Force

We know that F = ma.
If I apply 10 N force (constant force) on an object of 1kg, it has acceleration of 10ms^−2.

So the situation is as follows:
At time = 0 sec, the force is just appled onto the object. The velocity is of the object is still zero.
At time = 1 sec, the velocity of the object is 10m/s.
At time = 2 sec, the veloicty of the object is 20m/s.

Acceleration = 10m/s/s means that:
Over one second, the velocity of the object will increase by 10m/s.

2010-07-31 23:51:02 補充：
Correction:
At time = 0 sec, the force is just appled onto the object. The velocity of the object is still zero.

The formula F = ma comes from Newton's Second Law of Motion. It states that the rate of change of momentum of a body is proportional to the applied force and takes place in the direction at which the force acts.

Mathematically, the Law can be written as,
F = m(v-u)/t
where m is the mass of the body
u and v are the initial and final velocity of the body
t is the duration of time at which the force acts.

By transposing the time t to the left hand side, we have,
F.t = m(v-u)
[this is just the relationship between impulse and change of momentum]

Notice that t is the "duration of time the force acts". If t = 0 s, it just means that the force doesn't act on the body at all. Surely, the body would have zero acceleration and zero velocity (i.e. there is a zero change of momentum).

Of course, if the force only acts at a short, but not zero, duration, say 0.01 s for example, then there is a finite impulse given by F x (0.01), and which would give a small change of momentum.

1. it happens spontaneously
it should happen instantly so that the time is zero

2. thus the increase in velocity = 10*0
this is wrong. v=a.t can be only used in uniform acceleration
in general case, acceleration,a = dv/dt.