Answer:
1.045 m from 120 kg
Step-by-step explanation:
m1 = 120 kg
m2 = 420 kg
m = 51 kg
d = 3 m
Let m is placed at a distance y from 120 kg so that the net force on 51 kg is zero.
By use of the gravitational force
Force on m due to m1 is equal to the force on m due to m2.
![(Gm_(1)m)/(y^(2))=(Gm_(2)m)/(\left ( d-y \right )^(2))](https://img.qammunity.org/2020/formulas/physics/high-school/bswa19w4brtj4faqgty8j9xhqsshox8q4d.png)
![(m_(1))/(y^(2))=(m_(2))/(\left ( d-y \right )^(2))](https://img.qammunity.org/2020/formulas/physics/high-school/q08t4tcsgfze30czsl72q7kflcqdai3dgf.png)
![(3-y)/(y)=\sqrt{(7)/(2)}](https://img.qammunity.org/2020/formulas/physics/high-school/63viht1smamapu830pjbjtit2wb3wqmvkq.png)
3 - y = 1.87 y
3 = 2.87 y
y = 1.045 m
Thus, the net force on 51 kg is zero if it is placed at a distance of 1.045 m from 120 kg.