1. Work and Energy Chapter 5

2. Work In physics: “the amount of energy transferred from one object to another” Measured in Joules (J) Mathematically: W = Fd – When force is constant – *Work is only done when motion is in same direction as applied force.* – Up is positive work; down is negative work.

3. When using forces at an angle: W = Fd(cos θ) θ is the angle between the applied force and the displacement Again, work only occurs when motion is in the direction of the applied force. Example: pushing a lawnmower.

4. Kinetic Energy Energy of motion Depends on mass and velocity KE = ½ mv 2

5. Work-Kinetic Energy Theorem W net = Δ KE W net = F net d

6. Potential Energy Stored energy of position 2 main types: – Gravitational – Elastic

7. Gravitational Potential Energy PE g =mgh h is vertical distance from “zero-level” “zero-level” is arbitrary level set as zero height. – Typically ground level

8. Elastic Potential Energy PE elastic = ½ kx 2 k = spring constant (measured in N/m) x = distance compressed or stretched Force of a spring F = -kx

9. Homework: Pg. 162 #1-4 Pg. 163 #1-6 Pg. 166 #1,2,5 Pg. 168 #1-4 Pg. 172 #1-3 Due Tuesday Nov. 19

10. Conservation of Energy Mechanical energy is conserved in a closed system. – ME= sum of kinetic and all forms of potential energy KE i + PE i = KE f + PE f In a non-closed system energy is introduced or withdrawn from the system in the form of work.

11. Power Power is the rate at which work is done. – The rate at which energy is transferred. P = W/t = Fd/t = Fv (if constant speed) Measured in Watts – 1 Watt = 1 J/s

12. Amy applies a force of 12 N on a spring to stretch it to a displacement of 8 cm. (Assume no friction) a) How much work was done? b) What is the total mechanical energy? c) What is the spring constant of the spring? d) How fast will a 200 gram mass move when it goes through the equilibrium position? e) How fast will the 200 gram mass be moving at a displacement of 3.5 cm?

13. Dave is standing on the top edge of 100 m cliff using a slingshot that contains a 55 g rock and pulls it back 35 cm (k=775 N/m) He proceeds to shoot the rock straight up. a) How much elastic energy will the rock have? b) What will the velocity of the rock be the instant it leaves the slingshot? c) What will be the maximum height the rock will attain? d) What is the hang time of the rock?

14. Homework Pg. 177 #1-5 Pg. 178 #1-3 Pg. 181 #1-5 Practice F – #1-3 Section Review – Due Friday November 22

15. Chapter Review Pgs. 184-187 #7,8,10,19,20,22,33,34,36,39,42,45,46,49,52 Due Tuesday December 3 rd