To use a force relationship on a living thing, find the force first, then divide by the area it acts on. The two equations are W = m × g and p = F ÷ A.
This lesson belongs to physical reasoning in mixed tasks because biology provides the object (a person, a deer, a leaf) and physics provides the method. All measurements below are invented for teaching, with g = 10 N/kg.
Why does area matter for living things?
Many organisms are shaped by pressure. A camel’s wide feet and a snowshoe hare’s large paws spread weight over a bigger area, so they sink less. A thorn or claw does the opposite and concentrates force into a tiny area.
To compare these cases, you convert a body measurement, usually mass and footprint area, into a pressure.
How do I calculate it, step by step?
- Convert mass to weight using W = m × g. The weight is the force.
- Convert the area to m² (1 m² = 10 000 cm²).
- Calculate pressure using p = F ÷ A.
- Write the unit (Pa) and interpret the result biologically.
Worked example
A student of mass 50 kg stands on soft soil. The area of both soles in contact with the ground is 400 cm². Find the pressure on the soil.
Step 1, weight: W = m × g = 50 × 10 = 500 N.
Step 2, area: 400 cm² ÷ 10 000 = 0.040 m².
Step 3, pressure: p = F ÷ A = 500 ÷ 0.040 = 12 500 Pa.
Check by estimating: 0.040 m² is about one twenty-fifth of a square metre, so the pressure should be about 25 times the force. 25 × 500 = 12 500. It agrees.
Step 4, interpretation: if the same student lies down so that 0.40 m² touches the soil, p = 500 ÷ 0.40 = 1250 Pa, ten times smaller. That is why lying flat spreads the load on thin ice or soft mud.
What mistake should I watch for?
A common slip is to use mass in place of force.
Mistaken working: p = 50 ÷ 0.040 = 1250 Pa
The value 50 is the mass in kg, not the force in newtons. The calculation gives the wrong number, even though it looks neat.
The correction is to calculate the weight first, then divide. Also convert the area to square metres: dividing by 400 instead of 0.040 would give an answer that is 10 000 times too small.
Check yourself
Take g = 10 N/kg and work these out.
1. A force of 600 N acts on an area of 0.015 m². Find the pressure.
Show answer
p = F ÷ A = 600 ÷ 0.015 = 40 000 Pa.
2. A tortoise has mass 4.0 kg and its feet touch the ground over a total area of 0.0020 m². Find the pressure it exerts.
Show answer
W = 4.0 × 10 = 40 N. p = 40 ÷ 0.0020 = 20 000 Pa.
3. An animal of weight 450 N stands on a total foot area of 150 cm². Find the pressure in Pa.
Show answer
Area = 150 ÷ 10 000 = 0.0150 m². p = 450 ÷ 0.0150 = 30 000 Pa.
Where this leads next
Before you meet this kind of question in a paper, decide whether it belongs to your syllabus tier in checking whether a numerical relationship belongs in the learner’s tier. If you want to test how a measurement was taken, try the scientific investigation critic.
Students often know both equations separately but freeze when a footprint arrives inside a biology question. That is the kind of cross-subject habit we build in online one-to-one Combined Science tuition.