“….. a pin allegedly dropped into Sydney’s busiest street, nearly killing bystanders…” read the sensationalist headlines. Is there truth in the headlines…maybe…lets apply the science component of being a Safety Practitioner (yes it is a philosophical science). If we apply the Lethality Criteria of Kinetic Energy versus Probability of fatality measured in Joules. There is a 98 % chance of dying if exposed to an energy source equal to or greater than 300 Joules. Let’s do a calculation on the kinetic energy of the the 10kg split pin, used to join tower crane sections, dropped 50m into a crowded Street around 11am. It narrowly missed pedestrians before slamming into the sidewalk and ricocheting into a waiting taxi.
To calculate use the standard formula from Newton physics to figure out how long before the falling object goes SPLAT:
- The force of gravity, g = 9.8 m/s2
Gravity accelerates you at 9.8 metres per second per second. After one second, you’re falling 9.8 m/s. After two seconds, you’re falling 19.6 m/s, and so on.
- Time to impact: sqrt ( 2 * height / 9.8 )
It’s the square root because you fall faster the longer you fall.
The more interesting question is why it’s times two: If you accelerate for 1 second, your average speed over that time is increased by only 9.8 / 2 m/s.
- Velocity at impact time: sqrt( 2 * g * height )
This is why falling from a higher height hurts more.
- Energy at impact time: 1/2 * mass * velocity2 = mass * g * height
A 10kg pin falling 50 m gives the following result
- 31.30 m/s
- 112.70 km/h
- 3.19 s
- 4900.00 joules
Given this result I believe headline is quite correct that there was a good possibility of killing bystanders.
So serendipity or luck not design was a factor here, which might of been measured in cm.
Should we investigate or analyse near miss/hits when the energy levels indicate a potential severe injury or fatality. I say yes as these are FREE lessons.
Design I will post about another time.
The original source of this story is http://www.cfmeuvic.com.au/news/where-will-it-end-another-grocon-stuff-up….
Contact: Guy LeNoir at firstname.lastname@example.org for more details