Which type of mechanical wave needs a source of energy to produce it?
a. a transverse wave
b. a longitudinal wave
c. a surface wave
d. all of the above

How does the electric potential energy between two charged particles change if one particle is reduced by a factor of 3? a. increased by a factor of 3 b. it is decreased by a factor of 9 c. it is reduced by a factor of 3 d. it is increased by a factor of 9