Potential difference in a uniform electric field

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Homework Statement

An insulating rod having linear
charge density l 5 40.0 mC/m and
linear mass density m 5 0.100 kg/m is
released from rest in a uniform electric
field E 5 100 V/m directed perpendicular
to the rod (Fig. P25.9). (a) Determine
the speed of the rod after it has
traveled 2.00 m. (b) What If? How does
your answer to part (a) change if the
electric field is not perpendicular to the rod? Explain.

Homework Equations

The Attempt at a Solution

I solved part A and got .4 m/s, but I don;t understand conceptually part b. if the rod is not perpendicular to the electric field, why would its motion behave exactly as if it was perpendicular?

 

[ehild]

Imagine the rod as a stack of point charges along its length. There is dq charge on dL length. What force acts on that charge? What force acts on the rod? Does it depend on the orientation of the rod with respect to the electric field?

ehild