# Physics 12 Harmonic Motion

In physics motion happens when something changes its position. We’ve watched toy cars and balls race down ramps for straight line motion. Nuts swung back and forth for pendulum motion. A nut went in a circle for circular motion. How else can something move?

Pick up a ball point pen and press the end. The knob pushes in then out again. It moved but why does it move that way?

If you take the pen apart, you find a tiny spring inside. You can compress or push the spring down then release it and it goes back to its original length.

Springs are very useful items. They show harmonic motion. They can be used to build a spring scale.

Question: What is harmonic motion?

Materials:

Spring as from inside a pen

A length of wire

Procedure:

Hold most of the Slinky in your hand allowing part of it to dangle down

Move your hand up and down then keep it still

Hold the small spring between your thumb and finger

A spring does show harmonic motion but only when force is directly applied to the coils to compress or pull apart the coils then easing off to allow the coils to return to their original position.

Press the spring down then let it loosen several times [Don’t let go of it, keep it between your thumb and finger.]

Pull the spring out a little longer and let it go back several times

Wrap the wire around something round like a marker or a pencil keeping the coils close together

The wire coils must be tight around the pencil and close together to create a spring.

Attempt to compress and release these coils

Attempt to pull and release these coils

Observations:

The coils of the Slinky go down and up in harmonic motion. All springs show some amount of this motion but this toy shows it very well.

Describe compressing and releasing the spring [amount of force needed etc.]

Describe stretching and releasing the spring

Describe how your coiled wire behaves

Conclusions:

Is speed constant in harmonic motion? Why do you think this?

Does the spring seek to maintain a certain length and shape? Why do you think so?

What do you think would happen if you compressed the spring then let it go?

Compressing an unsupported spring shows a problem engineers face building columns, the spring buckles to the side. If the compression continues, the spring sill shoot off to the side.

What do you think would happen if you pulled the spring out to twice its length?

How does a spring keep its ability to produce harmonic motion?

Does a tightly coiled wire behave differently from a straight wire?

Does the shape of a spring affect how it behaves?

What I Found Out:

The end of my Slinky went down then up over and over. Eventually it stopped but it took a long time.

When I looked at the pictures of the Slinky in motion, I could see the coils stretching out from top to bottom. They didn’t stretch out very far. Then the coils pulled back together as the Slinky pulled back up.

Watching the end of the Slinky, the wire loops zoomed down then stopped, zoomed back up and stopped. The speed was not constant as the loops slowed to a stop, sped up then stopped to start over again.

Once the Slinky stopped moving, gravity pulled the coils out a little. Otherwise the Slinky tried to keep its coils close together.

The small spring compressed down until the coils touched each other and returned to its original length. Pulling the spring stretched out the coils. When the spring was released it returned to its original length.

If I pulled the spring until it was twice its length, the coils straightened a little. The spring returned a little but not to its original length.

Coiling a wire seems to make the wire try to behave like a spring.

A spring seems to need to keep its coils in a certain position. The harmonic motion is produced when the coils are pushed or pulled out of position.

A straight wire stayed bent when I pushed it over. It didn’t straighten out again until I straightened it out.

My wire was hard to wrap around a fat pencil. When I pushed the coils together, it got harder as the coils got closer together. After releasing the coils, they moved out a little way then stopped. These coils acted a little like a spring. They would compress and return to place, pull a little and return to place.

The wire I used had been heated and cooled. From a Chemistry Project, wire that is heated and cooled behaves differently from wire that hasn’t been heated and cooled. It is stiffer and more brittle.

That makes me think, if I had new wire and wrapped it around a pencil, it would act more like a spring.