1. Rotational Mechanics
Ch. 11

2. 11.1 Torque Torque produces rotation. NOT a force
List times when you make an object turn or rotate
NOT a force
To make an object move you apply a force, force causes acceleration
Torque is produced when a force is applied with leverage
The longer the handle the more leverage can be applied

3. 11.1 continued
Lever arm-the distance from the turning axis to the point of contact
Torque = Fd
W=Fd
d in torque is ┴ to the force
d in work is parallel to the force
Units= newton-meter

4. 11.2 Balanced Torques
Use a triple beam balance to weight different masses
What relationships do you notice?
Torque in 2 different directions
Counter clockwise torque opposes clockwise torque
If the two are equal the scale will be balanced
F1d1=F2d2

5. 11.2 Balance Torque Cont. 200N 2 m 2 m τ= 400 N-m τ= 400 N-m 200N 2 m

6. 11.3 Torque and Center of Gravity
Why can’t a person stand against the wall and then bend forward without falling forward?
Your feet are not beneath your center of gravity
This creates torque
τ= Fd
How could the torque be balanced so that a person standing against the wall doesn’t fall over?
answer
Why do pregnant women get back pain?

7. 11.3
When a football is kicked or throw why does is rotate end over end instead of spiral?
If a force is applied off center then torque is created thus causing the ball to move end over end
How do you get it to spiral?
answer

8. 11.4 Rotational Inertia What is inertia?
Rotational inertia- the resistance of an object to changes in its rotational motion
An object rotating wants to keep rotating, an object that is not rotating wants to continue to not rotate
Torque is required to change the rotational state of motion
Rotational inertia depends on mass
And the distribution of mass
The greater the distance between the axis and the mass the greater the rotational inertia

9. 11.4 Think about a baseball bat
Why do some people use a shorter bat than other people?
What does it mean to “choke up”?
Why would a baseball player need to “choke up”?
Compare the legs of a giraffe with that of a mouse
List some difference
How does rotational inertia apply?

10. 11.4 Formulas for rotational inertia: pg. 157
If both cylinders were rolling down a ramp which would have more acceleration? (same mass, same radius)

11. 11.5 Rotational Inertia & Gymnasics
Human body has 3 axes of rotation
Median (z)
Transverse (x)
Longitudinal (y)
All at 90° angle to one another
Rotational inertia is least about the longitudinal axis thus it is the easiest rotation to perform
To increase longitudinal inertia extend an arm or a leg while spinning
A summersault rotates about the transverse axis
Least when in tuck position
Greatest when fully extended
A cartwheel rotates about the median axis

12. 11.6 Angular Momentum Angular momentum- inertia of rotation
Vector quantity
Has direction and magnitude
Rotational velocity- when a direction is assigned a speed
Angular momentum= rotational inertia x rotational velocity
Angular momentum = I x ω

13. 11.6 An external net torque is required to change angular momentum
A moving bicycle is easier to balance on because the wheels have angular momentum there fore more torque is required.

14. 11.7 conservation of angular momentum
Law of conservation of angular momentum- if no unbalanced external torque acts on a rotating system, the angular momentum of that system is constant
With no external torque, the product of rotational inertia and rotational velocity at one time will be the same as at any other time
Initial conditions
What happens to V as you increase the velocity bar but keep the radius the same?
What happens to V as you increase the radius but keep the velocity the same?
What happens to the snowman as you change the radius under run time controls?