Physics in the playground-IV: Spinning

The phone clamp I use to mount my phone on things has two ¼" sockets, so I put a nut and lock washer on one of the eye bolts of a cable stretcher. Then I looped a 2 meter long cord through the other eye.

Since the bolt is standard ¼" hardware, it fit into the phone clamp and I fixed it securely with the lock washer.

With the phone, the whole assembly looked like this:

And then I could swing my old phone around my head on a cord. I recorded the process using Science Journal and the accelerometers in the phone and the recordings look like this.

There are some surprising things here, but the acceleration in the z direction isn't one of them. The recording shows that there's a downward acceleration averaging around -7 m/s/s. Since the acceleration due to gravity is 9.8 m/s/s, that's not too surprising considering that there has to be a balancing force keeping it up (the spin).

The acceleration in the x and y directions are a little counter intuitive. Acceleration in the y direction is negative. Velocity is in the positive direction so, frankly, I don't know why there should be a measured deceleration in the phone.

 A lot of people assume that velocity in a circular motion is along the diameter of the circle but that's not right. There's two components of motion. One is straight ahead. If you don't believe it, sling an object on a rope and let go of it. It won't continue in a curve, it will continue straight ahead.

The real shocker is the x recording (this one is correct). That negative 13 m/s/s  acceleration is inward along the cord. It's very tempting to think that the force (and thus the acceleration) on a spinning object is outward, but it's actually inward.

Think about it. If the acceleration were outward, that's where the phone would be going. The phone is constantly being pulled inward by the cord. Like Douglas Adams says, flying is throwing yourself at the Earth and missing. So is orbiting.

Physics textbooks used to talk about a "centrifugal force" which spun things outward in circular motion. If they talk about it any more, they call it an apparent, fictitious, or pseudo-force.

That stuff between seconds 20 and 24 is me pulling the cord in while I continued to spin the phone.

Keep in mind that this is just a preview. We'll be looking a lot closer at all these kinds of motion in future installments.

In the meantime, can anyone suggest why my y acceleration was negative?

I'm sorta stuck with the data until someone replicates it and says,"I got something different. Your phone's messed up." Scientists follow the data. So here's my hypothesis.

I tried to keep the velocity of the phone constant once I got it whirling. Obviously, I failed because constant velocity means zero acceleration. Remember that velocity involves two things, speed and direction. As long as direction is changing, velocity is also changing, regardless of the speed. Also notice that the tracing is spiky at the bottom of the y accelerometer recording. That means that, regardless of my attempt, the phone's speed around the circle wasn't even constant.

In order to keep the phone in motion, I had to pump energy into it, otherwise it would fall. I suspect that, at each revolution, I gave it a jerk forward and each time, it resisted. All these factors together added up to a negative acceleration that I had to work against to keep the phone spinning.

Also notice that the phone accelerometers have a maximum. They clip measurements at -20 m/s/s. 

So, if you have any other ideas, send me a comment. There's a button at the bottom of the blog post that lets you do that. (If you're on a phone, you might have to change to the computer version of the blog.)

Spinning things on cords - gas powered airplanes, slingshot, or just a ball on a rope - can be lots of fun, especially if you pay attention to what's going on, what's pulling what, and where things go when you let go.

Physics in the playground III: Crash on a spiral slide

There was a spiral slide in the playground - in fact, this spiral slide.

I sent this cart:

(remember it?) with my old phone clamped to it, down the slide. Of course, being top heavy, it turned over when it hit the first twist.

The old phone also has Science Journal on it, so I recorded it's disastrous descent. Here's the recording.

It's pretty obvious that there were two big crashes and you can probably guess where they were by looking at the top photo. 

Remember that the x direction is left-right. If you were in the cart, you would have been thrown to one side when the cart hit the side of the slide but, when it flipped, you would have been on the other side and would have been thrown that way. The tracings show those spikes, first in the positive direction, and then in the negative.

The y (forward-back) tracing shows a lot of deceleration, but gravity is still working so, as soon as the cart comes away from the side, it continues it's downward acceleration.

The z direction is the direction of spin and, what might confuse some folks is that most of the acceleration is toward the center of spin. Wait and see what happens when I put the phone on a rope and spin it around my head!

A nice thing about these tracings is that they can be spread out to see details. Let's look at the second crash from the x tracing.

There's a lot of vibration happening here on the order of about five spikes per second.

This is the kind of data that engineers get when they study things like crash tests and black boxes from planes that have crashed.

We'll be looking a lot closer at these motions in future blogs. This is just a preview!

Keep in mind that I'm not crashing my new phone - the one I use for a phone - down crash courses, but it's nice to have an old smartphone that I can still use as a pocket computer. It's holding up well under the abuse, but I'm also pretty careful not to overdo it. For instance, I won't just throw it as hard as I can against a tree.

Next time, we'll see what happens when something spins around at the end of a rope. Expect g-forces.