1. Mr. Jean April 27 th, 2012 Physics 11

2. The plan:  Video clip of the day  Potential Energy  Kinetic Energy  Restoring forces  Hooke’s Law  Elastic Potential Energy

3. Physics wins!  http://latimesblogs.latimes.com/lanow/201 2/04/ucsd-scientist-evades-400-traffic- ticket-with-research-paper-.html http://latimesblogs.latimes.com/lanow/201 2/04/ucsd-scientist-evades-400-traffic- ticket-with-research-paper-.html http://latimesblogs.latimes.com/lanow/201 2/04/ucsd-scientist-evades-400-traffic- ticket-with-research-paper-.html

4. Who has the most E k and by how much more?

5. How much kinetic energy does each racer have?

8. FYI: Gallows’ Execution  Ironically, all of the energy required to execute the accused is done by the accused as he or she walks up the stairs to the platform.

9. Elastic Potential Energy in Springs  If you pull on a spring and stretch it out, you do work on the spring.  W = Fd  Since work is a transfer of energy, then energy must be transferred into the spring.

10.  Work becomes stored in the spring as potential energy.  When you stretch a spring, it has the potential to “spring” back. This is stored energy.  When you compress a spring, it has the potential to “spring” forwards. This is stored energy.

11. Elastic Potential Energy:  E e = ½ k x 2  E e = elastic potential energy in J (joules)  k = spring constant N/m (newtons per meters)  x = length of extension m (meters)

12. Energy Stored in a Spring  If a spring’s stretch/compression is directly proportional to the the amount of force applied to it then the elastic potential energy stored in a spring is given by:  Where x is the DISTANCE the spring is stretched or compressed  K is called a “spring constant”.

14.  Hookes Law: The force exerted by a spring is proportional to the distance the spring is stretched or compressed from its relaxed position. F X = -k x  Where x is the displacement from the relaxed position and k is the constant of proportionality.  (often called “spring constant”) x > 0

15.  If a spring is not stretched or compressed, then there is no energy stored in it.  It is in its equilibrium position. (it’s natural position)

16. Problem  It requires 100 J of work to stretch a spring out 0.10 m. Find the spring constant of the spring.

18. Conservation of Energy: m y y=0 m x x=0 E total = 1/2 mv 2 + 1/2 kx 2 = constant KE PE

19. Questions to do:  P. 229  Question #11  P. 239-241  Read Work and Kinetic Energy (Especially p. 239 but especially p. 240!)  P. 245  Question #22 - 24