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.

Pull object up the length of the ramp at a constant velocity. A constant force will be applied over the entire distance.

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.

Δx

Use several ramps of same height, but different lengths. Measure force required to travel up each ramp

In order to develop the relationship between force, work and distance, we need to take readings for several ramps and compare them

0

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8 1

App

lied

Forc

e (N

)

Ramp length, distance Δx (m)

Force vs. distance traveled on ramp 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

Table: Force F required for different lengths of ramp, Δx (height of ramp =16.5 cm)

  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

Hei

ght =

0.1

65 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

Δx

F

  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

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