Consider an air-bubble level that is sitting on a horizontal surface. Initially, the air bubble is in the middle of the horizontal glass tube. (a) If the level is pushed and a force is applied to accelerate it, which way would the bubble move? Which way would the bubble move if the force is then removed and the level slows down, due to friction? (b) Such a level is sometimes used as an “accelerometer” to indicate the direction of the acceleration. Explain the principle involved. [Hint: Think about pushing a pan of water.]
(a) When a force is applied to accelerate the air-bubble level, the bubble will not move. This is because the force applied results in a tendency for the entire level to accelerate together. The air bubble, being in the middle of the horizontal glass tube, remains in the same position relative to the level.
If the force is then removed and the level slows down due to friction, the bubble will move towards the direction of the deceleration. This is because the deceleration causes the level to tilt slightly, and the bubble will move towards the higher end of the glass tube.
(b) The principle involved in using the air-bubble level as an accelerometer is based on the property of fluids to seek their own level. When the level is accelerated, the inertia of the fluid pushes against the glass tube, causing the fluid to respond by slightly tilting. As a result, the air bubble moves towards the higher end of the glass tube. This displacement of the bubble indicates the direction of the acceleration.
This principle can be understood by imagining pushing a pan of water. If the pan is pushed in one direction, the water inside the pan will initially remain at rest due to inertia. However, as the pan accelerates, the water will start to move in the direction opposite to the acceleration. Similarly, when the pan slows down, the water will move towards the direction of deceleration.
In the case of the air-bubble level, the air bubble acts as a reference point to indicate the changes in the level's orientation due to acceleration or deceleration. This makes it a useful tool for measuring and indicating the direction of acceleration.