f1 + f2 + f3 + f4 + ... = m*a. I hope you know this, the sum of forces in one direction is the mass times the acceleration of the body in that direction.
For gravity, f1, the force is m*g*sin(20°). Accleration from gravity is g. Use the proper sign of course to get gravity pulling in the proper direction. The angle is there to adjust for how much gravity is pulling parallel to the incline. Check: when the angle is 0°, the ground is level, and gravity should have no acceleration horizontally, thus f1 = m*g*sin(0°) = 0. Double check: when the angle is 90°, gravity should have its full effect as the object is in free fall, thus f1 = m*g*sin(90°) = m*g. Yep, in both cases the angle works as it should.
For friction, f2, the force is 0.090*m*g*cos(20°). The angle is there to adjust for how much gravity is pushing directly perpendicular to the incline. Check: When the ground is horzontal (0°), the friction should have its full effect: f2 = 0.090*m*g*cos(0°) = 0.090*m*g. Check: When the angle is (90°), there should be minimal contact and the body should fall without friction being a problem (assuming it isn't glued on in some form, but then the coefficient of friction would be infinity), thus f2 = 0.090*m*g*cos(90°) = 0. Yep the angle works as it should.
Solve for the acceleration.
Use a= dv/dt and v = dx/dt to solve for the velocity at the end.
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