Hey, just wanted to check my method. 1)What is the distance between 2 protons experiencing a mutually repelling force of magnitude 4*10-11? so for this, we have to use the formula for coulombs law--F=kq1q2/r^2 we know F and k. And q1=q2= 1.602*10-19. hence we can find r. 2)Two point charges, 4*10-5 C and -1.8*10-5 C are placed 24 cm apart. What is the force on a third small charge of -2.5*10-6 C, if it is placed on the line joining the other two midway between the originally given pair of charges. For this what i did was: Find Fq3q1. The force for this will be in the left direction. then i found Fq3q2. Force for this will be in the right direction. Then i found the sum of the forces, and that is the force on q3. Note: q3 is in the middle of q1 and q2. distance between q3 and q1 is 0.12 m. the same is tru for the distance between q3 and q2. Sounds right?