Finding Theta Angle of a Pendulum in a Moving Car | Accelerometer Question"

[Frankenstein19]

Homework Statement

A small mass m hangs from a thin string and can swing like a pendulum. You attach it above the window of your car. When the car is at rest, the string hands vertically. What angle θ does the string make when the car accelerates at a constant a=1.20m/s^2?

Homework Equations

I assume a relevant equation is ƩF = ma

The Attempt at a Solution

This is already solved in my book but I was trying to do it by myself[/B]
I know I need to find ƩF = ma_y and ƩF = ma_x

And I know that ƩF = ma_y is F_Tcosθ -mg=ma_y

And that ƩF = ma_x is F_Tsinθ =ma_x

My book says that F_Tcosθ -mg =0, and my question is why? is a_y=0? How can I know that?

 

[Doc Al]

My book says that FTcosθ -mg =0, and my question is why? is ay=0? How can I know that?

Presumably the car is accelerating horizontally, not vertically.