Using Vector in Determining Period of Pendulum Inside a Moving Train

[science_world]

Hi all, i have a question relating to the period of pendulum. I got this question:
A bob of mass 0.5 kg is suspended by a string from the ceiling inside a train moving on a straight level rail (to the right). If the train has an acceleration of 0.2g, what is the tension in the string when the bob is at rest with respect to the train? (picture 1)

Someone explained to me that we must draw the tension as a vector joining the force by train to the pendulum and the pendulum's weight (picture 2). I don't understand. Isn't the pendulum supposed to be the resultant of those 2 forces (picture 3) and not the force by train who become the resultant (picture 2)?

Please do explain. Thanks a lot.

 

[kuruman]

Isn't the pendulum supposed to be the resultant of those 2 forces (picture 3) and not the force by train who become the resultant (picture 2)?

The net force is not the tension. The forces acting on the pendulum bob are gravity and tension. They add together to give s horizontal net force that you have labeled as F in your diagram. So you add a tension that is "up" and "to the right" to the weight that is "down". The "up" part of the tension exactly cancels the "down" weight and all that's left is the "to the right" part of the tension. There is no force exerted by the train.