Using the ideas of torque and center of gravity, explain why a ball rolls down a hill.
The center of gravity of the ball is perpendicular to the earth. If it is on a level surface, the center of gravity coincides with where the ball touched the surface. Thus, there is no lever arm and 0 torque to start rolling.
If the surface is at a slope, the point of contact of the ball on the surface is NOT in line with the center of gravity, The distance between the two lines is the lever arm, through which the mass of the ball is able to exert torque --> THE BALL CAN ROLL!