Why Is µ Equal to tan θ in Friction Calculations?

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SUMMARY

The equation µ = tan θ is derived from the balance of forces acting on a block placed on an inclined wooden plank. When the plank is lifted, the frictional force (Ff) equals the component of the gravitational force acting parallel to the incline, expressed as Ff = -mgsinθ. This leads to the conclusion that the static friction coefficient (µ) is equal to the tangent of the angle of inclination (θ), as shown in the equation µ = (-mgsinθ) / (-mgcosθ). The block remains stationary, indicating that the frictional force counteracts the gravitational component along the incline.

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Homework Statement



Deriv µs = tan θ
For when a block is put on a wooden plank and the plank is lifted at one side.

Homework Equations



tan = sin/cos
Ff = µFn

The Attempt at a Solution


Ff = µFn
-mgsinθ= µ (-mgcosθ)
µ =(-mgsinθ) /(-mgcosθ)
µ = tan θ

Where i am confused is why Ff = -mgsinθ. Is it because the block has an acceleration of zero when falling down the plank?
 
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1010 said:
Where i am confused is why Ff = -mgsinθ. Is it because the block has an acceleration of zero when falling down the plank?
Yes, the block is assumed not to move, and so the frictional force must balance the component of the weight of the block parallel to the plank.
 
The book claims the answer is that all the magnitudes are the same because "the gravitational force on the penguin is the same". I'm having trouble understanding this. I thought the buoyant force was equal to the weight of the fluid displaced. Weight depends on mass which depends on density. Therefore, due to the differing densities the buoyant force will be different in each case? Is this incorrect?

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