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Physics 218 Lecture 11 Dr. David Toback Physics 218, Lecture XI 1 Chapters 6 & 7 Seven Cont Last time: • Work This time • Work and Energy – Work using the Work-Energy relationship • Potential Energy • Conservation of Mechanical Energy Physics 218, Lecture XI 2 Physics 218, Lecture XI 3 Physics 218, Lecture XI 4 Kinetic Energy and Work-Energy • Energy is another big concept in physics • If I do work, I’ve expended energy – It takes energy to do work (I get tired) • If net work is done on a stationary box it speeds up. It now has energy • We say this box has “kinetic” energy! Think of it as Mechanical Energy or the Energy of Motion 2 Kinetic Energy = ½mV Physics 218, Lecture XI 5 Work-Energy Relationship • If net work has been done on an object, then it has a change in its kinetic energy (usually this means that the speed changes) • Equivalent statement: If there is a change in kinetic energy then there has been net work on an object Can use the change in energy to calculate the work Physics 218, Lecture XI 6 Summary of equations Kinetic Energy = 2 ½mV W= DKE Can use change in speed to calculate the work, or the work to calculate the speed Physics 218, Lecture XI 7 Multiple ways to calculate the work done Multiple ways to calculate the velocity Physics 218, Lecture XI 8 Multiple ways to calculate work 1. If the force and direction is constant – F.d 2. If the force isn’t constant, or the angles change – Integrate 3. If we don’t know much about the forces – Use the change in kinetic energy Physics 218, Lecture XI 9 Multiple ways to calculate velocity If we know the forces: • If the force is constant –F=ma →V=V0+at, or V2 - V02 = 2ad • If the force isn’t constant –Integrate the work, and look at the change in kinetic energy W= DKE = KEf-KEi = ½mVf2 -½mVi2 Physics 218, Lecture XI 10 Quick Problem I can do work on an object and it doesn’t change the kinetic energy. How? Example? Physics 218, Lecture XI 11 Problem Solving How do you solve Work and Energy problems? BEFORE and AFTER Diagrams Physics 218, Lecture XI 12 Problem Solving Before and After diagrams 1. What’s going on before work is done 2. What’s going on after work is done Look at the energy before and the energy after Physics 218, Lecture XI 13 Before… Physics 218, Lecture XI 14 After… Physics 218, Lecture XI 15 Compressing a Spring A horizontal spring has spring constant k 1. How much work must you do to compress it from its uncompressed length (x=0) to a distance x= -D with no acceleration? 2. You then place a block of mass m against the compressed spring. Then you let go. Assuming no friction, what will be the speed of the block when it separates at x=0? Physics 218, Lecture XI 16 Potential Energy • Things with potential: COULD do work – “This woman has great potential as an engineer!” • Here we kinda mean the same thing • E.g. Gravitation potential energy: –If you lift up a brick it has the potential to do damage Physics 218, Lecture XI 17 Example: Gravity & Potential Energy You lift up a brick (at rest) from the ground and then hold it at a height Z. • How much work has been done on the brick? • How much work did you do? • If you let it go, how much work will be done by gravity by the time it hits the ground? We say it has potential energy: U=mgZ – Gravitational potential energy Physics 218, Lecture XI 18 Mechanical Energy • We define the total mechanical energy in a system to be the kinetic energy plus the potential energy • Define E≡K+U Physics 218, Lecture XI 19 Conservation of Mechanical Energy • For some types of problems, Mechanical Energy is conserved (more on this next week) • E.g. Mechanical energy before you drop a brick is equal to the mechanical energy after you drop the brick K2+U2 = K1+U1 Conservation of Mechanical Energy E2=E1 Physics 218, Lecture XI 20 Problem Solving • What are the types of examples we’ll encounter? – Gravity – Things falling – Springs • Converting their potential energy into kinetic energy and back again E=K+U= 2 ½mv Physics 218, Lecture XI + mgy 21 Problem Solving For Conservation of Energy problems: BEFORE and AFTER diagrams Physics 218, Lecture XI 22 Quick Problem We drop a ball from a height D above the ground Using Conservation of Energy, what is the speed just before it hits the ground? Physics 218, Lecture XI 23 Next Week • Reading for Next Time: –Finish Chapters 6 and 7 if you haven’t already –Non-conservative forces & Energy • Homework 5 Due Monday • Start working on HW6 Physics 218, Lecture XI 24 Physics 218, Lecture XI 25 Compressing a Spring A horizontal spring has spring constant k 1. How much work must you do to compress it from its uncompressed length (x=0) to a distance x= -D with no acceleration? 2. You then place a block of mass m against the compressed spring. Then you let go. Assuming no friction, what will be the speed of the block when it separates at x=0? 3. What is the speed if there is friction with coefficient m? Physics 218, Lecture XI 26 Roller Coaster A Roller Coaster of mass M=1000kg starts at point A. We set Y(A)=0. What is the potential energy at height A, U(A)? What about at B and C? What is the change in potential energy as we go from B to C? If we set Y(C)=0, then what is the potential energy at A, B and C? Change from B to C Physics 218, Lecture XI 27 Kinetic Energy Take a body at rest, with mass m, accelerate for a while (say with constant force over a distance d). Do W=Fd=mad: • V2- V02 = 2ad= V2 ad = ½V2 • W = F.d = (ma) .d= mad ad = ½V2 mad = ½ mV2 W = mad = ½ mV2 Kinetic Energy = ½ mV2 Physics 218, Lecture XI 28 Work and Kinetic Energy • If V0 not equal to 0 then • V2 - V02 = 2ad • W=F.d = mad = ½m (V2 - V02) = ½mV2- ½mV02 = D(Kinetic Energy) W= DKE Net Work on an object (All forces) Physics 218, Lecture XI 29 A football is thrown A 145g football starts at rest and is thrown with a speed of 25m/s. 1. What is the final kinetic energy? 2. How much work was done to reach this velocity? We don’t know the forces exerted by the arm as a function of time, but this allows us to sum them all up to calculate the work Physics 218, Lecture XI 30 Example: Gravity • Work by Gravity Physics 218, Lecture XI 31 Potential Energy in General • Is the potential energy always equal to the work done on the object? – No, non-conservative forces – Other cases? • What about for conservative forces? Physics 218, Lecture XI 32 Water Slide Who hits the bottom with a faster speed? Physics 218, Lecture XI 33 Mechanical Energy • Consider a Conservative System • Wnet = DK (work done ON an object) DUTotal = -Wnet Combine DK = Wnet = -DUTotal => DK + DU = 0 Conservation of Energy Physics 218, Lecture XI 34 Conservation of Energy • Define E=K+U DK + DU = 0 => (K2-K1) +(U2-U1)=0 K2+U2 = K1+U1 Conservation of Mechanical Energy E2=E1 Physics 218, Lecture XI 35 Conservative vs. Non-Conservative Forces • Nature likes to “conserve” certain types of things • Keep them the same • Kinda like conservative politicians • Conservationists Physics 218, Lecture XI 36 Conservative Forces • Physics has the same meaning. Except nature ENFORCES the conservation. It’s not optional, or to be fought for. “A force is conservative if the work done by a force on an object moving from one point to another point depends only on the initial and final positions and is independent of the particular path taken” • (We’ll see why we use this definition later) Physics 218, Lecture XI 37 Closed Loops Another definition: A force is conservative if the net work done by the force on an object moving around any closed path is zero This definition and the previous one give the same answer. Why? Physics 218, Lecture XI 38 Is Friction a Conservative Force? Physics 218, Lecture XI 39 Integral Examples we know… V a dt at 0 dt Where a is a constant a( t 01 0 1 ) v0 a( t1 1 ) v0 V at v0 X (at v0 ) dt a( t 11 11 (at ) v0( 1 v0t ) dt t 01 0 1 0 ) x0 a( t2 2 ) v0t x0 X at v0t x0 1 2 2 Physics 218, Lecture XI 40 Work done to stretch a Spring Stretch a spring from x 0 to x X Work by Person W Xf X Xi 0 W F dl FP dx X 1 2X 1 2 kx dx k x kX 0 2 2 0 1 2 W kX 2 Physics 218, Lecture XI 41 Robot Arm A robot arm has a funny Force equation in 1-dimension 3x F(x) F0 1 2 x0 2 where F0 and X0 are constants. What is the work done to move a block from position X1 to position X2? Physics 218, Lecture XI 42 Stretch a Spring A person pulls on a spring and stretches it a from the equilibrium point for a total distance D. At this distance the force required to keep the spring stretched is F. How much work is done on the spring in terms of the given variables? Can we use F.d? Physics 218, Lecture XI 43