4.Main Content
experiments
Bernoulli effect demonstration
Demonstration
The Bernoulli effect results in some unexpected behaviours.
Apparatus and materials
- Rubber tube attached to glass tube with narrow jet
- Air blower
- Polystyrene sphere
- Ping-pong ball
- Glass funnel and tubing
- Cardboard tube (approx 5 cm diameter, 30 cm long)
- Cork ball to fit inside the smaller diameter cardboard tube
- Cardboard tube, small (approx 2.5 cm in diameter, 30 cm long)
- Rubber cord
- Cloth, piece of
Technical notes
A toy air blower will not suffice for these experiments. A cylinder vacuum cleaner with the flexible hose put on the 'blowing' end (as opposed to the 'sucking' end) works well.
"Jets" will have to be made from glass tubing before the lesson.
The cloth should be used to wipe up splashes.
Safety
Read our standard health & safety guidance
Procedure
Ball supported by an air jet
a Blow air down the rubber tubing, on the end of which is a glass tube with a jet at the end. You can support a ping-pong ball or light polystyrene sphere (3 cm diameter), or even a beach ball, on the air flow and it will continue to be 'held' even if the jet is tilted over.
Ball supported by an air jet
b You can hold a ping-pong ball similarly in a water jet connected to a water tap. Again, when you tilt the jet the ball will still be supported.
Ball picked up by a funnel
c Connect the glass funnel by rubber tubing to the air blower.
Put the lightweight ball on the table and hold the funnel over it. The air blast through the funnel picks up the ball and holds it.
Experiments involving spin
d Place a light ball, such as a 2.5-cm diameter ball of cork (used by anglers) in a long cardboard tube closed at the lower end. Hold the tube upright by the lower end, with arm stretched up behind your head.
Sweep your out-stretched arm quickly forward and down. The ball rolls out along the upper inside surface of the tube and emerges, spinning fast, around a horizontal axis. The ball's flight then shows a marked curve (upward).
It may help to line the tube with fine sandpaper to make sure the ball rolls rather than slides.
e Fix a length of rubber cord to a bench at two points, A and C. At its centre, B, attach a length of paper tape. Wrap the other end of the tape several times round the middle of a light cardboard cylinder (say, 2.5 cm to 5 cm in diameter and 30 cm long, with its ends closed by paper or Sellotape).
Roll the cylinder along the bench to continue the wrapping until all the tape has been coiled up round it. Then pull the cylinder back across the bench, stretching the elastic cord. Let go of the cylinder so that that the cord catapults it forward. The tape sets it spinning at the same time.
The cylinder will move in a' distorted' path. The Bernoulli effect may even be big enough to make the cylinder loop the loop.
Teaching notes
The Bernoulli effect is the reduction in pressure exerted by a fluid due to its flow. In general, the faster the flow the greater the effect.
In step c the flow of air is fastest in the narrow space and so the pressure is less. The greater pressure below supports the ball.
In step d, a spinning ball (or tube) carries the air around with it. As it flies along a projectile's path it has a combined air flow faster above it than below, so the pressure below the ball is higher and the ball rises.
In step a, the flow of air from the centre of the jet is faster, and so the pressure is less. When the jet is tilted, the ball is pushed into the low pressure, so it doesn't fall until the angle becomes too great.
This experiment was safety-checked in March 2005