M.5.1 Paradox of Forces

A 1 kg mass weighs 9.8 N as checked by a spring scale. Two 1 kg masses are hooked on the ends of a string which is passed over a pair of pulleys, and the spring scale is placed in the center of the string to measure the tension as shown. The scale is covered with a cloth to hide its reading, and the class is then asked to predict whether the scale will read 0. 9.8, or 19.6 N.