A 2-kg block is on top of a 5-kg block, which sits on a frictionless surface. A horizontal force of 21 N is applied to the bottom block. If both blocks move together (no slipping), what is their common acceleration?
A3.0 m/s²
B4.2 m/s²
C10.5 m/s²
D2.1 m/s²
Explanation
📌 Treat both blocks as a single system with total mass 7 kg. a = F/m_total = 21/7 = 3.0 m/s². Distractor B used only bottom block's mass (21/5 = 4.2); C used top block's mass (21/2 = 10.5); D used difference.