# Physics Stop

## Blue skyMarcus WilsonAug 31

I’m still working on the problem of why is the sky blue?  Now, I’ve already told you its because the short-wavelength blue light is scattered more than the long-wavelength red light, but why are short wavelengths scattered more than longer ones? In words suitable for a blog.

I could do some maths to show you that’s the case, but that’s really a cop-out. (Plus most of you wouldn’t follow it.) I think if a physicist has to resort to maths to explain something, it means he doesn’t understand it himself. (That said, you will struggle to do physics without being reasonably competent at maths – since maths provides the language in which physics is often best expressed.)

Help!

## The scientific methodMarcus WilsonAug 28

This morning we had a school group visit us from Whakatane – about 30 year 10 students (14 and 15 year olds) – they carried out some activities in Chemistry, Earth Sciences and Physics. I led them (as two groups) in a physics activity involving catapults.

After doing the boring bit (talking about how energy is transferred from one form to another as the catapult is loaded and fired) we got to actually using the things. Our catapults have a metal arm, that can be bent back and locked in place. A projectile is placed on the arm, and then the arm is released. As it springs back into position, it throws the projectile about fifteen metres (if you get it right) across the room. Students can vary the release angle of the catapult, the mass of the projectile they put on it, and the effective length of the spring arm. They need to think through the best combination to get the projectile going the desired length.

As part of this session, I put a packet of sweets (lollies, candy, depending on your nationality – to me they are sweets) on the ground about 11 metres from the catapult and the rules were that if the group could get the projectile to land on the packet of sweets they could keep them. They could have as many goes as they wanted in ten minutes.

As I said, the students came through in two groups. The first group had a very careful, systematic approach. They varied one thing at a time, and carefully recorded the results – where the projectile landed in relation to the packet. After about eight minutes of narrowing down the possibilities, they scored a direct hit.

The second group took a rather different view of the problem. Essentially, this was to fire the catapult as many times as they could, and hope they got it.

Despite many more launches than the first group, the sweets remained very safe.

What the first group did is a lot like how science is done. (Well, it IS how science is done.) Carefully, and methodically. Think about what is being measured, and why it is important; the effect of one variable on another is considered. Slowly we uncover things. We may have a lot of failures on the way to developing an understanding about something, but they are not failures without purpose. In this way science moves forward. It is true that occasionally something a discovery is made seemingly at random, but this is not so often, and when it does, it is usually backed up by a lot of careful and systematic measurement. Like Becquerel did when he discovered radioactivity.

## Landing gear failureMarcus WilsonAug 27

Last night our cat failed to live up to the reputation of his species for executing four-footed landings when he lept off a perch in hot pursuit of a piece of string, landed on his front paws with too much forward rotation, performed a graceful flick-flack and thuded head first into the CD rack. After a couple of seconds looking very dazed he got to his feet again and decided (to our relief) he was still fit enough to teach that bit of string a lesson.

Which led me to looking up videos of cats landing on their feet. I think this one is one of the best. (How did he get the cat to co-operate? – I have no idea!)

It’s amazing how they do it. An Olympic high-board diver has the advantage of a push-off from the board in order to start his twisting motion, but a cat that is just released has no such luxury. There has to be conservation of angular (rotating) momentum as he falls – if one bit of him rotates clockwise, some other bit has to rotate anticlockwise. Watch the cat’s tail here – it whips round the other direction to which the cat is rotating, while at the same time the legs are first drawn in to decrease its moment of inertia (like a spinning ice skater draws in his arms to rotate faster) and then opened out to slow the rotation and prepare for landing.

Maybe our puddy could benefit from watching. (NB no signs of lasting injury when we left him this morning…)