A wooden box of mass 2 kg and dimensions (30 cm × 15 cm × 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 — D, 666.6 N/m². The box rests on the face made by the 30 cm and 10 cm sides, so the contact area A = 0.30 m × 0.10 m = 0.03 m². Its weight (the thrust on the table) is F = mg = 2 kg × 10 m/s² = 20 N. Pressure = force ÷ area = 20 ÷ 0.03 ≈ 666.6 N/m². (Using g ≈ 9.8 m/s² gives about 653 N/m², which rounds to the same option.)
- (a)111.1 N/m² — Far too small — it corresponds to dividing the weight by a much larger area than the true 0.03 m² base; the correct pressure is 666.6 N/m².
- (b)222.2 N/m² — This comes from using a wrong (too large) contact area or a unit-conversion slip; with the 30 cm × 10 cm base of 0.03 m² the pressure is 666.6 N/m².
- (c)333.3 N/m² — This is half the correct value — it results from taking a larger face (e.g., 30 cm × 15 cm) or halving the weight, not the 30 cm × 10 cm base that is actually in contact.
Pressure is the force (thrust) acting per unit area, P = F/A, with SI unit the pascal (Pa = N/m²). For a block resting on a surface the relevant area is the face in contact, and the force is the block's weight, mg. For the same weight, a smaller contact area gives a larger pressure.
The step that decides the answer is picking the correct contact face — the 30 cm × 10 cm base — and converting it to 0.03 m². Dividing the 20 N weight by this area gives about 666.6 N/m².
- Pressure P = F/A; SI unit pascal (Pa) = N/m².
- The relevant area is the face in contact with the surface: 30 cm × 10 cm = 0.03 m².
- Weight (thrust) = mg = 2 × 10 = 20 N.
- P = 20 ÷ 0.03 ≈ 666.6 N/m².
- Using the wrong face for the contact area.
- Forgetting to convert cm² to m² (divide by 10 000).
A numerical on P = F/A — identify the contact face (0.03 m²) and divide the 20 N weight by it.
No directly related past PYQ was found.
- practice — not a real PYQ
The SI unit of pressure is the
- (a)newton
- (b)joule
- (c)pascal
- (d)watt
Answer(c) pascal (N/m²).
- practice — not a real PYQ
For a fixed force, if the contact area is halved, the pressure
- (a)halves
- (b)doubles
- (c)stays the same
- (d)becomes zero
Answer(b) doubles.