Question

Three charges q, 2q, and 3q are arragend to form a equileteral triangle. A charge Q is placed at the center of the triagle. What´s the Net Force about Q. Give the modulus and the direcction of the force.

Answer 1

The forces are 120 degrees apart. Lets let the q be the reference charge for angles, measured from the center, and postive angles toward 2q is at 120, and 3q is at 240.

Force=kq/s^2 (q at 0 + 2q at 120 + 3q at 240)

now, but the magic of superposition, we can take out q from each corner, because the resultant force from those three equally spaced charges is zero)
Force=kq/s^2 (q at 120 + 2 q at 240)
=kq^2/s^2 (1 @ 120 + 2@240)

= kq^2/s^2 sqrt5 is the magnitude
direction(1sin120@180+1cos120@180+2sin240@90+2 cos240 @90)
combine like terms in the ( ) YOu have now components which are 90 degrees apart, which can be added to find the direction. Draw a sketch of this, you can see it visually.