1. PHY221 Ch18: Rotational Statics II
Applications:
Examples: ladder and Box in truck
Stability of object w/respect to gravity and CM location

2. PHY221 Ch18: Rotational Statics II
Examples
Example: Ladder of length L. Static  needed? Assume no friction on vertical wall. 

3. PHY221 Ch18: Rotational Statics
Examples
Example: Box in truck
Truck accelerates forward
Assume friction large enough so box doesn’t slide
But: it can tip!
Problem: what is the max acc. for no tipping?
Draw picture with forces and choose axes (remember that accel. not zero)
Force equation:
Torque equation: (be careful about choosing O; not all points valid!)

4. PHY221 Ch18: Rotational Statics
Stability
Stability of equilibrium and torque:
Equilibrium requires net force and net torque equal to zero on object
But some configurations are stable and some unstable.
Consider following 2 configs of a red bar that can freely rotate around a pivot pt O:
Forces applied?
Net force?
Net torque?
Let’s look at what happens when the red bar is pushed slightly from equil?
Force point of view:
Torque point of view:
Relation of stability of equil. to potential energy:

5. PHY221 Ch18: Rotational Statics II
Stability
Application: Stability and CM position for object resting on surfaces (and relation to potential energy change)
Look at tipping around O (clearly the interesting point here)
Assume object barely off surface (so that N is all applied at O) and study motion:
Now let’s relate to what happens to pot energy.
On the left the rotation around O would increase or decrease the pot energy?
On the right the rotation around O would increase or decrease the pot energy?