1. Energy 4 – Elastic Energy Mr. Jean Physics 11

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

3. 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.

4.  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.

5. Work & 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)

6. 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”.

8.  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)

9. Hooke’s Law:

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

11.  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

13. At Rest: m x x=0

14. Extended (Potential Energy) m x x=0

15. Compressed (Potential Energy) m x x=0

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

17. Conservation of Energy:  E k1 + E p1 + E e1 = E k2 + E p2 + E e2  E k1 = kinetic energy before event (J)  E p1 = gravitational potential energy before event (J)  E e1 = elastic potential energy before event. (J)  E k2 = kinetic energy after event (J)  E p2 = gravitational potential energy after event (J)  E e2 = elastic potential energy after event. (J)

18. Questions to do:

19. Hooke’s Law Investigation:  Tomorrow we are doing a mini-lab on Hooke’s law and spring constants.