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Potential Energy Lecture 29 Friday November 14 Chapter 10: 6-10.

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1 Potential Energy Lecture 29 Friday November 14 Chapter 10: 6-10

2 Quiz In 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 = m, F LIFT = 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 x i =0 to x f = 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, U S = ½kx 2.

8 Using Energy Conservation 1.Define system 2.Set E i =E f if system is isolated

9 Hoop Race Use energy conservation

10 Smallest I will be the fastest.

11 Power Power is the rate of transformation of energy Unit is 1 Watt=1W = 1 J/s If energy being transformed is work, W then

12 Is the work done by F + or - ? 1.Positive 2.negative F d

13 Is the work done by F + or - ? 1.Positive 2.negative F d

14 Is the work done by F + or - ? 1.Positive 2.negative F d

15 Problem 10:20 A pendulum is made by tying a 500g ball to a 75-cm-long string. The pendulum is pulled 30 0 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 10:20 Use energy conservation 30 0 L=0.75 m Δy=L-Lcos 30 0

17 Problem 10:20 cont Set y=0 at lowest point of swing

18 Problem 10:24 A 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 10:24 Use energy conservation We want to find K f.

20 Problem 10:24 Using the initial position as the compressed spring, final after book leaves spring:

21 Problem 10:24 Finally

22 Monday Oscillations Read 14:1-3 Problems 10: 14,15,20,21,24,27,31,36,38,41, 45


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