IPC Station 4(D) Efficiency in Machines

Transcription

1 Correlations to Texas Essential Knowledge and Skills: Integrated Physics and Chemistry (4) Science concepts. The student knows concepts of force and motion evident in everyday life. The student is expected to: (D) investigate and demonstrate mechanical advantage and efficiency of various machines such as levers, motors, wheels and axles, pulleys, and ramps. (2) Scientific processes. The student uses scientific methods during field and laboratory investigations. The student is expected to: (A) plan and implement investigative procedures including asking questions, formulating testable hypotheses, and selecting equipment and technology; (B) collect data and make measurements with precision; (C) organize, analyze, evaluate, make inferences, and predict trends from data; and (D) communicate valid conclusions. Purpose: The purpose of this station is to reinforce students understanding of how the efficiency of a machine can change. Note: Text with a line through it indicates this part of the TEKS is not being addressed in this activity. Some TEKS statements printed here end with a ; or and and nothing thereafter this indicates that further TEKS statements follow but are not included here. (3) Scientific processes. The student uses critical thinking and scientific problem solving to make informed decisions. The student is expected to: (A) analyze, review, and critique scientific explanations, including hypotheses and theories, as to their strengths and weaknesses using scientific evidence and information; Teacher Notes About the TEKS In the science TEKS, the study of simple machines begins at grade 7. At grade 7, however, the level of understanding pertains to the relationship between force and motion within the machine. IPC takes that basic understanding and expands on it by addressing energy transformations within a simple machine. By comparing the input work to the output work of a simple machine, the student is expected to determine the overall efficiency of the machine. Background Information for the Teacher This station requires students to examine the structures of a simple machine and hypothesize about how effectively the machine can do work. IPC students familiar with the law of conservation of energy should recognize that work cannot be increased by a machine. The efficiency of a machine is the ratio of the useful energy output of a machine to the total energy input. Efficiency measures the degree to which friction and other factors reduce the Charles A. Dana Center at The University of Texas at Austin 1

2 Essential Science Concepts for Exit-Level TAKS actual work output of a machine. In this activity, students are to compare the parts of a simple machine and determine its efficiency. Materials Pulleys (2) 1-kg weight Spring scale (in Newtons) Calculator Twine Meterstick Station information sheet (included in blackline masters for this station) Student pages, including glossary (included in blackline masters for this station) TAKS Formulas chart (included in blackline masters for this station) Question card (see Advance Preparation) Advance Preparation 1. Using the pulleys, twine, and spring scale, assemble the pulley system as shown below. Attach the pulley system to a stationary support. 2. For each student make a copy of the student pages (including glossary), station information sheet, and TAKS Formulas chart; for the station table, make a copy of the station information sheet and the TAKS Formulas chart. 3. Make a question card using question 10 from the Released TAKS Test for Exit Level Science, July This released test can be found at 2 Charles A. Dana Center at The University of Texas at Austin

3 Essential Science Concepts for Exit-Level TAKS Station Setup 1. Place the pulley system (attached to a stationary support), 1-kg weight, metric ruler, and a copy of the TAKS Formulas chart at the station table. 2. Tape a copy of the station information sheet to the table. Students will use this sheet to confirm the station is set up correctly. Procedures 1. When students arrive at the station, they should check the station setup against the station information sheet at the table. If anything is missing or out of place, the students should notify the teacher. 2. Students should read the procedures in the student pages and answer the questions. Guide to Student Responses Focus Question: The two inclined planes shown below are identical. A 100-kilogram block is being pulled up each inclined plane for a distance of 1 meter. Explain why more energy is required to move the block on inclined plane A 1 meter up the plane, and less energy is required to move the object on inclined plane B 1 meter up the plane. The block on inclined plane A loses more energy to friction than the block on inclined plane B, which is on wheels. Charles A. Dana Center at The University of Texas at Austin 3

4 Essential Science Concepts for Exit-Level TAKS Finding Work Output Work output (W o) is the product of the force of an object (mass of object x 9.8 m/s 2 ) and the distance it is moved; or, work output = (force)(distance the object moves). What is the work output when a pulley system moves a 10N object 0.1 m? (Write the formula for work from the TAKS Formulas chart and use it to solve for work output.) Work output = force x distance Force = 10N Distance = 0.1 m Work output = 10N x 0.1 m Work output = 1 Joule Finding Work Input (Remember, 1 Newton meter = 1 Joule) Work input (W i) is the applied force multiplied by the distance the applied force moves an object; or, work input = (force applied to the object)(distance the force moves). What is the work input when a 7N force is moved a distance of 0.2 m. (Write the formula for work from the TAKS Formulas chart and use it to solve for work input.) Work input = force x distance Force = 7N Distance = 0.2 m Work input = 7N x 0.2 m (Remember, 1 Newton meter = 1 Joule) Work input = 1.4 Joules NOTE: Work input (W i) is equal to work output (W o) only for perfect machines, which are 100% efficient. In practice, however, Wo is less than Wi and efficiency is less than 100% due to friction. Questions 1. Based on the diagram above, find the work input needed to lift the 1-kg weight 0.1 m using the pulley system. First, determine the force (read from the spring scale) needed to lift the 1-kg weight using the pulley system Next, find the distance the spring scale moves to lift the weight 0.1 m. Show your work. The answers that your students give will depend on the calibration of the spring scale and the elasticity of the twine. However, a work input of about 1.00 Joules is a reasonable answer for this pulley system. 2. Find the work output needed to lift the 1-kg weight 0.1 m using the spring scale. The answers that your students give will depend on the calibration of the spring scale and the elasticity of the twine. However, a work output of about.98 Joules is a reasonable answer for this pulley system. 4 Charles A. Dana Center at The University of Texas at Austin

5 Essential Science Concepts for Exit-Level TAKS 3. Why aren t the work input and the work output equal? Some of the energy is transformed into heat energy from the friction. 4. Find the efficiency of the pulley system. (Use the formula for efficiency on the TAKS Formulas chart.) Show your work. Efficiency = (work output/work input) x 100 Efficiency = (.98 Joules/1.00 Joules) x 100 NOTE: The values are taken from the answers to questions 1 and 2 above. Student answers will vary, but the efficiency should be less than 100% Efficiency = 98% Charles A. Dana Center at The University of Texas at Austin 5

6 Essential Science Concepts for Exit-Level TAKS Blackline Masters for Contents: TAKS Formulas chart (1 copy for station table and 1 copy for each student) Station information sheet (1 copy for station table and 1 copy for each student) Student pages (1 copy for each student) Glossary (1 copy for each student) 6 Charles A. Dana Center at The University of Texas at Austin

7 Essential Science Concepts for Exit-Level TAKS Charles A. Dana Center at The University of Texas at Austin 7

8 Essential Science Concepts for Exit-Level TAKS Station Information Sheet 8 Charles A. Dana Center at The University of Texas at Austin

9 Student Pages Before You Begin Check to see that all the items are present and organized according to the station information sheet. If you notice a problem, notify your teacher immediately. Materials Pulley system (2 pulleys and spring scale, strung with twine) 1-kg weight Meterstick Calculator Glossary TAKS Formulas chart Procedures Focus Question: The two inclined planes shown below are identical. A 100-kilogram block is being pulled up each inclined plane for a distance of 1 meter. Explain why more energy is required to move the block on inclined plane A 1 meter up the plane, and less energy is required to move the object on inclined plane B 1 meter up the plane. Charles A. Dana Center at The University of Texas at Austin Student Page 1

10 Essential Science Concepts for Exit-Level TAKS Discuss the focus question with your teammate(s) and record your answer. Finding Work Output Work output (W o ) is the product of the force of an object (mass of object x 9.8 m/s 2 ) and the distance it is moved; or, work output = (force)(distance the object moves). What is the work output when a pulley system moves a 10N object 0.1 m? (Write the formula for work from the TAKS Formulas chart and use it to solve for work output.) (Remember, 1 Newton meter = 1 Joule) Finding Work Input Work input (W i ) is the applied force multiplied by the distance the applied force moves an object; or, work input = (force applied to the object)(distance the force moves). Find the work input required to lift a 7N weight a distance of 0.2 m. (Write the formula for work from the TAKS Formulas chart and use it to solve for work input.) (Remember, 1 Newton meter = 1 Joule) Student Page 2 Charles A. Dana Center at The University of Texas at Austin

11 Essential Science Concepts for Exit-Level TAKS Questions Use the equipment at this station to answer the following questions. Begin by attaching the 1-kg weight to the pulley system. (See diagram below.) 1. Based on the diagram above, find the work input needed to lift the 1-kg weight 0.1 m using the pulley system. First, determine the force (read from the spring scale) needed to lift the 1-kg weight using the pulley system Next, find the distance the spring scale moves to lift the weight 0.1 m. Show your work. Charles A. Dana Center at The University of Texas at Austin Student Page 3

12 Essential Science Concepts for Exit-Level TAKS 2. Find the work output needed to lift the 1-kg weight 0.1 m using the spring scale. Show your work. 3. Why aren t the work input and the work output equal? Student Page 4 Charles A. Dana Center at The University of Texas at Austin

13 Essential Science Concepts for Exit-Level TAKS 4. Find the efficiency of the pulley system. (Use the formula for efficiency on the TAKS Formulas chart.) Show your work. NOTE: Because other students are going to do the activity after you, be sure to put all the materials at the station back as you found them. Sometimes there will be materials that need to be renewed or replaced. If you need assistance or have any questions, ask your teacher. Charles A. Dana Center at The University of Texas at Austin Student Page 5

14 Essential Science Concepts for Exit-Level TAKS Question Card 1. Obtain the question card from your instructor. 2. Read the question and discuss the answer with your teammate(s). 3. Record the team s consensus in your study folder and justify your answer. 4. Record the team s answer on the instructor s diagnostic wall chart. I Need to Remember Complete this part AFTER the class discussion of the station. I need to remember... Student Page 6 Charles A. Dana Center at The University of Texas at Austin

15 Essential Science Concepts for Exit-Level TAKS Glossary for Efficiency Efficiency is equal to work output (W o ) divided by the work input (W i ) expressed as a percentage. Mechanical advantage Mechanical advantage is the ratio derived from the output force divided by the input force. Work Work is the product of force and distance moved in the direction of the force. Work input Work input is the applied force multiplied by the distance the applied force moves an object; or, work input = (force applied to object) (distance object moves). Work output Work output is the product of the force of an object and the distance it is moved; or, work output = (force)(distance object moves). Charles A. Dana Center at The University of Texas at Austin Student Page 7