Presentation on theme: "Lecture 29 Friday November 14 Chapter 10: 6-10"— Presentation transcript:
1 Lecture 29 Friday November 14 Chapter 10: 6-10 Potential EnergyLecture 29 Friday November 14Chapter 10: 6-10
2 QuizIn lab, you lift a 2 kg mass from the floor and put it on top of the lab bench which is 92.5 cm high.A) How much work did you do?B) How much work did the gravitational force do?C) Where did the energy go that you put in as work?
3 Answers d = 0.925 m, FLIFT = mg Work done by gravity Where did the energy you transferred go?
4 Gravitational Potential Energy Book and the earth are the system, but do not form an isolated system because we are reaching in through the walls of the box.
5 Note that ΔU depends only upon Δy, not on the path the object took moving from the floor to the bench top. No dependence upon x.Problem 10:1 During an etiquette class, you walk slowly and steadily at 0.20 m/s for 2.5 m with a 0.75 kg book balanced on your head. How much work does your head do on the book?
6 Elastic Potential Energy Force exerted by a spring depends on how far it has been stretched. Not a constant force.Stretch spring from xi =0 to xf = x. The average force during this process of stretching is
7 Work done by us as we stretch spring is Where does this energy go?Stored in spring and we can get it back out. Hence we call the energy stored in the spring potential energy, US = ½kx2.
8 Using Energy Conservation Define systemSet Ei =Ef if system is isolated
11 Power Power is the rate of transformation of energy Unit is 1 Watt=1W = 1 J/sIf energy being transformed is work, W then
12 Is the work done by F + or - ? PositivenegativeFd
13 Is the work done by F + or - ? PositivenegativeFd
14 Is the work done by F + or - ? PositivenegativeFd
15 Problem 10:20A pendulum is made by tying a 500g ball to a 75-cm-long string. The pendulum is pulled 300 to one side and then released.A) What is the ball’s speed at the lowest point in its trajectory?B) To what angle does the pendulum swing on the other side.
16 Problem 10:20Use energy conservationL=0.75 m300Δy=L-Lcos 300
17 Problem 10:20 contSet y=0 at lowest point of swing
18 Problem 10:24A student places her 500g physics textbook on a frictionless table. She pushes the book against a spring 4.00cm and then releases the book. What is the book’s speed as it slides away? The spring constant is k = 1250 N/m.
19 Problem 10:24Use energy conservationWe want to find Kf .
20 Problem 10:24Using the initial position as the compressed spring, final after book leaves spring: