Physics 151 Week 11 Day 3 Topics: Forces, Apparent Weight, & Friction  Energy  Work  Conservation of Energy with Work  Work-Energy Theorem  Green House Effect  Momentum  Conservation of Momentum  Collisions  1-D Collisions  2-D Collisions  Work and Impulse

Conservation of Energy Slide KE i + PE gi +Pe si +  E sys = KE f + PE gf + Pe sf +  E th Three ways to increase the energy of a system 1.Work by external forces on the system - working 2.Transfer of thermal energy - heating 3.Radiation - radiating

Work Examples: In-class demonstrations Slide 10-23

The Greenhouse Effect Visible Light Radiation from Sun absorbed by Earth’s surface and reemitted Radiation from surface absorbed by greenhouse gas molecules and reemitted Green house gases like holey blanket - keep some heat in Higher concentrations of green house gases like repairing holes in blanket - keeps more heat in Main greenhouse gases are H 2 O and CO 2. If no greenhouse effect, surface would be 40 o C cooler!

Green House Effect: Earth Radiation Energy Balance Adding up the Energies show E in = E out

Greenhouse gases CO 2 and water vapor keeps surface teachers on earth pleasant for us. How is this a problem? CO 2 levels have been rising since the start of the industrial revolution and today are at highest level in 400,000 years => 388 PPM => like patching the blanket, E th increases (Data from analysis of gases trapped in ice cores)

Look at greenhouse effect on Venus (this would not happen here if you burned all fossil fuels on earth) Slide Pressure at surface is 90 x that of Earth's => much more gas in atmosphere. No oceans. - Consequence - meteoroids burn up easily. No impact craters less than ~3 km. - Atmosphere is 96.5% CO 2 - Yellowish color from sulfuric acid clouds and haze. - Hot at surface K! Almost hot enough to melt rock - Why so hot? Huge amount of CO 2 leads to strong greenhouse effect.

For more information on Climate Change and Greenhouse Effect, try these links Slide Science Web Sites American Physical Society NASA National Earth Science Teachers Association: Windows to the Universe house_effect_gases.html house_effect_gases.html

Energy and Momentum Slide Energy (scalar)Momentum (vector) When to useWhen time and direction are not needed Collisions, explosions, and large forces in short  t Inertia with m & v Changing the system Newton’s 2nd Law Application Work-Energy TheoremImpulse-momentum Theorem Newton’s 3rd Law Application Conservation of EnergyConservation of Momentum

EXAMPLE 11.6 Calculating work using the dot product QUESTION:

EXAMPLE 11.6 Calculating work using the dot product

Work Energy Theorem Slide W net =  ma*Delta x * cos(  )= 1/2 mv f 2 - 1/2 mv i 2 2 * ma*Delta x = 2 * (1/2 mv f 2 - 1/2 mv i 2 ) v f 2 - v i 2 = 2a*Delta x v f 2 = v i 2 + 2a*Delta x Look familiar

Work Energy Theorem Slide Answer these questions: Does KE increase or decrease? A) increase B) decrease C) can’t tell What is the sign of  KE? (A) positive, (B) negative, or (C) zero What forces act on the object in question? For each of these forces, is the work (A) positive, (B) negative, (C) zero? Is Wnet (A) positive, (B) negative, or (C) zero?

Work Energy Problem 1 Slide Solve this problem two ways, with Newton's 2nd Law and with the Work-Energy Theorem 1. A 200 g Ball is lifted upward on a string. It goes from rest to a speed of 2 m/s in a distance of 1 m. What is the tension (assumed to stay constant) in the string?