Non-acceleration and 0 force If a mass is moving at the rate of 30/ft. per minute for 5 minutes on a straight line and it strikes a second stationary mass and effects a change of position to this second mass, then we know from F=ma that the force is 0 since the acceleration is 0. Then why do we say because it is 0 that there is no force when in fact the first mass changed the position of the second.
 A: We don't say it is $0$, why is $F=0$? If the second mass was stationary, then sometime after being struck by the first mass it is in a new position, it must have undergone acceleration $\to F\neq0$ precisely because $F=ma$.
A: We never say force acting is zero.We just say there is no external forces to the two masses(external force to the system is zero).This is because only if the external forces are zero(or their effect should be negligible), momentum conservation can be applied.But the force applied by one mass on the other(internal forces) is not zero.Since this is the only force acting and it acts for the same time(a short time) on both the bodies, we say the magnitude impulse on one body equals the other(opposite in direction) and thus the momentum change is also equal in magnitude but opposite in direction(since impulse equals change in momentum).
A: It appears you are mixing the force at one given time with the acceleration at a different given time.
Don't feel bad; it is a common beginner mistake to think that there is "one force" and "one acceleration" in a given problem.  Part of learning physics is learning how to treat values from different times, as well as values from different objects.
During the 5-minute interval, the first mass stays at the same velocity.  This means its acceleration is zero.  By $F_{net}=ma$, the net force on this object during that time interval is zero.
During this same time interval, the second mass stays at rest.  Its acceleration is zero, and thus the net force on it is zero.
The collision occurs at a different time.  Neither force nor acceleration care about what happened during the 5-minute interval, they only depend on what is happening now.  The first object is now slowing down -- a form of acceleration -- so $a \ne 0$.  This means the net force on the first object $F_{net}=ma\ne 0$.  This net force comes from the second mass pushing on the first mass.
By the Third Law, during the collision the first mass pushes back on the second mass.  This creates a net force on the second mass, which accelerates the second mass during the collision.
A: Hey when the collison occur the force will act on it this force will persist until they are in contact which is given by 
F =(m1v1-m1u1)/t=(m2v2-m2u2)/t=impulse/t 
 Here v1 v2 are final velocities and u1 u2 are initial velocities t is the time upto which force acts or the time of contact
Here we can see that force is not 0 it will act during collison for very short period of time .Here F is the average force acting for time t.This force will be equal and opposite for both particles and force will cause the change in momentum
