Aforce that opposes motion in moving parts is friction.

a. true
b false

A force that opposes motion in moving parts is friction. A

This is true. Friction is the resistance that one surface or object encounters when moving over another. The definition described above is worded differently but both definitions still mean the same thing.

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Lexy is correct because angle r is between the incident wave,  n  = 0, and a diffracted wave.