Aball of mass 2.0 kg moves with a velocity of 4.0 m/s. a force of -6.0 n is applied for 3.0 sec. find the final velocity.

M = 2kg, u = 4 m/s, F = -6 N, t = 3 s

From Newton's second law of motion,
F = m * a
   = m * (v-u)/t
Therefore,
-6 = 2 * (v-4)/3
(v-4) = -9
v = -5 m/s

(Note : The negative velocity indicates that the ball has a net velocity in the direction opposite of it's initial motion, i.e it is stopping)

It is increased due to the doppler effect. (apex)