Applications of Atmospheric Pressure in Everyday Life
The air pressure inside the rubber sucker becomes very low (and a partial vacuum is created). The much greater atmospheric pressure acting on the rubber sucker from outside fixes the rubber sucker firmly on the flat surface. Thus, a rubber sucker stays attached firmly to a flat surface due to the atmospheric pressure. In order to pull away the fixed rubber sucker from a surface, we will have to apply a force which is large enough to overcome the atmospheric pressure holding it onto the surface. Rubber suckers are usually used to hold glass ‘table tops’ onto the wooden freames of tables. Rubber suckers are also used for making suction hooks which are fixed on walls, doors and almirahs, etc., to hold various things. A suction hook is a rubber (or plastic) sucker having a plastic (or metal) hook attached to it. A suction hook is show in Fig. We are now in a position to answer the following questions :
When we push a box lying on the floor with a small amount of force, it does not move at all. It means that the surface of floor, on which the box is resting, exerts some force on the box which acts in a direction opposite to the force of our push. In other words, some force is acting on the bottom of the stationary box which opposes its motion (due to which the box does not move). The natural force between the floor and the bottom of the box is the opposite of the movement of the box on the floor. Let us give another example. A ball moving on the ground slows down gradually and stops after covering some distance. We know that a force is required to stop a moving ball. This means that a force is exerted by the ground on the moving ball which opposes its motion of ball on the ground is also friction. We can now define friction as follows : The force which always opposes the motion of one object over another object in contact with it, is called friction. Friction occurs between the two surfaces which are in contact with each other. For example, when we push a heavy box kept on floor, the force of friction occurs between the surface of floor and bottom of the box. And in the case of a ball rolling on ground, friction occurs between the surface of ground and the surface of ball. In fact, the friction acts on the two surfaces that are in contact with each other. Friction is a force which occurs when the two objects tend to slide over each other and even when they are actually sliding (moving) over each other.
Direction of Force of Friction
The force of friction always opposes the motion of one object over another object. So, the force of friction acts in a direction opposite to the direction in which an object moves (or tends to move). This will become more clear from the following activity.