Consider an air-bubble level that is sitting on a horizontal surface (Fig. 4.27). 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 €œaccelerometer€ to indicate the direction of the acceleration. Explain the principle involved.