Calculating Coefficient of Kinetic Friction for Inclined Plane Problem

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The problem involves calculating the coefficient of kinetic friction for a block sliding down an inclined plane with another block on top, connected by a cord. To solve it, Newton's second law is applied, but confusion arises regarding the direction of the frictional force between the two blocks. It's essential to define a coordinate system to clarify the motion; the block on the incline is attempting to slide down, while the plank opposes this motion. Understanding that friction acts against the direction of sliding is crucial for accurately determining the forces at play. The discussion emphasizes the importance of correctly identifying the frictional forces to solve for the coefficient of kinetic friction.
SanityMan
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Okay, here is the problem I am working on...

A block of a certain weight(w2) is sliding down an inclined plane at angle theta. There is a plank on top of the block with a certain weight (w2) that is attacked to a cord at the top of the plane so that it does not move.

I need to find the coefficient of kinetic friction in terms of the masses of each block and the angle theta. The coeffecient is the same between both surface contacts.

I am using Newton's 2nd law and I'm stuck with the friction. In my free body diagram I am not sure which direction the force of friction is going between the block and the plank nor how it affects both.
 
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Since you're trying to figure out directions, hopefully you defined a coordinate system first. After you've done that, think about this: in what direction is the block trying to slide past the plank? Friction always opposes this motion Likewise for the block sliding down the inclined plane.
 
Its better to say that Friction is a force that opposes sliding.
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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