This is a simple demo to help visualise the effect of resonance, but it also makes a nice alternative Christmas decoration. It has three different length pairs of vertical rods with weighted baubles on the top and when you pull one rod only its partner will move. The Bloodhound SSC quote below explains the importance of understanding resonance and its effect if you get it wrong!
"This simplified (CFD) model took around a week to solve and gave us a more accurate prediction of the drag generated by the airbrakes, as well as giving us an approximation of the frequencies of the flow around the car, so we know roughly how stiff the airbrake and rear structure needs to be in order to avoid those frequencies, which might otherwise resonate and shake themselves to bits!" Chris Hannon - Stress/Research Engineer
- Senior hacksaw
- Pencil and tape measure
- Centre punch and hammer
- Metal file
- Pliers, spanners and/or a small adjustable
- Bench or pillar drill
- Drill bits
- Laminator (optional)
- Floor lamp base (http://www.argos.co.uk/static/Product/partNumber/4...
- 25mm x 25mm steel bar - 50cm
- M4 or M5 threaded rod - 3m (or can be pre-cut to lengths of 2x 65cm, 2x 50cm and 2x 35cm)
- 12 nuts and 6 spring washers (either M4 or M5 on which sized rod you go for)
- Large nut, bolt and washers to suit the floor lamp base
- 6 coloured Christmas baubles (plastic) or wooden balls
- Picture of Bloodhound SSC (optional)
- Laminating pouch (optional)
Step 1: Metal Cutting and Drilling
- Use a senior hacksaw and cut your metal bar to 50cm, as most are supplied in 1m lengths. Use the metal file to deburr it and remove any sharp edges
- Carefully mark the centre using the punch and hammer, then mark the 6 remaining points. I started from the edge and worked inwards starting 0.5" from the edge, followed by 3" and then 3". Then I repeated it from the opposite end and you get a nice even spread for the rods. I always add a cross using the pencil to make the mark easier to see
- The threaded rod also usually comes in 1m lengths, which makes it easy to cut them to the correct length. Cut the first one in half, then mark the other two at the 65cm mark and cut them. That way you'll end up with two lengths of 50cm, two of 65cm and two of 35cm
- Drill the centre hole according to the size of your lamp base and bolt you're planning to use. Then drill the 6 smaller holes all the way through the metal bar. Deburr the metal bar
Step 2: Prepare the Base
- Take your floor lamp base to pieces
- Use a nut and bolt combo that is long enough to fit through the base and your metal bar
Step 3: Fit It Together
- Mount the innermost and shortest rods (35cm) first, I used Nylocks underneath and a regular nut and spring washer on top
- You can either mount the metal bar on to the floor lamp base now or fit the remaining threaded rods
- Securely add either baubles, wooden balls or something else to the top. If they are loose they will affect the performance of the resonance demo, but the main reason they are there is to prevent you poking your eye out on the exposed threaded rods. They also help the rods to oscillate from side to side.
Step 4: Adding Details and Testing
I thought this demo would look better with a picture of the Bloodhound SSC car on it (I worked on their Education team for 2yrs). I just printed out a hi-res copy of Bloodhound, laminated it and then used Blu Tack to stick it to the metal beam. This demo would have been a useful workshop addition for older students, although younger students find it fascinating too.
When testing it, it works better on softer surfaces (carpet, towels), rather than tables or wooden floors and I'm sure a resonance expert will be able to explain exactly why that is. Overall, I'm pretty pleased with it and using skinnier rods will help the shortest ones to work better - a glow in the dark version would be quite amusing!