[quote author=leeyiren link=topic=1960.msg7251#msg7251 date=1285513252]
In the case of a horizontal motion, if the push force=resistive force, will the ball even move, let alone move at constant velocity?  ???
The original starting condition determines the outcome of the motion.

your answer is true if the initial velocity = 0.

however, it is agreed by the physics community that if the initial velocity of the object is non-zero, it will continue in the state of motion due to the evidences of data with real life experiments.

so even if the push=resistive force at the beginning,

using Fnet = m*a
P - f = m*a
0 = a
but v is the starting velocity, thus it could be still traveling at some speed without any direction change  [color=blue](this will happen but not likely to be observed because it is an idealized motion)
to extend this knowledge to real life situation, you could relate this to a ice sledge on very smooth ice horizontal ground.
when the ice sledge is already moving at constant velocity, say v=10 m/s

ideally does not require any pushing force to continue at constant v

in the presence of air resistance, P - f  = m*a where f >0
if P =0
then -f = m*a, therefore, it will -ve = a and it will decelerate. [color=blue](this is observed in everyday life motion)[/color]

in this case, to maintain v = constant,
P - f = m*a
P- f = 0
P = f so a small Push P of equal magnitude to f is required for Constant v [color=blue](this is observed in everyday life motion like the motion of say a bicyle in constant v on horizontal ground)[/color]