Alright so there is this positively charged ball hanging from a string, and the ball weighs 9.2 grams. To the right is a flat metal plate which has a negative charge, causing the ball to be pulled because of the attraction and to hang at a slight angle from perpendicular, e.g. when it is hanging loose without the charged plate there. The vertical distance is 71.7 centimeters and the horizontally displaced distance is 4.3 centimeters. How much force is pulling on the ball?
I assured my son that I could figure this out. Sure. He had been stumped the whole morning, and the fact that he even asked for my help was a sign that it was indeed a tricky problem. I struggled and strained my brain cells, but quickly realized I was not as intrepid nor sharp as I used to be in the good old student days. I got a headache and felt frustrated. Drats.
But I was not about to admit it nor was I going to give up. Never ever.
So I retired to my study, sat down at my desk and drew pictures with arrows and force vectors and so forth. I checked it and double checked it. The answer came to me in about thirty minutes or so. In the past I could have solved this trivial physics problem in no time. With age time slows down. But I had the answer, and it was right.
The answer is: 0.005407 N (= 4.3 cm / 71.7 cm x 0.0092 kg x 9.8 N/kg)