Why do people on bicycles get so agitated about close passes, when they’re happy passing other vehicles closely? Isaac Newton has the answer.
Sameness is not equality
A few days ago, someone posted this on Twitter:
Now, if you read the thread, you’ll note that I was keen to jump on the objectionable idea that someone would wilfully endanger me because they’d seen someone else filtering through stationary traffic.
But this post isn’t about that. It’s to do with the deeply problematic principle of treating road users as the same rather than equal, a principle reinforced by the reply that Greater Manchester Police gave:
Fortunately, whilst the idea that I shouldn’t be made to pay for someone else’s actions (let alone their perfectly legal actions) is too complex for some people to grasp, the difference between sameness and equality largely boils down to some very basic physics.
And that can be explained with nice simple pictures.
So let’s go.
I’m going to introduce a device that I’m sure you’ve seen before: Newton’s cradle.
Newton’s cradle is primarily intended to illustrate conservation of momentum and energy, but if you’ve ever used one you’ll be well aware that the more steel balls you lift the more energy there is in the system – the more of a bang you get when they crash together – and also that the higher you lift them the more energy there is.
So. Here’s your common or garden Newton’s cradle in schematic form.
Before we start, we need to make a slight change to the standard Newton’s cradle. My bicycle and I together have a mass of pretty much exactly 100kg, whilst – say – a Ford Mondeo plus one occupant have a mass of around 1500kg. The ratio of mass is 1:15.
So in order to get the ratio of masses correct, we’ll need a 16-ball cradle: one ball for the person on a bicycle, 15 for the person in a car.
When a bicycle, or a car, or a steel ball moves, it has kinetic energy. I’m going to consider two scenarios: a person on a bicycle at 10mph (a speed which would be typical when filtering through stationary traffic) and a person in a car driving at 30mph (a typical speed when passing someone on a bike).
In order to represent the energy of our person on a bicycle, we’ll raise the relevant ball by a height h.
Good. We now have some unit of energy that is proportional to someone on a bicycle at 10mph. If you’ve used a Newton’s cradle before, you’ll know that the above will result in: a pretty gentle “clack” as the balls meet. Naturally, with one ball falling from quite a low height, the 15 other balls will barely move. (Imagine that they’re all stuck together, like a car is.)
Next, we need to look at the energy that a car has at 30mph. We’ve accounted for the mass already with the number of balls; all that’s left is to account for speed.
Now, although the energy in the balls of Newton’s cradle rise linearly with height, the energy of a body of mass rises with the square of its speed. At 30mph, three times 10mph, any given mass has nine times the kinetic energy it has at 10mph. So we need to lift the balls to nine times the height of our previous example, 9h.
Again, if you’ve used a Newton’s cradle, you probably get the idea—BANG! (Though, of course, you’ll perhaps only really have a good idea if you’ve welded three cradles together.)
Imagine what it’s like if the car’s doing 60mph. We’d need to suspend the balls on wires four times as long in order to achieve the height of 36h that would represent the equivalent energy.
And, of course, whilst some of those 15 balls on the right make up the incredibly protective outer shell of the car, none of the single ball on the left represents protection of the person on the bicycle. In a way, you’re slamming 15 standard steel balls into an egg.
The big bang
This basic physics is why people riding bicycles at 10mph inches away from cars present no greater risk than scratched paint, whilst those who drive cars at 30mph inches away from people on bicycles risk killing others.
It is why a legal system and a policing view that is based on sameness is wrong, and why we need to understand equality and build a legal and policing system that counteracts the inequity of the physics.
And it is why people who do things like this should, quite simply, not be on the road.