The following problems are based on the topic **Work Energy Theorem**.

**Example WET.1**: An Object of mass *m* is tied to a string of length *l* and a variable force *F* is applied on it which brings the string gradually at angle θwith the vertical. Find the work done by the force *F*.

**Solution WET.1**:
*STEP 1:* Draw a rough sketch if possible
*STEP 2:* Solution
In this case three forces are acting on the object:

- Tension (
*T*) - weight (
*mg*) and - applied force (
*F*)

Using Work Energy Theorem,

- or
- as
- because

Further, , as tension is always perpendicular to displacement. or

*NOTE:* the applied force *F* is variable. So, if we do not apply Work energy theorem here we will have to use method of integration.