Ring on the Resonance!
Have you ever been on a swing set and suddenly noticed that the person on the swing next to you seems to be swinging almost exactly in time with you? You go up and down at either the same time, or exactly opposite each other. This might seem random – but it’s actually physics! Like many things in nature, swing sets have a ‘resonant frequency’, which means that they have a ‘favorite’ frequency (or speed) of movement. The swing set will naturally want to swing at its favorite speed – you may have experienced this if someone has ever tried to push you too fast on the swing – it can actually make you go slower!
In this activity we’ll use paper rings and (lots of shaking) to examine resonant frequencies for ourselves!
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
If you’ve ever played a guitar, violin, or other string instrument, you’ve seen resonant frequency in action! A single guitar string, when plucked, will vibrate at its resonant, or favorite frequency. The vibration of the string creates a sound wave, which we hear as a note. It’s always the same note for the same string, because that string always vibrates at its resonant frequency.
Resonant frequency is determined by a several factors, including the mass of the object (how big is it) and the stiffness of the object. Again, if you’ve ever played a guitar, you may have noticed that the strings aren’t all the same. The ‘low E’ string is much thicker than the ‘high E’ string, and because its bigger, the low E string’s resonant frequency is lower (or slower) than the high E string.
In this activity, we’ll observe how mass and stiffness affect the resonant frequency of different size rings. Get ready to shake things up!
Extra: Repeat this activity, but this time hold the board above the table and move it up and down. Experiment with increasing the speed and distance that you’re moving the cardboard. Notice how this affects the shape and movement of the rings.
Extra: Repeat this activity using different materials to make the rings. Some materials you might try include aluminum foil, thin floral wire (be sure to ask for an adult’s help!), and paper with different thicknesses. Notice how the stiffness of the material affects the movement and shape of the rings.
Observations and Results
Did you notice that at each distance, each ring seemed to have a ‘favorite’ speed, a speed where that ring in particular seemed to have a stronger movement than the others? This is what we expect to see. The largest ring has the most mass, and it’s also the floppiest (or least stiff). Just like with the low E guitar string, having more mass means the biggest ring has the lowest resonant frequency. Therefore, when you were moving the cardboard slowly, the biggest ring was probably more ‘excited’ than the other rings. In contrast, the smallest ring has the smallest mass, and is the least floppy (or the most stiff). As a result, the small ring has a higher resonant frequency, and was the most ‘excited’ when you were moving the cardboard faster.
If you experimented with the speed of the movement, you may have noticed that at least some of the rings had more than one resonant frequency. For example, the big ring vibrated strongly when you were moving the cardboard slowly, but as you sped up it didn’t move as well. Then, when you got fast enough, the big ring seemed to get excited again! This is because the rings (like many objects) have multiple resonant frequencies. However, if you pay close attention you will notice that the shape of the big ring is different at the low resonant frequency, compared to the higher one. At the low frequency it flattens itself out, whereas at the high one it might almost look like a square!
As you increased the distance of the movement, the resonant frequencies didn’t change, but it was probably easier to see how the rings changed shape in response to the movement.
If you moved the cardboard up and down, you probably noticed that the rings followed this movement, instead of moving side to side they seemed to get skinny and fat. The biggest ring still has the lowest resonant frequency, but you may have noticed that it was a little harder to get the smallest ring to move, compared to when you moved the cardboard side to side. This is because when you’re moving the board up and down, the cardboard is adding it’s own stiffness to the rings, making them less floppy in that direction.
More to Explore
Teisha Rowland, PhD, Science Buddies
Science Buddies |
Resonance, resonant frequency, vibration
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