A wooden box of mass 2 kg and dimensions (30 cm x 15 cm x 10 cm) is placed on a table with sides 30 cm and 10 cm touching the tabletop. Which one of the following is the approximate pressure exerted on the table?
- A.111.1 N/m²
- B.222.2 N/m²
- C.333.3 N/m²
- D.666.6 N/m²
Correct answer
Explanation
The correct answer is D, 666.6 N/m². Pressure is force divided by the area in contact: the weight is 2 kg × 10 m/s² = 20 N and the face touching the table is 0.30 m × 0.10 m = 0.03 m², so the pressure is 20 ÷ 0.03 ≈ 666.6 N/m². Pressure means thrust per unit area, P = F ÷ A, and its SI unit is the pascal (1 Pa = 1 N/m²). Here the force on the table is the weight of the box, mg. The same box presses harder when it stands on a smaller face, because the same weight is spread over less area; that is why knives are sharpened and why camels have broad feet. Option A is wrong because 111.1 N/m² would need a contact area of 0.18 m², larger than any face of the box. Option B is wrong because 222.2 N/m² would need an area of 0.09 m², which no face of this box has. Option C is wrong because 333.3 N/m² would come from an area of 0.06 m², twice the real contact area. Exam tip: P = mg ÷ A; convert cm to m before multiplying (30 cm × 10 cm = 0.03 m²).
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