Connected Bodies And Tension

Question

Two blocks of masses 3 kg and 2 kg are connected by a light string and pulled along a smooth horizontal table by a force of 20 N applied to the 3 kg block. What is the tension in the connecting string?

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Using NCERT Class 11, Chapter 5 (Laws of Motion), for connected bodies on a frictionless surface we first treat the whole system as one unit to find the common acceleration, then isolate one block to find the internal tension. The total mass is 3 + 2 = 5 kg, and the applied force is 20 N, so the common acceleration is a = F / (m₁ + m₂) = 20 / 5 = 4 m/s². The string tension is the only horizontal force on the 2 kg block, so applying Newton's Second Law to it alone gives T = m₂ a = 2 × 4 = 8 N. Option 12 N is the force on the 3 kg block side (F − T), not the tension. Option 20 N wrongly equates tension to the full applied force, ignoring that the string only pulls the rear block. Option 4 N mistakes the acceleration value for the tension. Plausibility check: the tension must be less than the applied 20 N because part of the force accelerates the front block, and 8 N being the force needed for the 2 kg block confirms internal consistency.

Connected Bodies And Tension

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