# Physics- Elastic Potential Energy and Hooke’s Law, Monday November 24, 2008

Elastic Potential Energy

– Many objects can stretch, compress or bend (ex. diving board)

– Once the force that was applied to it is removed, some objects can return to its original shape

– This suggests that there was energy stored in the object due to its condition- known as Elastic Potential Energy

Hooke’s Law

Springs are an example of a type of object with elastic potential energy
When a force causes a spring to stretch, the spring exerts a force in a direction that will return it to its original length
– This is known as a restoring force
– Always acts in a direction opposite that in which the spring is stretched or compressed

Property of elastic objects is known as Hooke’s Law

Hooke’s Law: The applied force is directly proportional to the extension or compression of a string

The data produces a straight line with the equation y=mx+b where m is the slope and b is the y intercept

o The slope of a line describing a spring is known as the spring constant (k)

o Spring constant is in the unit N/m

o Each spring has its own constant; it tells us the amount of force that is necessary to stretch or compress the spring a given amount

o The zero position is generally designated to the spring when there is no force applied (thus the y intercept is zero)

Hooke’s Law

Fa= -kx

Fa= force applied (N)

K= spring constant (N/m)

X= amount of compression or stretch (m)

The equation is negative because the restoring force always acts in opposite direction of the displacement (x)

Elastic Potential Energy

The elastic potential energy of a perfectly elastic material is ½ of the product of the spring constant and the square of the length of extension or compression.

Ee= ½kx2

For homework, please complete:

Page 258, #35-37

Page 261, #38-40, Section Review Questions 7, 9