Stuff you asked about: I thought I understood the concepts in the pre-lecture until they went over the three examples. I felt that it moved too fast especially when the video was showing manipulations of equations. I needed so much help with this. Please explain all of the checkpoint and lecture questions. Also, give more examples!!! I think work and energy is my favorite topic. I think i know the answers, but don't know why and how to explain why it is right. Well, the day I've been dreading has come. We have finished everything I've previously known about physics and the rest of the semester is new. Time to start learning. We should discuss calc 2 because it is much more difficult than this. This isnt that confusing to me Many of the concepts of work are very confusing. I hope to go over this in detail tomorrow 1. This stuff is super confusing. Discuss all of it. 2. I think everyone will be glad to know the second I read about centrifugal force in my MCB lab manual, I dropped the class. DON'T THEY KNOW PHYSICS?!?! That was a lot to cover in one prelecture. How is dot product different from vector sum? “It seems like work was defined differently to me in high school than it is here. Then again, I didn't pay much attention in high school” You are old enough & smart enough to see where the formulas come from now. “I really just don't understand work. Please try to cover this in a very understandable way. The pre-lecture just didn't do it for me! :)” You aren't supposed to know everything after the prelecture Physics 211 Lecture 7 Today’s Concepts: Work & Kinetic Energy Work-Kinetic Energy Theorem r2 W F dl r1 “Integrals are a little scary, but the concepts themselves don't seem to difficult.!” The integral IS the concept !! Work-Kinetic Energy Theorem The work done by force F as it acts on an object that moves between positions r1 and r2 is equal to the change in the object’s kinetic energy: W K TOT r2 W F dl NET TOT r1 1 2 K mv 2 “Doesn't work-kinetic energy theorem have anything to do with potential energy?." Clicker Question W K Does the work-kinetic energy theorem have anything to do with potential energy? A) YES B) NO The Dot Product “Direction and magnitude of the total work done is confusing..” “the vector dot thing. what is the point of it? Will we have to use this in problems.” Work-Kinetic Energy Theorem: 1-D Example If the force is constant and the directions aren’t changing then this is very simple to evaluate: car F d r2 W F dl F d r1 In this case = Fd since cos(0)=1 This is probably what you remember from High School. Clicker Question A lighter car and a heavier van, each initially at rest, are pushed with the same constant force F. After both vehicles travel a distance d, which of the following statements is true? (Ignore friction) F d W= Fd car same F d van K= W Same too A) They will have the same velocity B) They will have the same kinetic energy C) They will have the same momentum r2 F dl K Derivation – not so important Concept – very important r1 r2 F dl r1 A force pushing over some distance will change the kinetic energy. K q W r2 F dl r1 Work done by gravity near the Earth’s surface “May we please discuss the concepts of gravitational work?” mg Work done by gravity near the Earth’s surface WTOT W1 W2 ... WN mg dl1 mg dl2 ... mg dlN dlN dl1 mg dl2 dy1 dl1 dx1 mg Work done by gravity near the Earth’s surface WTOT W1 W2 ... WN mg dl1 mg dl2 ... mg dlN mgdy1 mgdy2 ... mgdyN mg y dlN y dl1 mg dl2 Wg mg ( y final yinitial ) dr rdq dr Fg r2 GM e m GM e m GM m 1 1 W F (r ) dr dr e 2 r r r r 2 1 r r 1 r1 r2 1 r2 Close to the Earth’s surface: r1 ~ r2 ~ Re: 1 1 Wg GM e m r2 r1 r1 r2 GM e m r1r2 r2 r1 GM e m r1 r2 2 Re mg -y So: Wg = -mgy Work-Kinetic Energy Theorem If there are several forces acting then W is the work done by the net (total) force: WNET K W1 W2 ... You can just add up the work done by each force WNET WTOT Checkpoint Three objects having the same mass begin at the same height, and all move down the same vertical distance H. One falls straight down, one slides down a frictionless inclined plane, and one swings on the end of a string. In which case does the object have the biggest net work done on it by all forces during its motion? H Free Fall A) Free Fall Frictionless incline B) Incline C) String String D) All the same Clicker Question Three objects having the same mass begin at the same height, and all move down the same vertical distance H. One falls straight down, one slides down a frictionless inclined plane, and one swings on the end of a string. What is the relationship between their speeds when they reach the bottom? H Free Fall Frictionless incline A) vf > vi > vp B) vf > vp > vi String C) vf = vp = vi Checkpoint A car drives up a hill with constant speed. Which statement best describes the total work WTOT done on the car by all forces as it moves up the hill? A) WTOT > 0 B) WTOT = 0 C) WTOT < 0 Less that 40% got this right… Clicker Question A car drives up a hill with constant speed. How does the kinetic energy of the car change as it moves up the hill? A) It increases B) It stays the same C) It decreases Clicker Question A car drives up a hill with constant speed. The acceleration of the car: A) Points up the hill B) Points down the hill C) Is zero Clicker Question A car drives up a hill with constant speed. The net force on the car: A) Points up the hill B) Points down the hill C) Is zero Reminder We know two expressions that involve WTOT W K TOT r2 W F dl NET TOT r1 Checkpoint A car drives up a hill with constant speed. Which statement best describes the total work WTOT done on the car by all forces as it moves up the hill? A) WTOT > 0 B) WTOT = 0 C) WTOT < 0 TOT A) The car moves upward, in the positive direction, therefore the force that caused it to move by a positive distance did positive work. B) change in kinetic energy is zero, total work done is zero. C) the work is being done by gravity, which is in the negative direction.. “Question 1 in the prelecture is wrong. It asks for the work done and not the change in kinetic energy. Since the Apple is raised, there is work from the change in gravitational potential energy.” Hand does positive work. Gravity does negative work. The sum of these is zero since Wtot = Whand + Wgravity = K = 0 Checkpoint A box sits on the horizontal bed of a moving truck. Static friction between the box and the truck keeps the box from sliding around as the truck drives. S a The work done on the box by the static frictional force as the truck moves a distance D is: Less that 40% got this right… A) Positive B) Negative C) Zero “How friction is related to work, and which direction work goes in various cases” From Last Lecture A box sits on the horizontal bed of a moving truck. Static friction between the box and the truck keeps the box from sliding around as the truck drives. S a If the truck moves with constant accelerating to the left as shown, which of the following diagrams best describes the static frictional force acting on the box: A B C Checkpoint F S a D The work done on the box by the static frictional force as the truck moves a distance D is: A) Positive B) Zero C) Negative A) since the static frictional force is acting in the same direction as the motion, i believe the work would be positive. B) Static means the box isn't moving, so the displacement is 0. C) Friction always does negative work. Work done by a Spring “Can you clarify the directions of positive or negativity for springs when they are compressing and uncompressing?” Use the formula to get the magnitude of the work Use a picture to get the sign (look at directions) In this example the spring does negative work since F and x are in opposite direction. The axes don’t matter.