The Doppler Effect

You know that noise a siren makes as it goes whizzing past atop of an ambulance or police car? As it approaches you the pitch of the siren changes. This is due to something we call the Doppler effect. But you don't have to wait for an emergency vehicle to hear it in action...

 All you need is:

• A length of string
• A kitchen timer

What to do:

1) Securely tie the string to the timer (Double check this, you don't want the timer flying around later)

2) Start the timer beeping.

3) Find a nice big open space and swing the timer round your head in a big circle.

Notice anything odd about the sound of the beeper?

Well you shouldn't, because if you are doing the swinging then the distance between you and the timer isn't changing, so you won't hear a change in pitch. Just like you wouldn't notice the sound of the siren changing if you were sitting in the emergency vehicle. 

However if you get someone else to listen (watch out they don't get hit by) , then they'll hear a different note depending on whether the timer is coming towards them or swinging away.

What's going on?
Sound travels as waves, and the pitch of note depends on how close together the peaks of these waves are. So a high note will be made of waves with peaks that are closer together that a low note.

The timer is emitting a constant  beep with a constant note i.e. the distance between the peaks of the waves are always the same. But when the timer is travelling towards an listener (i.e. not the person doing the spinning) each wave peak is emitted from a point that is closer to the listener than the previous peak.   Therefore the time between each peak arriving at the listener is reduced and so the note sounds higher. Then the reverse happens when the timer is moving away from the listener and the note sounds lower.

And of course the person doing the spinning doesn't notice any change because the distance between him and the timer doesn't change.

Garlic Challenge, the results show!

In my last post I posed a question: Does rubbing your hands on stainless steel gets rid of garlic smells? There's clearly plenty who believe it does, you can even buy stainless steel 'soap' .

Various theories as to how steel may achieve this were posited. But I wanted to know if there was a real effect in the first place.

Kitchen chemists everywhere helped answer this by taking part in a stinky citizen science challenge. And the results are, well, interesting.

I asked people to conduct a quick experiment whilst prepping dinner. The task was simply to rub the palms of their hands with garlic. Then treat one hand with a wipe from a stainless steel spoon and the other with a wooden spoon. Finally participants asked some other poor soul to take a sniff of their hands and report on whether there was a discernible difference. Thanks to everyone who took up the garlic challenge (especially the person who did their experimenting whilst cooking Christmas dinner).

And so to the results. These were collected via surveymonkey, with the question "Which hand smelt more of garlic?" and the answer choices a) The hand rubbed with the wooden spoon, b) The hand rubbed with the stainless steel spoon, c) Couldn't tell the difference. 44 allium lovers responded. Of those 17 thought the hand treated with the wooden spoon smelt more garlicky, 6 said the stainless steel treated hand was the stinkier. So far, so good. Looks like the stainless steel effect might be real. But here's the rub, there's still the other 21 responses, none of whom could tell the difference between the smelly hands.  

So we've got results that are significantly different from an even distribution between the options (the two-tailed P value equals 0.0163 ,according to a chi squared test). However, the stainless steel treatment seems to be only about 38% effective, assuming the wooden spoon is a good negative control. But maybe the abrasive, absorbent wooden spoon is also quite good at removing garlic smells? In which case the effectiveness of the stainless steel is an underestimate. Oh well, sorry people, but it looks like I can't really offer a definitive answer. In hind sight I think the experimental design could have been better. A before and after spoon treatment sniff test would have been a good idea. And maybe a better negative control was in order. Is another round of experiments could be in order?