The free body diagram (FBD) for block \( m_1 \) represents all the forces acting on it. These include:
- Tension (\( T_1 \)): The force exerted by the string pulling upward on \( m_1 \).
- Weight (\( m_1 g \)): The gravitational force acting downward on \( m_1 \).
- Acceleration (\( a \)): The block \( m_1 \) accelerates upward, as shown by the arrow labeled \( a \).