Question

A 20 kg sphere is at the origin and a 10 kg sphere is at x = 20 cm. At what position on the x-axis could you place a small mass such that the net gravitational force on it due to the spheres is zero?

Answer 1

The small mass should be placed at x = 11.72 cm or x = 68.28 cm

Step-by-step explanation:

Let the small mass be m and position on x axis be y.

A 20 kg sphere is at the origin.

Distance to 20 kg mass = y

A 10 kg sphere is at x = 20 cm

Distance to 10 kg sphere = 20 - y

We have forces between them are equal

We have gravitational force   

                    $F=\frac{GMm}{r^2}$              

          Where G =  6.67 x 10⁻¹¹ N m²/kg²

                      M = Mass of body 1

                      M = Mass of body 2

                      r = Distance between them

Here we have

            $\frac{G\times 20\times m}{y^2}=\frac{G\times 10\times m}{(20-y)^2}\\\\800-80y+2y^2=y^2\\\\y^2-80y+800=0\\\\y=68.28cm\texttt{ or }y=11.72cm$

So the small mass should be placed at x = 11.72 cm or x = 68.28 cm