Physics - NLM Question with Solution | TestHub

If both blocks moved together, $a=\frac{40-27}{9}=\frac{13}{9}m/{\mathrm{s}}^2$, needing friction of 34.2 N on the 4 kg block.

The limiting friction between the blocks is only 20 N, so the 4 kg block slides.

For the 4 kg block: $a=\frac{40-20}{4}=5m/{\mathrm{s}}^2$; the 5 kg block does not move.

Answer: D