A car is moving with a constant speed of 20m s ^ - 1 in a circular...

Let the frame of reference to be attached to the car with x-axis along horizontal direction and y-axis along vertical direction. Since the frame of reference is a rotating frame, a centrifugal force term should be added while applying Newton's second law.

Let $\theta$ be the angle made by the pendulum with the vertical.

Applying Newton's second law for horizontal direction:

Tsin\theta - \dfrac{m{v}^{2}}{R} = 0$(since the acceleration of the pendulum in the rotating frame is 0).</p><p>$∴ Tsin\theta = \dfrac{m{v}^{2}}{R}$..........(1)</p><p>Applying Newton's second law for vertical direction:</p><p>Tcos\theta - mg = 0$

$⇒ Tcos\theta = mg$ ..........(2)

Dividing equation 1 by equation 2:

$tan\theta = \dfrac{{v}^{2}}{Rg} = \dfrac{{\left(20\right)}^{2}}{\left(40\right)\left(10\right)} = 1 ⇒ \theta = \dfrac{\pi}{4}$