Meera Chandrasekhar University of Missouri, Columbia Dorina Kosztin University of Missouri, Columbia Gabriel de La Paz Clayton High School, St. Louis, MO Support: Missouri Department of Elementary and Secondary Education Math-Science Partnership Grant www.physicsfirstmo.org
Physics First is a national movement to teach a year-long Physics course in 9th grade In Missouri, MO-DESE has funded a partnership led by Columbia Public Schools and Univ. of Missouri-Columbia to develop curriculum and conduct professional development (PD) Three sessions of PD were conducted in 2006, 2007, and 2008 for approx. 70 teachers from 25 districts
Year 1: Uniform and Accelerated Motion, Forces, and Newton s Laws Year 2: Motion in 2D, Energy, Momentum, Astronomy, and Electricity Year 3: Electromagnetism, Heat, Light, Waves Pedagogy - based on Modeling, Inquiry & 5E Today - parts of Unit 6: Energy
Student Beliefs and Big Ideas Exploring Energy Lab What is Work? Lab Representing Energy transfer and transformation Elastic Energy Lab
From the non-scientific point of view, "work" is synonymous with "labor". Energy gets used up or runs out. Objects that are not moving have no energy. Energy is destroyed or created (making energy, using energy). Energy is a force. The terms "energy" and "force" are interchangeable. If energy is conserved, why are we running out of it?
Work is defined as force x distance moved along direction of force For a closed system, energy is conserved. Energy can be stored, transferred or transformed.
Stations: identify the system, initial and final states, and the process it undergoes. Distinguish between energy transformation and energy transfer.
We study work, elastic energy, PE, KE, in the unit Today Work and Elastic Energy
Pre-lab discussion: What happens when A block falls on clay? A car smashes into a lump of clay? A lump of clay is shot from a slingshot onto the wall? These are examples of a force acting over a distance and producing an effect. An experiment to obtain the relationship between force, distance and work is then conducted.
A car is pulled up a ramp so it reaches the top. Compare the force to pull it up the ramp to the force to lift it up vertically. Compare the work to pull it up the ramp to the work to lift it up vertically. Pull object up the length of the ramp at a constant velocity. A constant force will be applied over the entire distance.
Δx
In order to develop the relationship between force, work and distance, we need to take readings for several ramps and compare them Use several ramps of same height, but different lengths. Measure force required to travel up each ramp
Table: Force F required for different lengths of ramp, Δx (height of ramp =16.5 cm) Δx (m) F (N) 1 0.24 0.9 0.26 0.8 0.28 0.7 0.32 0.6 0.4 0.5 0.46 Applied Force (N) Force vs. distance traveled on ramp 0.5 0.4 0.3 0.2 0.1 0 0 0.2 0.4 0.6 0.8 1 Ramp length, distance Δx (m)
And finally, what if it just traveled vertically up? F = 1.4N (weight) Δx = 0.165 m (height) W = FΔx = 0.23J Δx (m) F (N) W = FΔx (J) 1 0.24 0.24 0.9 0.26 0.234 0.8 0.28 0.224 0.7 0.32 0.224 0.6 0.4 0.24 0.5 0.46 0.23 0.165 1.4 0.23 F = weight = 1.4 N Height = 0.165 m
Work = force x distance traveled inline with the force W = F Δx in units of N.m or J When you travel on a ramp, the force is less than traveling vertically; the work done is the same F Δx
Students learn to define the system represent transformation of energy from one form to another using pie charts and bar graphs (conservation of energy) represent the transfer of energy in and out of the system using bar graphs
System: ball + earth
System: spring + box + earth + surface
System: spring + box + earth
Design and conduct an experiment to determine a mathematical model for calculating the elastic potential energy stored in a spring. Relationship between the amount of force applied to the spring and the amount of deformation Relationship between the amount of deformation of the spring and the amount of energy stored in the spring Relationship between the amount of work done by the spring and the amount of energy stored in the spring
Analyze energy storage and transformations in a spring + earth system. In the initial position the spring is not stretched; in the final position the spring is stretched.
The data below is obtained by stretching 2.5 N and 10 N spring scales. Stretch (cm) Force (N) (2.5 N spring scale) Force (N) (10 N spring scale) 0 0 0 0.5 0.5 0.8 1.0 1.0 1.6 1.5 1.6 2.3 2.0 2.0 3.6 2.5 2.4 4.4
Slope (2.5 N spring scale) = k = 0.01 N/cm Slope (10 N spring scale)= k = 0.16 N/cm
F = kδx
Design experiment, collect data, draw Force F vs. stretch Δx graph Interpret graph and relationship between F and Δx Calculate work done as area under the F vs. Δx graph Recognize that work transfers energy into the system and stores it as elastic potential energy. Develop a mathematical expression for elastic potential energy
Practice problems (lots!) Labs to connect potential and kinetic energy, and to develop their formulae Lab to develop formula for power, and to connect work, power and energy
MeeraC@missouri.edu KosztinD@missouri.edu www.physicsfirstmo.org click on Meeting Archives