[quote author=leeyiren link=topic=1960.msg7234#msg7234 date=1285386022]


To move an object, I have to apply a force.
[/quote]
actually, if object is already in uniform motion, no external force is require. or simply Newton 1st Law
to change the motion of an object, yes a force is needed or simply Newton 2nd Law, F[sub]net[/sub] = m*a


[quote author=leeyiren link=topic=1960.msg7234#msg7234 date=1285386022]
But F=ma
Hence every time I apply a force there will be acceleration.
So how do you move an object at constant velocity?
[/quote]
if we look at F[sub]net[/sub] = m*a
let P be the push force
f be the friction force opposes motion v
then F[sub]net[/sub] = m*a
imply P - f = m*a
constant v imply a = 0,
therefore, it is possible if P - f = 0

how can we relate this to real life situation?
when a ball falls under gravity from a tall building for a long time, terminal velocity is reached
let mg is weight of mass, m under gravity g
let D be drag force on mass m
using F[sub]net[/sub] = m*a
mg - D = m*a
at terminal velocity, a = 0
mg - D = 0



[quote author=leeyiren link=topic=1960.msg7234#msg7234 date=1285386022]
PS: Why are there so few ppl are posting questions in this section?
[/quote]
no everyone leaves a question,but there many views.  ;D