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## Homework Statement

At a construction site, a small crane is raising two boxes of nails on a plank to the roof. One box has already been opened and is half full, while the other box is new. The boxes, including the nails, weigh 10 kg and 20 kg, respectively, and are both the same size.

c) If the coefficient of kinetic friction is 0.3, how fast will the boxes accelerate along the plank, once they start to slide?

## Homework Equations

a = mgsinΘ - mgcosΘ(μk)

## The Attempt at a Solution

a = mgsinΘ - mgcosΘ(μk)

= g(sinΘ-cosΘ(μk))

= 9.8sin(21.8) - 0.3cos(21.8)

=0.91m/s^2

Note that in the previous part of the question, I found that the angle at which both boxes slide are at 21.8 degrees. I went on yahoo answers and saw 2 people solve this question using the equation above. I'm really confused as to how that equation was formed. In the previous part to this question, to find the angle at which the boxes slide, I used Fnetx = mgsinΘ - mgcosΘμs. The mass and gravity cancelled out so I just solved for the angle that way. Not sure why that equation was used once again for this question. Is this the right formula and if so why?