Transcript Rotational Mechanics

• "I think that was the game we discovered Gary Russell as a potential short-yardage
and goal-line runner," Tomlin said. "And that's been solid for us. We don't pretend
that it's something mystical. We've just got to formulate good plans, call good plays
and execute them."
• Tomlin called Russell "a one-trick pony. He's down hill. It doesn't matter what play
you call for him. It's down hill. That happens to be a positive attribute for those
circumstances."
• Russell, signed as an undrafted rookie in 2007, is no Bus. But, at 5-11, he is stocky
and heavier than his listed 215 pounds.
• "Very rarely is he knocked back," Tomlin said. "He's
got a nice center of gravity."
ROTATIONAL MECHANICS
What causes rotation? Force?
• Let’s name it…
– Torque
• Pick a letter to represent it…
– Tau
• How do we calculate it?
– Well…what makes it easier or more difficult to
rotate an object?
– Why is a doorknob always in the same spot?
Torque
• Think about it!
– Ever use a wrench?
Torque=Force (perpendicular)
times Lever Arm
How can a see-saw be balanced?
Which object will roll the fastest?
What is rotational inertia?
• Compared to linear inertia?
• Resistance to changes in rotational
motion
An object rotating about an axis tends to
keep rotating about that axis.
What does rotational inertia depend on?
•
What did inertia depend on?
•
The greater the distance between the bulk of the mass of an object and the axis about which rotation takes place, the greater the rotational inertia
(Baseball Bat)
Rotational inertia also depends on the
distribution of the mass!
Rotational Inertia
The farther the mass is from the axis…the less likely the object is to rotate.
I  kmr
2
ANGULAR MOMENTUM
• What was momentum before?
– Linear
– p=mv
• What is angular momentum?
L  I
– “inertia of rotation”
Angular momentum equals rotational
inertia times angular velocity

Angular Momentum
An object or system
of objects will
maintain its angular
momentum unless
acted upon by an
unbalanced external
torque.
Angular Momentum
• There is another way to calculate angular
momentum.
– Can you help me?
L  I
I?
?
L  I
v
L  mr
r
L  mvr
2
Law of Conservation of Angular Momentum
If no unbalanced external torque
acts on a rotating system, the
angular momentum of that system
is constant.
I ω
ω=I
Who spins faster? Why?
Why does a top tip over?