Chapter 5.2 Hooke’s Law

WOD are underlined

Question What is the net force on this mass?

Question What about now? What direction will the force be?

Hooke’s Law Springs are objects that store energy

and create forces in order to restore

themselves to equilibrium.

Springs create NON-CONSTANT forces that are always towards the direction of equilibrium.

Hooke’s Law (WOD) F = - k Δx

New Symbol: “k”

k is Spring constant. “Stiffness” of the spring. Depends on each spring’s dimensions and material.

In N/m

Hooke’s Law F = - k Δx

Force = stiffness of spring (or k) times

How far you stretch it

The negative sign reverses the direction of Δx.

More about the negative sign The force exerted *BY* a spring is opposed to the displacement.

The force applied *ON* a spring will be equal and opposite to that. You have to push on a spring to compress it.

You have to pull on a spring to stretch it.

Problem A: A spring with spring constant 10 N/m has a force of 40 N applied to it (stretching it). How much does the spring stretch?

F = - k Δx

Problem A: A spring with spring constant 10 N/m has a force of 40 N applied to it (stretching it). How much does the spring stretch?

X = 40N / (-10N/m)

Another problem B: A force of 600 Newtons will compress a spring 0.5 meters. What is the spring constant of the spring?

F = - k Δx

Another problem B: A force of 600 Newtons will compress a spring 0.5 meters. What is the spring constant of the spring?

k = -F / Δx = -600N / (.5m)

Question If I let go, what will happen to the mass? Then what? Then what?

Question If I let go, what will happen to the mass?

For how long? Why would it stop?

Question For how long? Why would it stop?

Go on forever, unless friction or until friction sucks away all the energy. Then it stops.

What friction is there?

Simple Harmonic Motion Motion that occurs when the net force obeys Hooke’s Law The force is proportional to the displacement and always directed toward the equilibrium position.

The motion of a spring mass system is an example of Simple Harmonic Motion

Classwork: Section 5-2 Pg 172 Prob 21-29 Staple to 5-2 Read and Write and turn in. HW due Tuesday. Read and Write for 13.1 and 13.2 HW due Weds. Read and Write 13.3-.5

You can play around with Bike wheels if you wish to do probs at home.