A 40 cm ruler with non-uniform mass is hung from its center of mass, at 21 cm. A force is applied at the 40 cm mark of the ruler. The length of the moment arm is . 40 19 21 None of the above

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                            

                                      ↓                 r                 ↓

                                      M                                    F

Moment about F,  = 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