A100-kg box is placed on a ramp. as one end of the ramp is raised, the box begins to move downward just as the angle of inclination reaches 25°. what is the coefficient of static friction between box and ramp?

Explanation:

Just before the box starts moving, according to Newton's first law, we have:

The sine of the angle (25°) of a right triangle is defined as the opposite cathetus () to that angle divided into the hypotenuse (W):

The cosine of the angle (25°) of a right triangle is defined as the adjacent cathetus () to that angle divided into the hypotenuse (W):

Recall that the maximum frictional force is defined as:

Replacing (3) in (1) and (4) in (2):

Replacing (7) and (6) in (5) and solvinf for :

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