A pendulum of mass 1.0 kg is suspended from the roof of a car travelling on a level road. An observer in the car notices that the pendulum string makes an angle of 10 deg with the vertical. What is the acceleration of the car?

I drew the FBD diagram (attached) and I wrote the vertical and horizontal force equations:

Vertically: Tcos 10 deg=mg

Horizontally: Fc=mv^2/r=Tsin10deg.

So I got he equation mgtan10deg=mv^2/r after putting the equations together. But I don't know the radius of the pendulum, so I don't know how to solve the problem.

Thanks!

Never mind, I'm not supposed to use centripetal force since the pendulum is not going in a circle. Any hints on how to start the problem?