Mr. JeanMr. Jean

April 27April 27thth, 2012, 2012

Physics 11Physics 11

The plan:The plan:

Video clip of the dayVideo clip of the day Potential EnergyPotential Energy Kinetic EnergyKinetic Energy Restoring forcesRestoring forces Hooke’s LawHooke’s Law Elastic Potential EnergyElastic Potential Energy

Physics wins!Physics wins!

http://latimesblogs.latimes.com/lanow/201http://latimesblogs.latimes.com/lanow/2012/04/ucsd-scientist-evades-400-traffic-tick2/04/ucsd-scientist-evades-400-traffic-ticket-with-research-paper-.htmlet-with-research-paper-.html

Who has the most EWho has the most Ekk and by how much more? and by how much more?

How much kinetic energy does each How much kinetic energy does each racer have?racer have?

FYI: Gallows’ ExecutionFYI: Gallows’ Execution

Ironically, all of the energy required to Ironically, all of the energy required to execute the accused is done by the execute the accused is done by the accused as he or she walks up the stairs accused as he or she walks up the stairs to the platform. to the platform.

Elastic Potential Energy in SpringsElastic Potential Energy in Springs

If you pull on a spring and If you pull on a spring and stretch it out, you do work stretch it out, you do work on the spring.on the spring.

W = FdW = Fd Since work is a transfer of Since work is a transfer of

energy, then energy must energy, then energy must be transferred into the be transferred into the spring.spring.

Work becomes stored in the Work becomes stored in the spring as potential energy.spring as potential energy.

When you stretch a spring, it When you stretch a spring, it has the potential to “spring” has the potential to “spring” back. This is stored energy.back. This is stored energy.

When you compress a spring, When you compress a spring, it has the potential to “spring” it has the potential to “spring” forwards. This is stored forwards. This is stored energy.energy.

Elastic Potential Energy:Elastic Potential Energy:

EEee = ½ k x = ½ k x22

EEee = elastic potential energy in J (joules) = elastic potential energy in J (joules)

k = spring constant N/m (newtons per meters)k = spring constant N/m (newtons per meters) x = length of extension m (meters)x = length of extension m (meters)

Energy Stored in a SpringEnergy Stored in a Spring If a spring’s stretch/compression is directly If a spring’s stretch/compression is directly

proportional to the the amount of force applied proportional to the the amount of force applied to it then the elastic potential energy stored in a to it then the elastic potential energy stored in a spring is given by:spring is given by:

Where x is the Where x is the DISTANCEDISTANCE the spring is the spring is stretched or compressedstretched or compressed

K is called a “spring constant”. K is called a “spring constant”.

2

2

1kxEe

Hookes Law:Hookes Law: The force exerted by a The force exerted by a spring is proportional to the distance spring is proportional to the distance the spring is stretched or compressed the spring is stretched or compressed from its relaxed position.from its relaxed position.

FFXX = -k x = -k x Where Where xx is the displacement from the is the displacement from the

relaxed position and relaxed position and kk is the constant of is the constant of proportionality.proportionality.

(often called “spring constant”)(often called “spring constant”)

x > 0

If a spring is not stretched or compressed, then If a spring is not stretched or compressed, then there is no energy stored in it.there is no energy stored in it.

It is in its equilibrium position. (it’s natural It is in its equilibrium position. (it’s natural position)position)

ProblemProblem It requires 100 J of work to stretch a spring It requires 100 J of work to stretch a spring

out 0.10 m. Find the spring constant of out 0.10 m. Find the spring constant of the spring.the spring.

kmJ

km

J

mkJ

mkJ

kxEe

2

2

2

2

2

/20000

010.0

200

)010.0(200

)10.0(2

1100

2

1

Conservation of Energy:Conservation of Energy:

m

y

y=0

m

xx=0

Etotal = 1/2 mv2 + 1/2 kx2 = constant

KE PE

Questions to do:Questions to do:

P. 229P. 229 Question #11Question #11

P. 239-241P. 239-241 Read Work and Kinetic Energy (Especially p. Read Work and Kinetic Energy (Especially p.

239 but especially p. 240!)239 but especially p. 240!)

P. 245P. 245 Question #22 - 24Question #22 - 24