Question

A student of mass 50 kg is standing on both feet. Estimate the pressure exerted by the student on the Earth. Assume reasonable value to any other quantity you need. Justify your assumption. You may use g = 10 m s⁻². By what factor will it change if the student lies on back?

Answer 1

Assume area of each foot = area of a 6 cm × 25 cm rectangle.

∴ Area of both feet = 0.03 m²

∴ The pressure due to the student’s weight = `(m g)/A`

= `(50 xx 10)/0.03`

= 16.7 kPa

According to the most widely used Du Bois formula for body surface area (BSA), the student’s BS = 1.5 m², so that the area of his back is less than half his BSA, i.e., < 0.75 m². When the student lies on his back, his area of contact is much smaller than this. So, estimating the area of contact to be 0.3 m², i.e., 10 times more than the area of his feet, the pressure will be less by a factor of 10 or more [Du Bois formula: BSA = 0.2025 × W^0.425 × H^0.725, where W is weight in kilograms and H is height in metres.]