Mr. Jean November 21 st, 2013 IB Physics 11 IB Physics 11

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

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

How much kinetic energy does each racer have?

Try This:  Prove that work, Potential Energy, and Kinetic energy are the same for the following:  A block sitting at 0meters is slide up a frictionless inclined plane to a height of 80 meters. The block is then held in place for 50 minutes. The block is then released.

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.

 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.

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)

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”.

 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)

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

 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

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

Hooke’s Law Investigation:  Start Hooke’s Law Investigation