1. ROTATIONAL
DYNAMICS
Section 8. 2

2. Rotational Dynamics
• How can I describe torque?
• What factors influence torque?

3. Opening A Door
• Easiest Way to do it?
• Why is the handle where it is?

4. Opening a Door

5. Rotational Dynamics
• A force must be
exerted to open a door
• Doorknob is near the
outer edge, as far from
the axis of rotation as
possible
• Pull at a right angle to
the door to apply the
most force

6. Rotational Dynamics
• So where we apply the
force changes how
effective it is
• In this section we will
be measuring how
effectively an applied
force turns something
• Pertains to doors,
pedals, wrenches…..

7. Rotational Dynamics
• The change in angular velocity depends on:
• the magnitude of the force
• the distance from the axis to the point where the force is exerted
• the direction of the force

8. Lever Arm
• Lever Arm: The
perpendicular distance
from the axis of
rotation to the point
where the force is
exerted

9. Lever Arm
• For the door, it is the
distance from the
hinges the point where
you exert the force
• If the force is
perpendicular to the
radius of rotation then
the lever arm is the
distance from the axis,
r

10. Reduced Lever Arm
• If force is not
perpendicular, lever arm
is reduced
• Lever Arm (L)
• L = r sin ɵ
• ɵ = angle between force
and the radius from the
axis of rotation to the
point where the force is
applied

11. Torque
•

12. Example
• A bolt on a car engine
needs to be tightened
with a torque of 35 Nm.
• You use a 25 cm long
wrench to pull on the
end of the wrench at an
angle of 60.0° from the
perpendicular.
• How long is the lever
arm and how much force
do you have to exert?

13. Homework
• Page 203
• # 11 - 15

14. Balancing Torques
• http://phet.colorado.edu/en/simulation/balancing-act
When we played with the balance, what
two things did we change?

15. Moment of Inertia
•

16. Moment of Inertia
• Hold your book at the
bottom and rotate
• Hold your book in the
middle and rotate
• Which one has a lower
torque?

17. Moment of
Inertia
• When something
is spinning, we
have not taken
into account
shape

18. Homework
• Read Page 205-207