Originally Posted by **Zman** I'm a bit confused on paragraphs 3 and 4
the paragraphs about force of gravity and friction
can you go into more detail please |

Gravity is a force between two masses: In this case we are talking about the weight of the ball. This force always points directly to the floor, and has a magnitude of w = mg. Since the mass is moving down the incline we need to break the force into components along the incline and perpendicular to the surface. Make a sketch of it and use some basic trigonometry.

Friction is a retarding force that points in the direction opposite to the direction of motion. There are two types: static and kinetic. From the problem statement we are taking the motion to be down the incline, so friction will act up the incline with a force of $\displaystyle f_k = \mu _k N$ where $\displaystyle \mu _k$ is the coefficient of kinetic friction and N is the normal force from the incline.

-Dan