Question

Three masses 5 kg, 4 kg and 7 kg are aligned in a straight line as shown below. Find the net gravitational force on 4 kg.

• A. Zero
• B. 33.5 N
• C. 46.69 N
• D. 13.19 N

Answer 1

The correct option is D

13.19 N

5 kg and 7 kg masses attract the 4 kg mass in opposite direction. So, the net force is subtraction.
The gravitation force due to 5 kg on 4 kg is:
F = $\frac{GmM}{d^2}$
F = 5 $\times$ G = 33.5 $\times$ ${10}^{-11}$ N (The direction of force is towards 5 kg mass)
The gravitational force due to 7 kg on 4 kg:
F = $\frac{GmM}{d^2}$
F = 7 $\times$ G = 46.69 $\times$ ${10}^{-11}$​N (The direction of force is towards 7 kg mass).
Since the force on 4 kg masses due to 5 kg mass is opposite to the force due to 7 kg on 4 kg mass. The net force is the subtraction of both the forces.
Net force on 4 kg mass = (46.69 - 33.5) $\times$ ${10}^{-11}$​​ N = 13.19 $\times$ ${10}^{-11}$ ​​N.