Keen-eared scientists watching the latest episode of Bang Goes The Theory might have spotted some unusual terminology when Jem and Professor Chris Kemp were discussing crowd pressure. When discussing the forces exerted by crowd surges, they quote various measurements such as:
... the pressure on the barrier was 2.67 kilonewtons per metre.
You may be surprised to hear a measurement of ‘pressure’ expressed in ‘kilonewtons per metre’. Of course, pressure can be measured using various units, but all essentially describe a force per unit area. What crowd scientists describe as ‘crowd pressure’ is actually a measure of force over a linear distance – a metre. They measure it using a linear barrier along the front of the crowd, recording the overall force exerted by the crowd, and giving them a figure per metre that can be compared between crowds of any size.
You might also have noticed that Jem and the Professor go on to discuss measurements such as '1.2 kilonewtons'. Don’t be confused. For convenience they are simply abbreviating the actual units used. In fact they are still quoting kilonewtons per metre in every case.
During the film, Jem also makes a comparison:
To translate, that’s over a quarter of a tonne of crowd pressure. I mean, that’s like a small horse falling on you!
Of course a quarter of a tonne is not a pressure either—it is a mass. But Jem makes this comparison so that it’s easier to appreciate what 2.67 kN/m might actually feel like if you were on the receiving end. Don’t worry, he’s not mistakenly suggesting that pressures are measured in tonnes.
It’s worth remembering that when scientists talk to each other, they often use abbreviations and sloppy jargon when they’re sure there’s no risk of being misunderstood. So when Jem's talking about a force of a quarter of a tonne (along a metre-length of crowd), he's talking like a professional, because both he and the Professor know he's referring to the weight that a quarter-tonne of something has, like the weight of a small horse. We hope this professional-style mixing of mass and weight isn't too confusing!
The Open University’s Stephen Serjeant and Ian Johnston explain further:
Among themselves, physicists sometimes refer to weights in mass units because it's understood that everyone involved will know the difference between mass and weight. Mass is how much stuff there is (e.g. an entire horse), and weight is how hard it's pushing down on the ground (for a horse —heavy!). Mass and weight go in proportion, meaning that if you double the mass (two horses) you double the weight. That's why weight units and mass units are sometimes mixed up, but strictly speaking they're different things.
You probably do something like this every time you weigh yourself at home. Your bathroom scales measure how much you weigh, but most scales are marked in kilogrammes, and kilogrammes are a measure of mass. Your scales will give you a true answer in the Earth's gravity, but if you take your scales to the Moon, where you weigh less, your scales will tell you the wrong mass in kg.
If it helps, remember that if a horse were on the Moon, it would weigh less, but it would still have a mass of a quarter of a tonne. Remember, mass is how much stuff there is, weight is how hard it's pushing down on the ground. And if it helps even more, consider this:
Q. What's the heaviest aircraft you can fly on a private pilot's licence?
A. A hot air balloon. Typical weight: zero. Typical mass: several tonnes!