# W16 Mechanics Week 9

Day 1: HEY! might not be a bad idea to start reviewing old problem sets and exams to prepare for the final exam!

Conserving Angular Momentum, when Sum of the Torque = zero

• Angular momentum = I*alpha, kind of like linear momentum = m*v
• Torque SOMETIMES = I * alpha, as stated above in rotational dynamics. Torque ALWAYS = rate of change of angular momentum
• For instance if Torque = zero, then angular momentum is conserved (just like we conserve momentum if there’s no outside force) it doesn’t mean that alpha = zero, like acceleration = 0 if the sum of the forces = 0, Think of the spinning skater pulling in her arms, yielding a positive angular acceleration.

Before Class

• Collect PS #7,
• Please watch this video on torque = time derivative of angular momentum
• Please see video on Coriolis Effect , made by students from Fall, 2014
• PS#9 due next week has been posted. Please take a look at it.
• I posted my summary and reflection from your week 8 feedback on the main webpage

In Class:

Day 2:
How can a point mass (with no radius and no moment of inertia) have angular momentum? It all depends on the impact parameter!!

Before Class

In Class

After Class

Day 3: More on how Torque = rate of change of angular momentum

Before Class

In Class

• Big Exam #6 – Solutions posted on main class page

After Class

Day 4:

Precession:
Torque is rate of change of angular momentum so dL = (torque)*dt. However, angular momentum is a vector, so you also need torque to change the direction of angular momentum.
Before Class

In Class

• Angular momentum and Gyroscopic stability. We have some cool demos for this.
• Why must we spin frisbees or footballs to make them fly stable? What is counter steering on a motorcycle?
• How is this related to precession?

Friday. You have until midnight tonight to post your link on the website for full credit.