Since the car is moving with constant speed, its acceleration is zero.
According to the second Newton's law, the net force acting on the car equals mass times acceleration:
`F = ma` , where F is the net force, or the vector sum of all forces applied to the car.
Since acceleration is zero, the net force is zero. In this case, the net force equals the sum of the force making car move (produced by the car's motor) and the force of friction, which opposes the motion of the car and is directed opposite to the 2.5 KN force.
Therefore, the force of friction has to equal 2.5 kN in order to balance out the force making the car move.
The force of friction equals 2.5 KN.