Answer:
The  length of the moment arm is 19 cm
Explanation:
Given;
The total length of the ruler, L = 40 cm
a mass is hung from its center mass, at 21 cm
a force, F is applied at 40 cm
The length of the moment arm is calculated as;
the center mass of the ruler is at 20 cm mark
0                   21cm             40cm              Â
--------------------------------------------------------------------
                   ↓         r         ↓
                   M                   F
Moment about F, Â = Fr
The length of the moment arm is = r
r = 40 cm - 21 cm
r = 19 cm
Therefore, the  length of the moment arm is 19 cm