On a Roll

This mural is funny because Mr. Curtis the physics teacher really does say that. He also calls himself a green entertainer-- he recycles his jokes! :)

There is a normal force from the slope perpendicular to the surface. The center of mass is moving down the hill (Vcom). The cylinder is rotating, so it has an angular velocity (omega w), which is related to its linear velocity because w=rv, where r is the radius of the cylinder. mg is the force of gravity acting straight down on the center of mass (it has to be broken into components that cancel the normal force and accelerate the cylinder down the slope). The friction force points up the hill because it is opposing the cylinder sliding. If it is rolling without sliding, there is static friction, so the equation is F<=uN, where u is the coefficient of friction and N is the normal force. If it's sliding, then it is kinetic friction and F=uN.