Question

A particle moves under constant external force F = F₀ ˆi. It is also subjected to a drag force, F_D = −C_DV. How will the particle move, qualitatively speaking? Describe in words before using equations. What will the particle's final velocity be? What is the sum of the external forces at the final velocity you found?

Answer 1

Imagine the particle is starting from rest. The constant force will give accelerate it and it will gain some velocity. After getting the velocity, the drag force will start to act in opposite direction. So the net force will be less than the constant force, but the particle will accelerate increasing the velocity of the particle. Which in turn increases the drag force.... This process will go on untill the net force becomes zero i.e. the drag force equates the constant force. Then the particle will move with uniform velocity (since no force is acting) . This velocity is called terminal velocity or final velocity.

The particle will move in final velocity until something stops it.