"An object of mass m is tied to a string of length L and a variable horizontal force is applied on it which starts from rest and pulled slowly until the string makes an angle $\theta$ with the vertical. Find the work done by force F."
All the solutions that I have seen in such type of questions is by using work-energy theorem. $$\Delta U+ \Delta K= W $$
Here they take $\Delta K$ to be zero and calculate the change in potential energy and give the answer which turns out to be correct.
But I can't understand how $\Delta K$ is zero. Initially the body is at rest and some force is applied due to which it starts moving. Thus it gains some velocity.So there should be some change in Kinetic energy.