25: Video Tutor: Charged Rod and Aluminum Can

Part A: 
Consider the situation in the figure below, where two charged rods are placed a distance d on either side of an aluminum can. What does the can do?

a) Rolls to the left
b) Stays still
c) Rolls to the right 

SOLUTION:
The positively charged rod induces a negative charge on the left side of the can, creating an attractive force between the rod and the can. However, the negatively charged rod induces an equal positive charge on the right side of the can, which creates an attractive force between the can and that rod. The net force acting on the can is zero.
So the answer is option b)

Part B:
Now, consider the situation shown in the figure below. What does the can do?

a) Stays still
b) Rolls to the right 
c) Rolls to the left

SOLUTION:
The polarization force is always attractive, so the can does not move, option a)

Part C:
Using the setup from the first question, imagine that you briefly touch the negatively charged rod to the can. You then hold the two rods at equal distances on either side of the can. What does the can do?

a) Does not move
b) Rolls toward the positively charged rod
c) Rolls away from the positively charged rod 

SOLUTION:
The can acquires a net negative charge after being touched, so it is then attracted to the positively charged rod, or option b)