Work Energy Theorem/Problems

The following problems are based on the topic .
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 with 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 ,
:: W_{net} = \Delta KE 
::or W_T + W_{mg} + W_F = 0 
::as \Delta KE = 0 
::because K_i = K_f = 0 
Further, W_T = 0, as tension is always perpendicular to displacement.
 W_{mg} = -mgh or W_{mg} = - mgl(l - \cos{\theta} )
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.
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Category:Mechanics