Speed and acceleration are NOT the same thing!
The Sunday Nation
Nairobi,
02 December 2012
Why is it so hard to understand that 100 kilometres per hour means that if you maintain that sped for one hour, you will travel 100km? I would think it is obvious, but apparently, it isn’t. Thus I keep getting questions from readers suggesting that vehicles with large wheels (buses, lorries etc) move faster than those with smaller ones even when both are travelling at the same speed!
The most recent suggestions is even more curious. Abraham Murithi
writes: “My friend and I have two types of [
Now Abraham is mixing up two quantities: speed and acceleration. The 100km/h that he is talking about is the speed. If the two cars are driven with their speedometers showing 100km/h and one overtakes the other, then my conclusion would be that at least one of them has a badly calibrated speedo. When it shows 100km/h, the car is could actually be doing something like 120km/h or even 80km/h.
To get an idea of which of the two has a bad speedometer, Abraham may invite a third friend with a totally different car. Chances are that, when all are showing 100km/h on the speedo, two will stay together while the third will either be left behind or move ahead. The one that gets out of the formation has a bad speedo.
But Abraham writes that “Remember you told us a small car and a bus each at the same acceleration the time taken to cover same distance is the same…” Now that is not true. I have never written or implied such a thing! However, I did write that if a car and a bus are moving at the same speed, the time taken over a given distance is the same – regardless of the tyre circumference.
Speed and acceleration are not the same! Acceleration is the rate at which speed changes. For example, when car manufacturers give the time taken to reach 100km/h, they are talking about acceleration. But when we state the time taken to cover a certain distance, this speed… or more accurately, average speed.
It is not clear which of these two quantities Abraham has in mind. But, from the reported experiment, I strongly suspect that he is referring to acceleration – the time taken to reach 100km/h. In that case, it is perfectly reasonable to expect that the car with a 3,000cc engine will accelerate faster than the one with 2,500cc.
But life does not go on straight lines: we should not assume that bigger engines always produce greater acceleration! After all; you wouldn’t expect a 7,000cc lorry to reach 100km/h faster that an ordinary 1,500cc saloon car, would you?
 |
 |