The student then decided to use the principle of moments to measure the weight of the metre rule. He suspended the ruler from a loop of cotton placed at the 30 cm mark (P), and balanced the ruler by suspending a 50 g mass from the 6 cm mark as shown in the diagram below. Р 50 g W (1) Calculate the weight of the 50 g mass and the moment it exerts about P. (g = 10 N/ kg) (2) If the weight of the ruler is W, what is the moment of this force about P? (3) Use the principle of moments to calculate the value of W. (4) Calculate the upwards force exerted on the ruler by the cotton at P, and explain why this force does not have to be included in the calculation of W.