work (Wnet) done by all the forces on an object?**Energy**What is the physics definition of energy? What is the unit for any kind of energy? How does this compare to the unit of Work?**Kinetic Energy**What is Kinetic Energy? What is the equation for Kinetic Energy? What would cause an object to have more KE than another object? What is the unit for Kinetic Energy? Is KE a vector or a scalar? (Does it have direction or not?)**Work – Energy Theorem**What is the Work-Energy Theorem? What would cause the net Work on an object to increase based upon the Work-Energy Theorem?**Work – Energy Theorem**What would cause the net Work to be positive based upon the Work-Energy Theorem? What would cause the net Work to be negative based upon the Work-Energy Theorem?**Power**What is the physics definition of Power? What is the variable for Power? What is the equation for Power?**Power**What is the unit for Power? What units make up the unit for Power? (look at the equation) How many Watts make 1 Horsepower?**Work**You are now walking down the hall at a constant velocity while holding the book in the elevated position. What can be said about the Work that you are doing on the book? • Positive • Negative • Work is done, but whether it is positive or negative can’t be determined • No Work is done • None of these can be determined**Work**When you are moving your huge entertainment center starts sliding down the ramp! You jump in and try to stop it by pushing on it before it hits the ground. While it is still sliding (there is friction), how many forces are doing work on the box? • No forces • 1 force • 2 forces • 3 forces • 4 forces • Can’t be determined**Work**When you are moving your huge entertainment center starts sliding down the ramp! You jump in and try to stop it by pushing on it before it hits the ground. While it is still sliding (there is friction), how many forces are doing negative work on the box? • no forces • 1 force • 2 forces • 3 forces • Can’t be determined**Work**When you are moving your huge entertainment center starts sliding down the ramp! You jump in and try to stop it by pushing on it before it hits the ground. While it is still sliding (there is friction), how many forces are doing positive work on the box? • no forces • 1 force • 2 forces • 3 forces • Can’t be determined**Work**Does Work have direction? • Yes • No • Sometimes**You are pulling a 50kg box by a rope at 40° with 600N**across a floor that has 200N of friction for 20m. How much Work is the Force of Tension doing on the box? • 12000 J • 9192 J • 5192 J • 0 J • None of these**You are pulling a 50kg box by a rope at 40° with 600N**across a floor that has 200N of friction for 20m. How much Work is friction doing on the box? • 4000 J • – 4000 J • 3064 J • – 3064 J • None of these**You are pulling a 50kg box by a rope at 40° with 600N**across a floor that has 200N of friction for 20m. How much Work does Fg do on the box? • 9800 J • 7507 J • 1000 J • 0 J • None of these**You are pulling a 50kg box by a rope at 40° with 600N**across a floor that has 200N of friction for 20m. What is the net (total) Work done on the box? • 5193 J • 8000 J • 13193 J • 16000 J • None of these**You are pulling a 50kg box by a rope at 40° with 600N**across a floor that has 200N of friction for 20m. How fast will the box be going after 20m if it started from rest? • 14.4 m/s • 19.2 m/s • 21.9 m/s • None of these • Can’t be determined**You are pulling 100kg crate by a rope at 40° with for 15m.**What is the net Work done on the box if it was initially sliding at 1m/s and ended up with a speed of 3m/s? • 200 J • 150 J • 10 J • None of these • Can’t be determined**You are pulling 100kg crate by a rope at 40° with for 15m.**What is the Work done by Tension on the box if it was initially sliding at 1m/s and ended up with a speed of 3m/s? • 153 J • 150 J • 115 J • None of these • Can’t be determined**You are pulling 100kg crate by a rope at 40° with for 15m.**What would cause the net Work done on the box to be negative if it was initially sliding at 1m/s? • If friction was greater than Tension • If the crate slowed down • Both of these • Neither of these • Net Work can’t be negative b/c it doesn’t have direction**Work**A truck is initially driving North at 60pmh. After some time the truck is now driving South at 60mph. During this process: 1) has unbalanced Forces (ΣF≠0) acted on the truck; 2) has there been net Work (Wnet ≠0) done on the truck? • Yes; Yes • Yes; No • No; Yes • No; No • Can’t be determined**Work**A ball is swung in a vertical circle such that the string always remains taut. The only forces acting on the ball are gravity and Tension. Which of the following is true about the given quantities as the ball goes from the top of the circle to the bottom? ΔKE WFg WFTWnet A. + + + + B. + + 0 + C. + – 0 + D. – – 0 – E. – + – –**You riding your bike at a certain speed and skid on your**drive-way to a stop. If you were going twice as fast, how much more distance will you skid until you come to a stop? • The same • 1.5x as far • 2x as far • 3x as far • 4x as far**Power**Arnold pushes a box at a constant speed of 4m/s for 40m. Palmer pushes an identical box at the same speed for 80m. Which demonstrated more Power? • Arnold • Palmer • Both the same • Can’t be determined**Power**Bubba could lift a 50kg crate of shrimp 2m in 0.25s. How much Power did he demonstrate? • 6.25 Watts • 25 Watts • 100 Watts • 400 Watts • None of these**Momentum vs Work**Momentum-Impulse Work-Energy Theorem Theorem**Lesson 2 – Gravitational Potential Energy & Conservation**of Mechanical Energy Demos: 4 colored tracks, pendulum**Gravitational Potential Energy (PEg)**What is the equation for gravitational potential energy (PEg)? What is the unit for PEg ?**Gravitational Potential Energy (PEg)**When does an object have PEg? When does an object NOT have PEg? What would cause an object to have more PEgthan another object?**Mechanical Energy (ME)**What is the physics definition of Mechanical Energy (ME)? What is the equation for ME?**Conservation of Mechanical Energy**When is Mechanical Energy conserved? What does it mean to say that Mechanical Energy is conserved?**Conservation of Mechanical Energy**What is a conservative force?**Conservation of Energy**What does the Law of Conservation of Energy state? (same one from elementary school) Is the Energy in this law the same as Mechanical Energy, or is Mechanical Energy just a type of energy that is only a part of the Energy that is refer to in this law?**Gravitational Potential Energy (PEg)**A 2kg book rests flat in the middle of a 1.5m tall table on the 2nd story of a house (5m above the ground). How much Gravitational Potential Energy (PEg) does it have at this point? • 0 J • 24.4 J • 98 J • Any of these • None of these**What did you learn?**Where did mgh come from?**Conservation of Mechanical Energy**Mechanical Energy is conserved when only conservative forces act on an object, or nonconservative forces at perpendicular to the object’s motion. Conservative ForcesNonconservative Forces**Conservation of Mech Energy?**A sled slides up an iced embankment until it momentary stops at the highest point (ignore friction). Is mechanical energy conserved? • Yes • No • Can’t be determined**Conservation of Mech Energy?**Your car begins to roll down a hill, because you accidently left your parking brake OFF! You push on it trying to stop it, but it keeps moving at a constant velocity down the hill. (ignore friction acting on the car). Is mechanical energy conserved? • Yes • No • Can’t be determined**Conservation of Mech Energy?**Starting from rest, Tarzan down on swings on a vine. (ignore air resistance) Is mechanical energy conserved? • Yes • No • Can’t be determined**Conservation of Mech Energy?**Two cars are rolling in neutral (ignore friction) and crash into each other. Is mechanical energy conserved of the two cars? • Yes • No • Can’t be determined**Conservation of Mech Energy?**Two carts are rolling towards each other (ignore friction). One has a spring bumper, causing them to bounce off each other. Is mechanical energy conserved for the system of the two carts? • Yes • No • Can’t be determined**Energy Analogies**Bar charts: Money:**Mech Energy Bar charts**A sled slides up an iced embankment until it momentary stops at the highest point (ignore friction). Sketch the energy bar charts starting at the bottom and ending at the top. Initial Final KE + PEg = ME KE + PEg = ME**Changes in Energies**A sled slides up an iced embankment until it momentary stops at the highest point (ignore friction). How did the change in KE compare to the change in PEg ? • KE lost more than PEg gained • KE lost less than PEggained • KE lost the same as PEggained • KE gained the same as PEglost • None of these