Kindergarten Energy - Part 1

http://www.thestar.com/article/653925

A 7-year-old boy … was struck by a car at a Cabbagetown crosswalk. The boy was riding a bicycle … the (car in the) curb lane continued and struck him," said Const. Mig Roberts.

The raw story - riding a bicycle across a pedestrian crosswalk.

There are two laws against this practice; one of them man-made, one universal. It is the universal law I want to discuss.

But first - it is absolutely terrible that a 7-year old is struck by someone disobeying the man-made law about stopping at crosswalks. It is also absolutely terrible that drivers don’t stop, ignore or don’t see the warning lights, get distracted, are impatient, and so on. The sadder truth is that these events will always be with us. See for example Driving For Dummies – 3 .

How can we guard against them? We pass other laws such as the one that requires all cyclists to dismount and walk their bicycle across the cross walk.

It is called a cross WALK for a reason. And it is further described as a PEDEstrian cross walk for the same reason.

I want to go into that reason in this and the next essay ( Kindergarten Energy - part 2 ).

Energy. Again.

The energy of a moving body is given by the formulae ½ *m*v^2; energy is proportional to the mass of the body, and energy is proportional to the square of the velocity.

A child walking across a crosswalk has a mass of say 100 pounds. Quantities may vary. The bicycle has a mass of say 50 pounds. Quantities may vary.

For the purpose of our argument a child-plus-bicycle has one and a half times the mass, and so, at any given velocity, will have one and a half times the energy of a child without a bicycle.

A child walks across the crosswalk at, say, 3 miles per hour. A child bicycles across the crosswalk at, say, 9 miles per hour.

Three times the velocity, three SQUARED, or NINE times the energy.

A child bicycling across a crosswalk has then about fifteen times as much energy to dissipate as a child walking without a bicycle.

How will the child-plus-bicycle dissipate so much energy in a short time?

By applying the handbrakes on the bicycle, and perhaps flying over the handlebars into the car, or by just sliding into the front of, or the side of, the car.

Either way the small mass of the child (100 pounds) is about to receive some momentum, and energy back from a much larger object with a mass of, say, 1,000 pounds.

It’s no contest.

It is why our law-makers have asked parents to raise their children to walk their bicycles across pedestrian crosswalks.

Search