One last question:
If a 12lb. block is slowly tilted from horizontal when the plank reaches 30 degrees the block begins to slide.
To find the coefficient of the static friction you take the tangent form of 30 degrees?
To find the friction force you multiply the coefficient of the static friction times the weight?
But how do you find the Fn?
This looks like a more carefully worded version of a questiona already answered
See the answer to http://www.jiskha.com/display.cgi?id=1201404660
To find the normal force (Fn) in this scenario, you need to consider the forces acting on the block. The normal force is the force exerted by a surface to support the weight of an object resting on it.
In this case, there are two forces acting on the block: the weight (mg) and the vertical component of the normal force (Fn).
The weight (mg) can be calculated by multiplying the mass (m) of the block by the acceleration due to gravity (g).
Once you know the weight, you can find the vertical component of the normal force (Fn) by applying the concept of equilibrium. At the point where the block is just about to slide, the vertical component of the normal force should be equal in magnitude and opposite in direction to the weight (mg) of the block.
Hence, to find the normal force (Fn), you need to find the weight (mg) first by multiplying the mass (m) of the block by the acceleration due to gravity (g). Then, the normal force (Fn) will have the same magnitude as the weight but in the opposite direction.
Remember that the coefficient of static friction (μ) is related to the maximum value of static friction, which is given by the equation Fs ≤ μFn. This equation indicates that the static friction force (Fs) is proportional to the normal force (Fn), and the coefficient of static friction (μ) determines the proportionality constant.
Once you have the normal force (Fn) and the coefficient of static friction (μ), you can calculate the maximum value of static friction by multiplying these two values together: Fs = μFn.