What is the meaning of “twice as slow”?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

28 January 2018

A few days ago, I was watching a movie in which some criminals were hunting down a woman in a forest. Along the way they noticed that she had picked up a bag containing some weapons and that scared them. But their leader shouted at them saying: “Don’t look so scared. She now has extra weight. She will be twice as slow.”

As I wondered about that last sentence, my mind went back to October 2005 when I wrote about the meaning of “twice as cold”. The question was: what temperature is twice as cold as zero degrees?

The answer is complicated by the fact that the “degrees” were not defined. That is; were they Celsius or Fahrenheit? In addition, zero degrees is not the coldest temperature. And as if that was not enough, what is the meaning of coldness? Still, I came to the conclusion that “twice as cold as zero degrees” can be negative 136.5 or positive 546 degrees Celsius.

So, what is the meaning of “twice as slow”? If it was “twice as fast”, we would simply multiply the speed by two. What about slowness: how can we measure it?

Electricians and electrical engineers often talk about the “resistance” of various electrical devices. This is a measure of how much the gadget resists the flow of electricity. The higher the resistance, the lower the amount of current that will flow.

However, like darkness and silence, electrical resistance is a negative entity. We don’t normally measure darkness and silence, we measure brightness and loudness, respectively.

Similarly, the positive counterpart of electrical resistance, is conductance. Resistance is the inverse (or the reciprocal) of conductance. Thus, the higher the conductance, the higher the amount of current that will flow.

In the same way, we can define slowness as the inverse or the reciprocal of speed. That is, since speed is the distance travelled divided by time taken, the slowness should be time taken over distance travelled. In other words, speed is kilometers per hour while slowness is hours per kilometer.

For example, if the speed of a car is 100km/h, we can say that its slowness is 0.01 hour per kilometre. This is equal to 36 seconds per km. If another one was doing 50km/h, its slowness would be 0.02h/km; or 72s/km. Clearly, 72 is twice 36; therefore, the slowness of the second car is twice that of the first one.

So, when we say that one car is twice as slow as another one, what we mean is that it is moving at half the speed. Clearly, the slower car has higher slowness and the faster one has higher speed.

But now think about this: suppose one car can do 10km per litre of fuel and a second one manages 15km per litre. Why do we say that the first one has higher fuel consumption yet 10 is greater than 15?