Moving Air Provides A Lift
Moving air can provide a lift. A simple experiment will help us explain lift.
Take a strip of paper and blow on top of it (Fig.) What happens?
The paper is pushed up. To understand this, let us look at the forces acting on the strip of paper. Firstly, there is gravity which tends to pull it down. So, when you hold the paper near your mouth without blowing air, the paper will hang down due to gravity.
However, when you blow the air above the paper, the pressure on on top of a strip of paper top of the strip of paper is reduced. The pressure below the paper is greater than the pressure above it. This causes the pressure acting on the underside of the paper strip to push the paper up. Thus, moving air provides a ‘lift’.
Activity
Aim: To show that air expands on heating
Materials needed: A small balloon, two buckets/mugs, hot water, and ice Method: ‘
- Blow air into the balloon so that its surface becomes smooth.
- Place the balloon in very cold ice water in a bucket/mug and leave it for some time. Observe what happens to the material of the balloon.
- Now take some very hot water (the water should be as hot as you can bear to touch) in a bucket/mug and place the balloon in it for some time.
Observation: When the balloon is placed in ice-cold water, it develops wrinkles. The wrinkles disappear and the balloon gets back its shape when it is placed in hot water.
Conclusion: The balloon develops wrinkles because the air inside the balloon contracts. It gets back to its original shape when it is heated because the air inside the balloon expands.
