Hey, look kids! There's Big Ben.

And there's parliament. Ok - sorry, I had to make a "Tom (Swans on Tea)" title for this one. Tom, forgive me.

Here are two great circular motion videos. First, this one is from Dale Basler. He made himself a fine little floater-type accelerometer. Better than just make it, he made a video of the accelerometer in his car going around a round about. Check it out.

Bobber Meets Roundabout from Dale Basler on Vimeo.

So, if he is driving at a constant 10 mph, how big is the round about (traffic circle)?

Next video - more silly kids

First, I saw this one on ZapperZ's Physics and Physicists who in turn references The Physics Buzz from Physics Central. When did Physics Central get a blog? How come I am the last to know these things? Here is the video:

Listen up video makers - this is real important. If you are going to go through this much trouble to possibly injure yourselves, why not get the video camera shot from directly above? This would it MUCH better for video analysis. Oh, and make sure that you include a meter stick or something so I can scale the video (or since you seem European, a metre stick would do).

At least the one guy was trying to set a good example by putting out the cigarette. Don't you know those things are killers?

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for the radius of the roundabout, my mad free body diagram skillz show*

r=v^2 / (g tanÎ)

where r is the radius of teh roundabout
v is the speed of the car
g is accel of gravity
Î is the angle the bob makes

v is 10 mph or about 6 m/s
g is about 10 m/s/s
Πis about 30°

so r is 36/(10*0.58) meters ~ 30 meters.

*i make no claims as to the accuracy of my mad skillz or calculator key pressing.

How about taking the simpler path.

Assumed speed 10 mph or 4.47 m/s
Time to complete one revolution 17.8 s
4.47 m/s * 17.8 s = 79.6 m circumference
79.6 m / 3.14 / 2 = 12.7 m radius


I don't know the answer - you will have to wait for Dale to reveal it.


If you are given a barometer and told to determine the height of a building, would you drop the barometer and time the fall?

Well done Bill. I salute you. (Although I don't know the actual experimental answer)


that is good a good technigue to figure it out. i wondered for about 1/2 minute where you got the 17.8 s value, then i realized you actually watched the video and timed it.


p.s. 60 mph is about 30 m/s so my v above should be about 5 m/s. so the answer i *really* meant to get is about 20 m.

For the merry-go-round:

Typical scooter length seems to be right around 2m, and it appears the scooter is roughly the same length as the diameter of the merry-go-round. Comparing one of the people on the ride to it also makes it look about 2m in diameter.

The slo-mo part of the video shows it frame-by-frame, and I count 14 frames per rotation at full speed. A typical video camera runs at 30 fps, and a stopwatch on the full speed seems to verify that 14/30s per rotation is about right.

v = 2*pi*r / T
v = 2*pi*1m / (14/30s)
v = ~13.5 m/s

Here's a link to a map of the roundabout.

I'd use the Distance Measurement Tool in Google Maps but Google has terrible maps of this area.

I put the map into LoggerPro and analyzed it using the 10 meter key at the bottom to get this:

With a radius of 14.55 meters and 18 seconds for one complete revolution, I get my velocity to be 5 m/s.

But why can't I get the bobber to back this up? I measured the angle to be 22 degrees:

Did you ever manage to get the bobber to back up the results Dale?

From my understanding of the problem it would be quite tricky to work out, as the reason the bobber moves to the left is because there is a pressure difference from the varying height of the water due to the centrifugal force. Is this how you were calculating it, if so how?

Perhaps refraction from the glass jar may also skew the angle measurement slightly?

By Daniel Brown (not verified) on 25 Jun 2010 #permalink