Work, Power, & Machines

Transcription

1 Physics Regular 1213 Williams Rollercoaster Physics: Work, Power, & Machines Packet 2 / Chapter 6 1

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3 Power Up! Name No: AMA = F out /F in, IMA = d in /d out, W = Fd, P = W/t, kilo = 1000, 1 hp = 746 W, PE = mgh, KE = ½ mv 2, wt = mg 3,600,000 J = 1 kw-hr g = 9.8 m/sec 2 1 m/sec = 2.24 mph, 1kg = 2.22 lbs 1 mile = 1610 m = 5280 feet 1. Power is the rate at which is done. 2. What is the advantage of using a power tool like a saw if the same job could be done by hand? 3. Power companies charge by the total amount of energy used, not by the rate energy is used. So why do power companies charge more for electricity used during the day then at night? 4. A student does 100 J of work in 2 seconds, then another 100 J in 4 seconds. How does her power during the last 4 seconds compare to the first 2 seconds? 5. What are the units for power? 6. How many watts is a 400 hp bumper car? 7. What power is exerted by an ice skater who lifts his 60 kg partner a height of 2m in 3 seconds? 8. What power is exerted by a set of brakes to bring a coaster having 35,000 J of KE to a stop? 9. How much work could be done by a 250 Watt engine in 3 minutes? 3

4 10. Construction workers must exert a lot of energy over a work day. Does this mean they must be powerful? 11. Katie, a 30.0 kg child, climbs a tree to rescue her cat who is afraid to jump 8.0 m to the ground. How much work does Katie do in order to reach her cat? (2352 J) 12. Marisa does 3.2 J of work to lower the window shade in her bedroom a distance of 0.8 m. How much force must Marisa exert on the window shade? (4 N) 13. Bud, a very large man of mass 130 kg, stands on a pogo stick. (A) What is Bud s Weight on Earth? (1274 N) (B) How much work is done as Bud compresses the spring of the pogo stick m? (63.7 J) 14. Atlas and Hercules, two carnival sideshow men, each lift 200 kg barbells 2.00 m off the ground. Atlas lifts his barbells 1.00 seconds and Hercules lifts his in 3.00 seconds. (A) How much work does each strong man exert on the barbells? (3920 J) (B) Calculate which man is more powerful. (Atlas = 3920 Watts; Hercules = 1307 Watts). (C) Find each man s power rating in horsepower. (Atlas = 5.25 hp; Hercules = 1.75 hp). 4

5 Name: No: Work and Skidding Cars Helpful equations: Kinetic Energy: KE = ½ mv 2 Work: W = F d 1. If work is defined as a push or pull that makes an object move faster or lifts it up to give it more potential energy, then would you say that brakes do positive or negative work? 2. When you slam on the brakes on your car, where does most of the energy of motion go? 3. If you double your speed, what happens to your kinetic energy? 4. If you double your speed, what happens to the amount of work done when you brake? 5. Assume that stronger brakes deliver larger stopping forces. If you get stronger brakes, what happens to the distance you skid for any particular starting speed? 6. How are kinetic energy and skidding distances related? 7. Suppose you skid four meters when you are traveling at 20mph. How far will you skid in the same car when you are moving at 40 mph? 8. How far would you skid when you are moving 80 mph? 5

6 Power/work review and simple machine basics 1. If you have a mass of 60 kg, how much do you weigh? (the force required to lift you) 2. If you lift up 60 kg 15 cm, how much work have you done? 3. If it takes you 0.6 seconds to do the above work, how powerful are you? 4. Imagine you have a mass of 70 kg and there are 20 steps, each 15 cm tall. How much work do you do to lift yourself up these 20 steps? (hint: find the TOTAL distance!) 5. If it takes you 8.0 seconds to do the above work, how powerful are you? 6. Suppose a wheelchair ramp is 20 m long (hypotenuse) and lifts a person + chair with a total mass of 80 kg up 1.5 m. (kinda hard problem, but it s guided, so follow along..) a. What weight is being lifted? (this is force you get from the machine: F out ) b. How much work is done against gravity? c. How hard must you push? (this is force you put into the machine: F in ) d. How powerful are you if you push the person up the ramp in 15 seconds? 6

7 POWER UP LAB! Name: No: Objective: Can you exert 1 horsepower? Some students can The purpose of this activity is to determine your power when going up stairs and comparing this power to other every day items. Variable list: your weight, your height, time to run up stairs, shoe size, height of each stair, mother's maiden name, number of stairs, length of stairs, acceleration of gravity, weight of the earth, Purpose Questions: (SI units for everything) 1. What variables do you need to know in order to determine your Power for running up a flight of stairs? 2. What variables do you need to consider in order to determine the Work you do to run up a flight of stairs? 3. What is the Force you need to exert to move up a flight of stairs equal to? Materials: stopwatch meter stick your muscles your brain calculator / pencil Method: Pick someone in your group will to run up the stairs. Time them going up the stairs for the following cases: Casual walk. Walking quickly (two stairs at a time), Running up the stairs (quickly and carefully!) Data: (show sample calculations for Trial #1) Trial # Time (s) Force (lbs) Force (N) Casual Distance (m) Work (J) Power (W) Power (hp) Fast walk Running Useful facts: pounds (lbs) = 9.81 Newtons (N) 100 cm = 1 meter (m) 746 Watts = 1 horse power (hp) Sample Calculations (4 Steps!): 7

8 Analysis: (use date from student who ran up the stairs) 1. How fast do you have to go up these stairs to equal the power of a 100 W light bulb? 2. A package of plain chocolate M&M s that has 240 Calories. There are 4,184 J in a Calorie. How many stairs do you need to climb to "burn off" those Calories? 3. According to Washington University website, 1 gallon of gas releases 125 MJ of energy. Assuming that cars are about 25% efficient with this energy, how many horsepower does your car demonstrate during a 1/2 hour drive at 60 mph? (assume 30 miles/gallon) c'mon, this is just mostly conversions; I'm hoping you are good at conversions by now! 8

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13 Name: No: Simple Machines Lab Purpose: The purpose of this lab is to investigate different types of simple machines. Directions: For each machine, measure the input force, output force, input distance, & output distance for each of the following set-ups. Then calculate work in, work out, IMA, AMA, and Efficiency. Use PENCIL or points will be deducted! CAUTION: WEIGH the masses used don t rely on what is stamped on them!! Fill in all data tables as you work ). Pulleys: Use 500 g masses. Use 5 N scales for more sensitivity and be careful to account for the weight of the scale when you take your readings (especially Figure 1)! m Figure 1 Figure 2 m Figure 3 m Levers: All distances for levers should be measured from the fulcrum (pivot point). Set the fulcrum to be where the meter stick just balances on its own (should be close to the 50 cm mark). Directions: Hang a heavier mass ( g) about 10 cm from the fulcrum. Apply a balancing force on the other side to level the ruler using a smaller mass ( g). You may need to hold the fulcrum (pivot point) to prevent tipping. 10 cm m Repeat for other distances at about 20 cm, 30cm, 40cm, and 50 cm from the fulcrum you may need to vary the masses to maintain balance or move the fulcrum. 13

14 Inclined plane ("ramp") Directions: Begin your cart on the incline. Measure the distance you pull the cart & the force you pull parallel along the incline. Measure the change in height of the car as you pull it up the incline (hint: measure to the center of the bottom wheels). The work you do is the energy you give the cart (lifting it up, you give it GPE). The work you do is also equal to the force you apply times the distance you apply it. Both values will be the same if there is no friction. If there is friction then you have to put in more work than you get out of the system (some of the work goes into internal energy - friction). Lever Directions: A lever, like any simple machine trades force for distance. The light weight is used to lift a much heavier weight; but, to do so it drops down a great height, while the heavier weight is lifted up only a small height. Set the light weight distances of 20, 30 and 40 cm from the pivot point (fulcrum). Move the heavy weight to "just balance" the heavy weight. Let the light weight fall to lift up the heavy weight and measure h light (the height the light weight falls) and h heavy (the height the heavy weight rises). In a perfect world with 100% efficiency and perfect measurements and no other factors, the work input would equal the work output. Find what you really have and measure the efficiency of this machine. Efficiency = W out /W in x 100%, IMA = d i /d o, AMA = F o /F i, W = Fd, Wt. = mg, 100 cm = 1 m Data Table / Analysis: Show a set of sample calculations for each machine on a SEPARATE SHEET of paper (4 STEPS!) & complete the table below. Machine Force Input (N) Force Output (N) Lever (20cm) Distance Input (m) Distance Output (m) Work Input (J) Work Output (J) IMA AMA Efficiency Lever (30cm) Lever (40cm) Incline Conclusions: 14

15 Unit 05 Physics Themed Vocabulary and Equations Work, Energy & Power in Rollercoasters Vocabulary: Joule Newton Kinetic energy (KE), Potential energy (PE), Work Gravitational potential energy (GPE or PE) Mechanical energy, Elastic potential energy Chemical energy, Nuclear energy, Electrical energy Internal energy, Deformation energy, Sound energy Light energy, Thermal energy, Total energy, Mechanical energy (ME) Efficiency, Friction, Frictionless Conservation of energy Pendulum, Perpetual motion machine Symbols: PE, KE, GPE, m, g, h, v Equations & constants: PE = mgh KE = ½mv 2 ME = KE + PE Weight = m * g g = 9.8 m/s 2 1 kg = 2.2 lbs 1 mile = 1609 m 60 mph = 27 m/s Unit Objectives - Williams 1. I understand all the vocabulary & math of this unit and all demos, videos, equations, and class assignments. 2. I remember objectives & vocabulary from previous units. 3. I understand that positive work requires a parallel force components and increases the energy of the object 4. I can compute work where forces are at various angles relative to the displacement of the object 5. I know that work can be negative and what this implies 6. I can compute work, power and various energy forms in absolute and relative terms based on speed, mass, height and force direction values or changes in these values 7. I know conservation of energy including ME & friction converts useful energy (ME) to internal energy 8. I know common forms of energy and can identify them 9. I am able to compute power and can explain how it differs from work 10. I have memorized the current cost of energy locally per kilowatt hour 11. Given information on work, power, time etc., I can compute energy cost using the factor label method 12. I understand the relationship between work, force, energy and distance for brakes 13. I can recognize the relationships between W, P, t, d, K, etc graphically 14. I understand elastic systems including how force and stored energy vary with elongation/compression 15. I understand what conservation energy means including common examples of real world exchanges of energy 16. I can to track the exchange energy for masses moving up and down hills while ME remains constant 17. Compute/graph energy totals & categories using GPE, KE, ME including using correct units and including how changes in mass, speed and height change the results 18. Compute weight/mass 19. Identify and understand other forms of energy in unambiguous situations 20. Follow energy flow in pendulums and rollercoasters including consequential changes in speed and position when heights are changed as well as novel situations; understand why first hill must be tallest 21. Conceptually, I can account for friction/thermal energy including the unavoidable energy flow in that direction DuPage ROE Objectives 401. I can identify if masses have kinetic and/or potential energy at a given instant I can identify potential energy as a function of position I can identify kinetic energy as a function of velocity I can calculate gravitational potential energy and kinetic energy I can identify an isolated system and analyze it I can identify that energy is transferred between different forms I can solve problems using conservation of mechanical energy I can apply the mathematical definition of work as the product of Force and displacement I can identify situations of positive work, negative work, zero work I can identify work as a change in energy I can analyze the rate of energy change of a system in terms of power. 15

16 Work, Power, Simple Machines Calendar: (Williams) Bold and underlined means put in journal notes. (12-01) Explain basis of Work/Power, HW prep:3,4,5,6,8,11,13 Review cost of energy for this year if necessary:10 Demo inclined plane (ramp) & lever Fr:04/12/13 Go over Energy quest, collect journals & start HW Work & skidding 1 ACT Satur. Do worksheet cars, p. 3/4 Go over HW Force demo: ramp with fish scale & cart (12-02) Skidding car work, simple machine F vs. d tradeoffs: Finish up P. 5, 6 examples of lever, ramp (no pulleys!):15 (12-03) Takes notes Do some problems, p. 5, 6 (skidding car, power review & on Reading Packet Mo:04/15/13 simple machine guided problem); notes HW start 2.6 (12-04) Conservation of energy (coaster example) including braking (Brakes do negative work!):7,9,12 Power, energy & Tu:04/16/13 Power up lab, p. 7, 8 (outdoors if weather is nice), or braking problems, due 3 College nt practice problems Friday (12-05) Simple machines equations (IMA, AMA), Work, power, energy review Possible worksheet handed out in class More simple machines examples (trade force for distance), preview tomorrow s lab? Have journal up to date for journal check 4 We:04/17/13 Parts of pages 9-14 (TBD) Thursday Extra problems handed out on Tuesday due tomorrow Go over simple machines lab as a class Have journal up to Begin lab: Short Simple machines lab, p (lever & date for journal check 5 Th:04/18/13 ramp only) tomorrow Possible simple machines lab quiz Finish pages/parts as Fr:04/19/13 Pairs review problems directed in class 6 Pep Rally QUEST MONDAY! QUEST MONDAY! 7 Mo:04/22/13 Work, power, simple machines quest (50 pts?) Good luck on PSAE! Tu:04/23/13 PSAE jrs 1st per.: Juniors to Field AM 12:10 PSAE (No instruction for juniors, 12:10 dismissal) Relax and do well on 8ED Dismissal Video PSAE! 9 We:04/24/13 PSAE jrs PSAE (No juniors til 5th per. +) Video 16