A constant force acting on a body of mass 3.0 kg changes its speed from 2.0 m s-1 to 3.5 m s-1 in 25 s. The direction of the motion of the body remains unchanged. What is the magnitude and direction of the force?
The first equation of motion is given as :
v = u + at …….(i)
where,
v is the final speed,
u is the initial speed,
a is the acceleration of the body.
t is the time taken by the body.
Given,
Mass of the body, m = 3 kg
Initial speed of the body, u = 2 m/s
Final speed of the body, v = 3.5 m/s
Time taken by the body, t = 25 sec
From equation (i), 
⇒ 
From Newton’s second law of motion, the magnitude of the force is given as,
F= ma
= 3 kg × 0.06 m/s2
⇒ F = 0.18 N
The magnitude of the force is 0.18 N and since the acceleration does not change the direction of the motion. The direction of the force is in the same direction of the motion.