Two blocks A and B, each of mass 2 kg, are in contact on a rough horizontal surface. Their coefficients of kinetic friction with ground are $\mu_{A}$ = 0.2 and $\mu_{B}$ = 0.4. A horizontal force 20 N is applied on A, pushing it towards B. Take g = 10 m/s². The contact force exerted by A on B is:
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Question

Two blocks A and B, each of mass 2 kg, are in contact on a rough horizontal surface. Their coefficients of kinetic friction with ground are $\mu_{A}$ = 0.2 and $\mu_{B}$ = 0.4. A horizontal force 20 N is applied on A, pushing it towards B. Take g = 10 m/s². The contact force exerted by A on B is:

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G1 Physics · Accepted Answer

Friction forces are f A = 0.2 × 2 × 10 = 4 f_A=0.2×2×10=4 f A ​ = 0.2 × 2 × 10 = 4 N and f B = 0.4 × 2 × 10 = 8 f_B=0.4×2×10=8 f B ​ = 0.4 × 2 × 10 = 8 N. Net external force on both =20-4-8=8 N, total mass=4 kg, so a=2 m/s². For B: contact N - 8 = 2×2, hence N=12 N.