Force = F = 20 Newtons
Mass = m = 4 kg
initial velocity u = 0
acceleration a = force / mass = F / m
= 20 / 4 = 5 meters/sec²
velocity v = u + a t
v = 0 + 5 m/sec² * 10 sec = 50 m/sec
During the first 10 seconds when the force is acting on the body, the body accelerates continuously. That is its speed increases every instant. At 1 sec its speed is 5 m/s, at 2 sec, its speed is 10 m/s, at 3 sec, its speed is 15 m/s ....
when the force ceases to act at 10 sec, then the body continues to move along the straight line (as per the 1st law of Newton). it moves with a constant velocity of 50 m/sec. this is the inertia of motion.