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Emil233
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Homework Statement
An A block of mass 8 kg, is lying over an horizontal table, joint to a bucket of mass 1 kg, by a rope which passes by a pulley. The masses of the rope and the pulley are insignificant. Over the A block is lying a B block, of mass 2 kg. The coefficient of static friction, and kinetic friction, between the A and B blocks, and between the A block and the table, are static µ = 0.5, and kinetic µ = 0.4
1) If the bucket is loaded with 10 kg of sand, calculate the acceleration of the blocks and the bucket.
2) Show that for this load of sand, the B block does not slide over the A block.
3) What is the maximum load of sand that you can put in the bucket, without making the B block slide over the A block?
[PLAIN]http://a.imageshack.us/img825/7114/problemax.jpg
Homework Equations
F = ma, Frk = µk N, Frs max = µs N
The Attempt at a Solution
1) First, making a free body diagram for the blocks (where m blocks is the mass of the 2 blocks, and m bucket is the mass of the bucket loaded with sand(:
i) ΣFy = 0 = N - m blocks * g
N = m blocks * g
ii) ΣFx = T - Frk = m blocks * a (T = tension of the rope, Frk = kinetic friction force, between the table and the blocks)
And for the bucket:
ii) ΣFy = m bucket * g - T = m bucket a
So, T = -m bucket a + m bucket * g
Now, replacing T in ii)
-m bucket * g + mbucket * g - Frk = m blocks * a
-m bucket * g + mbucket * g - Frk - m blocks * a = 0
Frk is µk N, and N = m blocks * g, so:
-m bucket * a + m bucket * g - µk * m blocks * g - m blocks * a = 0
a (-m bucket - m blocks) + g (m bucket - µk * m blocks) = 0
a = (-g (m bucket - µk * m blocks)/(-m bucket - m blocks))
Now, m bucket is 11 kg, µk is 0.4, m blocks is 10 kg, and g = 9.81 m/s², so
a = (-9.81 m/s² (11 kg - 0.4 * 10 kg)/(-11 kg - 10kg)) = 3.27 m/s²
2) Now, making a free body diagram for the B block (m b block is the mass of the B block):
ΣFy = 0 = n - m B block * g
n = m B block * g
ΣFx = Fr s = m B block * a
m B block is 2 kg, and a = 3.27 m/s², so
Fr s = 2 kg * 3.27 m/s² = 6.54 N
Now, the maximum static friction force that can move the B block, is given by µs * n, where n is the normal force between the B block and the A block, and is equal to m B block * g.
So: Frs max = µs m B block * g = 0.5 * 2 kg * g = 9.81 N
The maximum static friction force, 9.81 N, is greater than the static force that moves the B block, which is 6.54 N. So, the B block does not slide over the A block.
3) Given that Frs = m B block * a, there is a maximum acceleration (a max) that the blocks can have, without making the B block slide over the A block.
So: Frs max = m B block * a max
µs * n = m B block * a max
µs * m B block * g = m B block * a max
µs * g = a max
Now, using this acceleration, and making a free body diagram for the bucket:
i) ΣFy = m bucket * g - T = m bucket * a max
And for the blocks:
ii) ΣFx = T - Frk = m blocks * a max
iii) ΣFy = N - m blocks * g = 0
T = m blocks * a max + Frk
Replacing T in i)
m bucket * g - m blocks * a max - Frk = m bucket * a max
m bucket * g - m blocks * a max - Frk - m bucket * a max = 0
m bucket * g - m blocks * a max - µk * m blocks * g - m bucket * a max = 0
m bucket (g - a max) - m blocks * a max - µk * m blocks * g = 0
m bucket = (+µk * m blocks * g + m blocks * a max) / (g - a max)
m bucket = (+0.4 * 10 kg * g + 10 kg * a max) / (g - a max)
m bucket = (+0.4 * 10 kg * g + 10 kg * µs * g) / (g - µs * g)
m bucket = (+0.4 * 10 kg * g + 10 kg * 0.5 * g) / (g - 0.5 * g) = 18 kg
Now, 18 kg is the maximum mass of the bucket allowed. The bucket alone has a mass of 1 kg, so, the maximum load of sand, would be 17 kg.
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