Lockdown Challenge: Wonders of whirlpools
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In this week’s Lockdown Challenge, Neil Downie experiments with centrifugal forces and shows how they can be used to control water and sand.
Lockdown Challenge #36 – Whirling water, centrifugal sand and buckets with holes
Maybe Dear Liza should have told Dear Henry to use centrifugal force. Here we use acceleration in a circle to oppose gravity and control the flow out of a holey bucket.
Vessels with holes in them don’t have to leak water. Visit an antique shop and you may find a ‘toddy lifter’. This is a bottle with a hole at both the top and bottom. You can dip it in a drink, put your finger over the top hole and lift out a measure. Lift your finger and you can drop the measure into a glass. More ingeniously, you can use centrifugal force to do something similar.
If you are whirling around on a carousel, you will feel a force just like gravity which is proportional to the rpm squared and the radius. Centrifugal force makes carousels fun to ride on, but can be used in other ways, like centrifugal casting of pipes, making parabolic telescope mirrors, or making a Whirling Water Lifter.
You’ll need a cylindrical plastic container, for example, 8-12cm diameter and 10-16cm long with a lid strong enough to fit a nut and long bolt to form a mounting spigot on it. Maybe strengthen the lid with large washers made from plywood or from other container lids. Drill a hole in the lid away from the edge – that is to let the air in when you fill with water.
Then, cut a circle one-third or so of the cylinder diameter out of its bottom. Then, fit the lifter cylinder spigot to a battery portable drill (a mains electric drill could be hazardous near water). Now, put the Whirling Water Lifter into the water, speed up the drill and then gently pull it out of the bucket. Some water will drain out, but a lot will lift out.
Measure how much you can lift. Don’t forget to look at the water surface shape if you have a clear container. It’s a parabola! When you slow or stop the drill, the water will swirl out. Speed up the drill and the remaining water is held again. With a one-litre (approximately) container, you should be able to dispense up to 500ml or so. You can put vanes inside to improve how quickly the water speeds up and slows down. This makes the Whirling Water Lifter much faster at picking up and dispensing.
Enough water play? Why not have a go at a Centrifugal Sand Sprinkler? You’ll need some fine sand from the beach or your sandpit. Wash and dry it: the sand must be free-flowing. Prop up a Whirling Water Lifter on the drill pointing upwards and half-fill with sand using a wide-mouth funnel (for example, the end of a soda bottle).
Then start the drill going fairly slowly and gently turn it until it is facing downwards. With luck, a lot of sand will stay inside until you want a shot of sand out of it. Slow down and the parabola grows less steep until a tube-shaped cascade of sand showers out, stopping as you speed up again.
When you pour sand out it doesn’t lie flat like water, it forms a conical heap with a certain angle – the ‘angle of repose’. Due to the angle of repose effects, you can hold up a very large amount of sand inside the cylinder, even at low speeds. Simple valves jam on sand and concrete and the like: could a version of the Centrifugal Sand Sprinkler be useful in industry?
If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org
There is also the back catalogue of Lockdown Challenges from 2020 to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.
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