Recently we investigated the influence of the air around us through a number of experiments. There were a lot of concepts that the children would not have had a chance to think hard about, but they had fun all the same. Hopefully the experiences will give cause for thought later. Most of these are easy to repeat at home.
Weight of air
Air really has something in it. The children inflated balloons and weighed them, and then weighed the balloons after the air was let out.
The deflated balloons were lighter than when they were filled, because of the extra air they held. (The air inside is under pressure, so is more dense than the surrounding atmosphere).
Air under pressure applies a force as it escapes a balloon. We cast a string across the room and threaded a straw onto the string. The children inflated balloons and taped them to the straw, then let go of the end of the balloon to let it rush down the string.
I brought in a variety of parachutes of different sizes, shapes and materials, ranging from paper napkins to plastic containers. Each had a small plastic toy suspended underneath. The junior scientists compared the effectiveness of the different parachutes by launching them from the second floor of the school building. The best parachutes were those that were lightest with the greatest area, since they caught more air and so experienced more drag.
Using hair-dryers (on a cold setting) to create a strong stream of air pointed directly upwards, the children placed various balls in the stream to see what would happen. Some light balls (ping pong balls, or even better the polystyrene balls available from craft stores) floated even in the stream, even when the hair-dryer was held at an angle. This is due to an effect that is tricky even for adults to understand, called Bernoulli’s principle: fast moving air is lower in pressure than slower-moving air. As the ball falls out of the stream it is pushed back in by the (high-pressure) stationary air outside the (low-pressure) stream. The point here is not to understand Bernoulli’s principle, just to appreciate this phenomenon exists.
One parent kindly supplied a pump from a bouncy castle, that allowed us to see this effect with a really large ball. I know what I want for my next birthday now…
Bernoulli’s principle makes flight possible, because it is the shape of an aeroplane’s wing that provides lift. The children tried to make a variety of wing shapes, threaded onto string, rise by blowing onto them with a hairdryer. Wings with circular or square cross-sections didn’t lift, but aerofoil shape did (just about). An aerofoil has a longer path over its top than under its base, so the pressure above is lower and the wing is pushed upwards.
Useful references for this week can be found on the web:
Floating balls and Bernoulli’s principle
The greatest parachute ride so far, made by Fearless Felix from the edge of space
Science Club facilitator