Question

what is the formula of external net force

Answer 1

Answer:

We can see from Figure that the engine thrusts add, while friction opposes the thrust. In equation form, the net external force is Fnet=4T−f. Substituting this into Newton's second law gives Fnet=ma=4T−f. Using a little algebra, we solve for the total thrust 4T: 4T=ma+f.