Question

A 12N block rests on a plane inclined 15 degrees. If the block is not moving what is the force of friction acting on that block?

Answer 1

Second Newton's law in projection on x-axis:

$$mg\sin\alpha = F_{fr}$$

where $mg = 15N$ is the weight of the block, $\alpha=15\degree$ is the angle and $F_{fr}$ is the frictional force. Thus, obtain:

$$F_{fr}=15\cdot \sin15\degree \approx 3.9N$$

Answer. 3.9 N.