Medical Mythbusting Commentary for August 31, 2026

Source:
These Ontario doctors know how dangerous e-scooters can be. But telling their own kids ‘tough s—-’ isn’t easy

Energy comparison (child at 20–25 km/h vs. a 20–40 kg cinder block dropped on the head)

Correct pairing of the two equations is:

$$\frac{1}{2} m_{\text{child}} v^{2} = m_{\text{block}} g h$$

so

$$h = \frac{m_{\text{child}}}{m_{\text{block}}} \cdot \frac{v^{2}}{2g}$$

(The “½mv² times mass” wording is not needed; mass of the child and mass of the block simply scale the height.)

Useful conversions:

• 20 km/h = 5.56 m/s → same-mass free-fall height $v^{2}/2g \approx 1.57$v2/2g≈1.57 m
• 25 km/h = 6.94 m/s → same-mass free-fall height $\approx 2.46$≈2.46 m
• $g = 9.81$g=9.81 m/s²

A typical 8–16″ hollow CMU is ~16–18 kg; a solid or filled block can reach the 20–40 kg range you specified.

Drop height hhh that gives the block the same kinetic energy the child has

Child mass	20 km/h, 20 kg block	20 km/h, 30 kg	20 km/h, 40 kg	25 km/h, 20 kg	25 km/h, 30 kg	25 km/h, 40 kg
25 kg	2.0 m	1.3 m	1.0 m	3.1 m	2.1 m	1.5 m
35 kg	2.8 m	1.8 m	1.4 m	4.3 m	2.9 m	2.2 m
45 kg	3.5 m	2.4 m	1.8 m	5.5 m	3.7 m	2.8 m
55 kg	4.3 m	2.9 m	2.2 m	6.8 m	4.5 m	3.4 m

How to read it. A 40–45 kg child (roughly a typical 10–13-year-old) traveling at the Ontario e-scooter cap of ~24–25 km/h carries the same energy as a 30 kg block dropped from about 3.5–3.7 m (a bit more than a standard residential ceiling, or a second-storey window sill). A lighter 20 kg block would have to fall from well over 5 m; a heavier 40 kg block from a little under 3 m.

Important limits of the analogy (this is a scale illustration, not an injury model):

• A fall from a bike or scooter rarely dumps 100% of the rider’s KE into the head. Arms, tumbling, helmet, and glancing contact absorb or redirect a large fraction.
• A rigid cinder block striking the skull is a short-duration, small-area, high-peak-force impact. A ground or vehicle strike is usually longer-duration and more distributed. Peak acceleration and focal loading can therefore differ even when total energy is the same.
• Helmets change the picture dramatically by increasing contact time and spreading force.
• Real crash energy can be higher if the child hits a moving vehicle or a fixed object while still carrying forward speed.

In short: at 20–25 km/h the energy involved is already in the same ballpark as dropping a construction block from roughly head-height to well above door-height, depending on the exact masses. That is why the physicians in the article treat these speeds as motor-vehicle-class rather than playground-class.