Physics 11 Mr. Jean November 23rd, 2011

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

Spring videos: Sling Shot champion

Calculate the potential energy?

How much kinetic energy does each racer have?

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: Ee = ½ k x2 Ee = 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. FX = -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 x x=0 m y y=0 Etotal = 1/2 mv2 + 1/2 kx2 = constant KE PE