Saturday, February 15, 2020

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.

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