When rolled down a mountainside at 7.0 m/s, the horizontal component of its velocity vector was 1.8 m/s. what was the angle of the mountain surface above the horizontal? 75â° 33â° 57 â° 15â°?

Given V = 7 m/s; Vx = 1.8 m/s.

Drawing a triangle, V becomes the hypotenuse and Vx becomes the adjacent side. From that image we derive the formula from the properties of a right triangle. We get cos x = Vx / V where x is the angle we are looking for.  Substituting the given values to the formula, we get cos x = 1.8 m/s / 7 m/s = 0.257. x = cos^-1 (0.257) = 75 degrees