How is this done?(physics/laws of motion)

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1 Answer
Gió
Jan 28, 2016

I think a) 15kg

Explanation:

Let us consider all the forces involved:
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Where:
W="weight"=mg
f="friction"=mu_sN
N="normal reaction";
T=tension.
and apply Newton`s Second law to the two blocks, CA and B with the condition that they must stay at rest (a=0).
We get:
for CA (along horizontal and vertical):
T-f=0
N-W_x=0
for B:
T-W_B=0

Let us expand a bit the three expressions:
T-mu_sN=0
N=(m_c+m_a)g
T=m_Bg
Let us use the fact that we have the same tension and substitute the third equation into the first and use the expression for N to get:
m_Bcancel(g)-mu_s(m_c+m_a)cancel(g)=0
and with our data:
5-0.2(10+m_c)=0
so m_c=15kg

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