The Plank Experiment

As shown in the image above, a physicist is experimenting upon a sliding object on a smooth inclined surface. He intends to find out at which angle of inclination $\theta$ does the object placed start to slide down. In other words, at which angle the force of gravity overcomes the force of friction.

Assume that the yellow plastic weighs $100$ $g$ . He observes that the angle of inclination at which the plastic slides down is $15^{o}$ .

Now he adds an additional $500$ $g$ to the plastic object. At which angle of inclination will the object slide down?