1. Physics 11
Mr. Jean
November 23rd, 2011

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

3. Spring videos:
Sling Shot champion

4. Calculate the potential energy?

5. How much kinetic energy does each racer have?

6. 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.

7. 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.

8. 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)

9. 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”.

11. 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)

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

14. 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

15. Conservation of Energy:m x
x=0 m y
y=0
Etotal = 1/2 mv2 + 1/2 kx2 = constant
KE PE