Kevin S. Huang

2010: Problem 10

Topic: Dynamics
Concepts: Newton’s laws

Solution:

Applying Newton’s 2nd law to the lower block, we have

F-f_1-f_2=m_2a_2

Recall kinetic friction is given by $f=\mu N$ so

f_1=\mu m_1g

f_2=\mu (m_1+m_2)g

Substituting into the first equation,

F-\mu (2m_1+m_2)g=m_2a_2

a_2=\frac{F-\mu (2m_1+m_2)g}{m_2}

so the answer is A.