What is speed? It seems obvious, it's how
fast you're travelling. But let's think
about this a little bit further, a little
bit deeper. Let's take two cars, one is speeding,
the other's not. What's the difference between
those two cars?
To help explain this I'm going to introduce
you to Sammy, Sammy the super snail. She goes
for a slide. How can we figure out how fast
she's travelling? How can we figure out her
speed? We need to know two things. One, we
need to know how far she's travelled. We're
going to call this distance. And we need to
know how much time she took to cover this
distance. Using these two figures, for example
Sammy covers 10 metres, we're going to use
metres here because to scientists these are
the most useful units. She covers the 10 metres
in 10 seconds. We're going to use seconds
also because they're very useful. So how
fast does she travel? We take the distance,
10m, and divide by the time, 10s. 10 divided
by 10 is 1m/s. Ok, the next day after eating
her way through my lettuces, she does the
same distance, 10m but in 5s. Is she faster
or slower? Well let's find out. We take the
distance in metres, 10m, and divide by the
time in seconds, 5s. 10 divided by 5 is 2m/s
which is twice as fast. Obviously very delighted
with this.
Ok, so let's go back to the cars. The speeding
car will be covering the same distance in
less time compared to the safe motorist.
Although scientists prefer m/s any combination
of distance and time, depending on what's
useful to you would be acceptable. So for
example, if you want to know how fast the
tectonic plates move against each other we
could use a unit more like cm/year, it would
still be acceptable.
