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PHYS-1600/2000 III4 Dissipation of Energy Is the tension bigger than, equal to, or smaller than the weight of B? DEAN SIEGLAFF NATHANIEL CUNNINGHAM NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 What does the net force look like? Does it depend upon the direction of motion? 1 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 AGENDA ITEM Introductory Concept Survey (Individual) The Work-Energy Theorem and Non-Conservative Work Is W-E Thm Always Valid? Analysis of Pushed Block w/Friction Bouncing Ball Analysis Fluid Drag and Terminal Velocity Motion Survey Re-vote (Group Discussion Mode) Dismissal DURATION START 0:10 0:15 0:25 0:25 0:20 0:10 0:00 0:10 0:25 0:50 1:15 1:35 1:45 ANNOUNCEMENTS • Consider the concept questions about Newton’s 2nd Law. DEAN SIEGLAFF NATHANIEL CUNNINGHAM 2 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 1 of 3 A balloon is dropped a distance of 100 m. Atmospheric drag is significant, causing the balloon to fall at constant speed. Through its flight the balloon lost 1 J of gravitational potential energy (DPEg = -1 J). The amount of kinetic energy gained (DKE) by the balloon is: y 100 v = const 1. 1 J. 2. Zero. 3. -1 J. 0 DEAN SIEGLAFF NATHANIEL CUNNINGHAM 3 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 2 of 3 A rocket (such as this SLS 130-ton-to-orbit cargo lifter) blasts off from ground level. During the first 10 seconds of its launch, it accelerates uniformly upward and gains 1 GJ of PEg. During that same time, the vehicle: y 1. Loses 1 GJ of KE. 2. Neither loses nor gains KE. 3. Gains KE. a 0 DEAN SIEGLAFF NATHANIEL CUNNINGHAM 4 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 3 of 3 A ball drops from rest from 1 m of altitude, bounces, then reaches a maximum final altitude of 0.5 m. It had 10 J of PEg initially, but only 5 J of PEg finally. The work done on the ball by the normal force of the rigid surface was 1. 2. 3. 4. DEAN SIEGLAFF –5 J. More than –5 J but less than zero. Zero. 5 J. NATHANIEL CUNNINGHAM 5 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 AGENDA ITEM Introductory Concept Survey (Individual) The Work-Energy Theorem and Non-Conservative Work Is W-E Thm Always Valid? Analysis of Pushed Block w/Friction Bouncing Ball Analysis Fluid Drag and Terminal Velocity Motion Survey Re-vote (Group Discussion Mode) Dismissal DURATION START 0:10 0:15 0:25 0:25 0:20 0:10 0:00 0:10 0:25 0:50 1:15 1:35 1:45 ANNOUNCEMENTS • Consider the concept questions about Newton’s 2nd Law. DEAN SIEGLAFF NATHANIEL CUNNINGHAM 6 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 Non-conservative Forces and the General Work – Energy Theorem The work done by a non-conservative (NC) force depends upon the path of motion. Therefore, when NC forces are involved, the total mechanical energy cannot be conserved. Rather, it can be dissipated or created. ROCKET THRUST AIR DRAG Fthrust Fdrag DKE 0 DPE 0 mg a DKE 0 DPE 0 mg DKE DPE WNC DEAN SIEGLAFF NATHANIEL CUNNINGHAM 7 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 Failure of the Work – Energy Theorem There are cases whereby the mechanical energy of a system cannot be fully accounted for (even taking into consideration the work of NC forces). • • Two blobs of clay of equal mass and opposite velocity colliding and sticking together. Two freight cars of equal mass and opposite velocity colliding and coupling. In these cases the WNC is ZERO. From the particle dynamics view, a direct transformation from mechanical energy E to thermal energy U has occurred. To find the dissipative forces responsible, one must peer into the internal structure of the bodies. DKE DPE DU 0 DEAN SIEGLAFF NATHANIEL CUNNINGHAM 8 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 Must be careful when applying work-energy principles to nonparticulate or non-rigid bodies: • • • • • Both particles and rigid bodies are structureless, ideal entities, not thought of as being composed of any smaller interacting structures. A particle or rigid body has no “internal degrees of freedom” in which to store internal energy (thermal energy). Real bodies such as blocks, crates, automobiles, and people, are obviously not particles or rigid bodies. The application of particle and rigid body mechanics to nonparticulate and non-rigid bodies will result in energetic inaccuracies. Augmenting the principles of work and energy to include systems with internal structure is called Thermodynamics. DEAN SIEGLAFF NATHANIEL CUNNINGHAM 9 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 The work of friction is not computable by particulate means (FDx), because its displacement is not known. The friction force arises from interactions between bits and pieces of the body and its environment that are in motion relative to the body. DEAN SIEGLAFF NATHANIEL CUNNINGHAM 10 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 III4 Exit Homework Problem #1 DEAN SIEGLAFF NATHANIEL CUNNINGHAM 11 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 III4 Exit Homework Problem #2 DEAN SIEGLAFF NATHANIEL CUNNINGHAM 12 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 1 of 3 A balloon is dropped a distance of 100 m. Atmospheric drag is significant, causing the balloon to fall at constant speed. Through its flight the balloon lost 1 J of gravitational potential energy (DPEg = -1 J). The amount of kinetic energy gained (DKE) by the balloon is: y 100 v = const 1. 1 J. 2. Zero. 3. -1 J. 0 DEAN SIEGLAFF NATHANIEL CUNNINGHAM 13 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 2 of 3 A rocket (such as this SLS 130 ton cargo lifter) blasts off from ground level. During the first 10 seconds of its launch, it accelerates uniformly upward and gains 1 GJ of PEg. During that same time, the vehicle: y 1. Loses 1 GJ of KE. 2. Neither loses nor gains KE. 3. Gains KE. a 0 DEAN SIEGLAFF NATHANIEL CUNNINGHAM 14 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 3 of 3 A ball drops from rest from 1 m of altitude, bounces, then reaches a maximum final altitude of 0.5 m. It had 10 J of PEg initially, but only 5 J of PEg finally. The work done on the ball by the normal force of the rigid surface was 1. 2. 3. 4. DEAN SIEGLAFF –5 J. More than –5 J but less than zero. Zero. 5 J. NATHANIEL CUNNINGHAM 15 of 15 PHYS-1600/2000 III4 Dissipation of Energy NEBRASKA WESLEYAN UNIVERSITY FALL 2014-2015 PROJECTION SCREEN 1 4 1 4 1 4 1 4 2 5 2 5 2 5 2 5 3 6 3 6 3 6 3 6 III4: MARKED SEATS PLEASE HAND IN TODAY’S ACTIVITIES SHEETS DEAN SIEGLAFF NATHANIEL CUNNINGHAM 16 of 15