Physics Question! Help!

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.

a) If the coefficient of static friction is 0.4, draw an FBD for each box of nails and use it to calculate the angle at which each box behinds to slide.

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

Help, I can't seem to get it started! For part a) you don't need to draw a FBD, I just don't know how to find the angle.

Thank you so much!

Can't do any FBD on here, but I can do the rest!

a) I will just do the calculations for the 10kg box, since they will both have identical answers for the angle.

Gravity = 10kg * 9.81m/s^2 = 98.1N
Normal Force = 98.1N * Cos (A) Where A is the angle off of the horizontal that the plane will be.
Friction Force = 98.1N * Cos (A) * 0.4

Tangential force (pushing it along the plane) = 98.1N*Sin (A)

Then equate Friction force to Tangential force;

98.1N * Cos(A) *0.4 = 98.1N*Sin(A) Cancel the 98.1N out of both sides
Cos(A) * 0.4 = Sin(A) note: sin/cos = Tan
0.4 = Sin(A)/Cos(A)
0.4 = Tan (A)
A = 21.8 degrees off the horizontal

b) The equations from a can be used, except the new friction force = 98.1*Cos(A)*0.3
Acceleration force = Tangential - Friction
Acceleration force = 98.1 Sin(A) - 98.1 Cos(A)*0.3
Acceleration force = 9.108N; mass = 10kg
Acceleration = 9.108/10 = 0.91m/s^2

Once Again, the mass cancels out and you would get the same acceleration for both the boxes.

Thanks for the Best Answer!

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