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Compact bone is dense and resists compression; its structural unit is the osteon, rings of calcified matrix around a central canal carrying blood vessels and nerves. Spongy bone is a lattice of trabeculae that form along lines of stress and make bones lighter.

Bone matrix is a composite: collagen fibres give flexibility so bone is not brittle, and hydroxyapatite crystals, formed from calcium phosphate and calcium carbonate, give hardness and strength.

Bone is living tissue that is constantly remodelled: osteoblasts form new bone, osteocytes maintain the matrix and osteoclasts break bone down. In the teens much more bone is deposited than removed, and bone mass usually peaks by the mid- to late 20s.

Experiment: A model forearm: how big is the 'biceps' force?

1. Fix the ruler so it can turn about a pivot at the 0 cm end. Hook the newton meter vertically at 10 cm (r₁) and adjust it so the ruler is horizontal.
2. With no mass hung, record the newton meter reading F₀ (it supports the ruler's own weight).
3. Hang 100 g (weight ≈ 0.98 N) at d = 15, 20, 25, 30, 35 and 40 cm. Each time keep the ruler horizontal and record F. Calculate F − F₀.
4. Predict F − F₀ from torques: (F − F₀) × r₁ = W × d. Compare measured and predicted values.
5. Plot F − F₀ against d. Is it a straight line through the origin? Estimate your uncertainties (newton meter resolution, reading the distance, keeping the ruler level).

d (cm)	F (N)	F − F₀ (N)	Predicted W × d / r₁ (N)
15	Predict the 'biceps' force when the 100 g mass is at 40 cm.
20	Predict the 'biceps' force when the 100 g mass is at 40 cm.
25	Predict the 'biceps' force when the 100 g mass is at 40 cm.
30	Predict the 'biceps' force when the 100 g mass is at 40 cm.
35	Predict the 'biceps' force when the 100 g mass is at 40 cm.
40	Predict the 'biceps' force when the 100 g mass is at 40 cm.

Questions

1. Compare compact and spongy bone: structure and function.

2. Using the textbook values (r₁ = 4.00 cm; forearm 2.50 kg at 16.0 cm; book 4.00 kg at 38.0 cm; g = 9.80 N/kg), show that the biceps force is about 470 N.

3. From your graph, how well did your measured values match W × d / r₁? Give one source of uncertainty and how you would reduce it.

4. Explain why the skeleton is described as living tissue, naming the three types of bone cell.

Sources for every fact on this sheet: echoesof.science/school/skeleton/sources/

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