A child sits on a dock and notices 8 wavelengths pass the end of the dock in 4 seconds. What is the frequency of the waves passing the dock?

a charge 3q is at the origin, and a charge −2q is on the positive x axis at x=a. part a where on the x-axis would you place a third charge so it would experience no net electric force? enter an expression for the exact answer, which will involve a square-root sign. (do not enter an approximate decimal answer.)

the body's situation won't change the forces are equal opposit and on the same body

i don't understand. explain it clearly