Tag Archives: Slinky

More Slinky Science

You can use slinkies to demonstrate all things waves! Start with the basics: wavelength, amplitude and frequency. Once you’ve got those down, then you can play around with some things that are a bit more complicated like wave type, standing waves, and superposition.

Longitudinal waves, like sound waves, expand and compress as they travel through a medium like air or water. Transverse waves are like ocean waves.  They have characteristics that we more commonly associate to a wave: wavelength, the distance from peak to peak or trough to trough, amplitude, the height of the wave, and frequency, the number of waves per given time.

As you increase the frequency of waves, you can also increase the number of nodes and antinodes in the standing wave. Nodes are the part of a standing wave that stay in place.  Antinodes are the part of a standing wave that move with maximum amplitude, like the peaks and troughs.  There is always one more node than antinode.

You can also cause superposition of waves.  Superposition is when you combine multiple waves together to get either constructive or destructive interference.  To get constructive interference, the waves must be in phase.  When call that coherent. Coherent light can often be found in lasers.  The light waves travel at the same frequency and are in phase, which results in a single point of light.  To get destructive interference, the waves would need to be sent out of phase, or be incoherent.  Light from a white flashlight is a great example of incoherence, because the light has many different frequencies, resulting in a cone shape of light.

In the case of the slinky. If you send one wave down to try to knock the water bottle over, on their own they do not carry enough energy. But, if both ends of the slinky are sent in phase and work together, then we can achieve constructive interference to knock the water bottle down!

What other waves can you think of, and is there a good way to demonstrate them?

The Slinky: Mystery of a Childhood Toy

Everyone loves a Slinky, especially scientists! Most people know them as cool springy things that can go down the stairs or end up in a giant tangled mess. We want to focus on something else: what happens when you dangle a slinky from the top until it is fully extended, then drop it?  You might expect the whole slinky to fall to the ground. As you can see in the video, it’s a little more interesting than that. The bottom of the slinky stays completely motionless until the top of the slinky catches up. But why?

Slinky Drop

The slinky isn’t breaking physics or ruining science. It’s really just a loose spring. The more a spring is stretched, the more it pulls back towards the center. When the slinky is extended vertically like this, gravity pulls down on every part of it, including the bottom. Before the drop, we allow the slinky to hang until it stops moving, which means that the gravity pulling down on it is exactly countered by the spring force pulling back up.

The whole slinky is pulled down by gravity with the same acceleration. Both ends are also pulled towards the middle due to its springy characteristics. The top is pulled down by gravity and the spring, so it falls extra fast. The bottom, even as the slinky is dropped, is still being pulled up by the spring. With the bottom going nowhere, and the top going faster, it all averages out. The whole slinky together is falling at the exact rate that gravity dictates.

There is a bit more to this explanation. The slinky actually shows a wave and the transmission of information as well. The information from the top (“we’re falling!”) needs to get to the bottom before it can take effect there. You can see this wave– called a compression wave– if you watch the top of the slinky in the gif below. The spring clumps up as the fast-falling top part catches up with the parts below it.

Slinky High Res

There is nothing special about our Slinky. It doesn’t need to be rainbow colored or giant, any old Slinky will do the trick. Don’t believe us? Try this one at home! And when you’re done, take it to the stairs!

 

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