Work-energy theorem

If a body moves to further, there must be change in its kinetic energy, how it happens? only when an external force is applied on the system of body. (or)When work is done by the unbalanced or resultant force on the body is equal to change in its kinetic energy.
But this theorem is applicable only when constant force is applied on the body. Therefore, there must be no change or no variation in the force, this is remember
We know the Newton's second law, that F=ma
The work done by the force W=F.S=ma.s
Here v and u are the final and initial velocities of the body when resultant or unbalanced force applied on the body. Do you have doubt? why constant force causes to change the velocity of the body? Here one important point to remember is that when constant force applied continuously, then the body overcomes the frictional force and viscous force gradually, that causes resultant force to accelerate the body, so that the body gets the velocity v.