CDS (II) 2021 General Knowledge · Question 5 of 120

Correct — A, Larger while standing than while lying. Pressure is thrust divided by the area over which the thrust acts, and thrust is the force acting perpendicular to the surface. In both postures the force pressing on the sand is the same — the person's weight, which does not care how the body is arranged. What changes is the area over which that weight is spread. Standing puts the whole weight on the soles of two feet, a few hundred square centimetres; lying down spreads it across the back or side, something like a square metre. Divide the same number by a much smaller area and the pressure comes out much larger, which is why feet sink into loose sand and a body lying on it does not. NCERT builds the section on exactly this comparison, noting that in both cases the force exerted on the sand is the weight of your body. The same arithmetic explains why a camel's broad feet keep it on desert sand, why heavy tanks run on continuous tracks and trucks on wide tyres, and why cutting tools are given sharp edges — small area for a large pressure when you want one, large area when you do not.

• (b)Smaller while standing than while lying — Reverses the relationship. Standing concentrates the same weight on a far smaller contact area, so the pressure has to be greater, not smaller.
• (c)Same while standing or lying — True of the force, false of the pressure. The weight is unchanged, but pressure is that force divided by the contact area, and the contact area changes by a large factor between the two postures.
• (d)Larger while standing during the daytime and smaller during the night time while lying — Gets the standing-versus-lying part right but ties it to the time of day, which has no bearing on either the weight or the contact area.

The force acting on an object perpendicular to its surface is called thrust, and pressure is thrust per unit area. Because area sits in the denominator, the same force can produce wildly different pressures: concentrate it and the pressure rises, spread it and the pressure falls. The SI unit is the pascal, one newton per square metre. This single relation explains the design of everything from drawing pins to snowshoes.

The item hands the student a constant and a variable and asks which one drives the answer. The constant is the weight; the variable is the contact area. Option (c) is the trap for anyone who reads 'effect on the sand' as 'force on the sand', because the force genuinely is the same either way. Sand is a good medium for the question because loose grains yield visibly under pressure, so the sinking of the feet is a direct readout. Pressure is also a scalar even though force is a vector, and the reason is visible in the definition — it is the component of force normal to the surface, divided by that area, and a ratio of a magnitude to an area carries no direction of its own.

• Pressure equals thrust divided by area; thrust is the force acting perpendicular to a surface.
• The SI unit of pressure is the pascal, equal to one newton per square metre.
• The weight pressing on the sand is the same whether a person stands or lies; only the contact area changes, and with it the pressure.
• Broad camel feet, wide truck tyres and the continuous tracks of a tank all reduce pressure by increasing contact area; a drawing pin and a knife edge raise it by shrinking the area.
• Pressure is a scalar quantity because it is the ratio of the normal component of the force to the area, not a directed quantity in its own right.

• Mistaking the constant force for a constant pressure — the weight is the same in both postures, the pressure is not.
• Assuming a heavier object always exerts more pressure; a heavy object on a very large base can exert less than a light one on a point.
• Treating pressure as a vector because force is one.

As a same-force-different-area comparison like this one, as a numerical dividing a weight in newtons by a stated contact area, or as an application item on camels, tanks, drawing pins and knife edges.

• practice — not a real PYQ

  A wooden block of weight 50 N rests on a table, first on a face of area 0.05 m² and then on a face of area 0.10 m². The pressure on the table in the two cases is respectively

  • (a)1000 Pa and 500 Pa
  • (b)500 Pa and 1000 Pa
  • (c)1000 Pa and 1000 Pa
  • (d)2.5 Pa and 5 Pa

  Answer(a) 1000 Pa and 500 Pa — the weight is unchanged, so pressure falls in proportion as the contact area doubles.

• practice — not a real PYQ

  Why is it easier to walk on soft snow wearing snowshoes than ordinary shoes?

  • (a)Snowshoes reduce the walker's weight
  • (b)Snowshoes increase the contact area and so reduce the pressure
  • (c)Snowshoes increase the thrust on the snow
  • (d)Snowshoes make the snow denser

  Answer(b) Snowshoes increase the contact area and so reduce the pressure — the weight stays the same while the area under it grows.