Friction (denoted by letter 'f') is the resistance to relative motion that occurs whenever two materials are in contact with each other, whether they are solids, liquids, or gases. Since it is a force therefore it is a vector quantity and has unit as newton (N).
Friction always acts in a direction to oppose motion. If you push a solid block along a floor to the right, the force of friction on the block will be to the left. When an object falls downward through the air, the force of friction, air resistance, acts upward.
Every surface is rough, even surfaces that appear to be highly polished can actually look quite rough when examined under a microscope as shown in figure 4.15. There is no such thing as a perfectly flat surface. As a result the two surfaces that are touching are not really touching across the entire area that appears to be touching.
Thus roughness of both surfaces interlock which makes friction.
Slidling friction is the resistance created by any two objects when sliding against each other. It is the sliding friction between the brake pads and our bike rims, that slows the rolling wheels so we can stop our bike in time.
Rim brakes, are the most effective and most popular bicycle brakes, as they provide adequate braking power without too much malintenance. They are controlled by hand lewer's which are attached to the actual brake by a cable. When the rider pulls on the brake lever the cable attached to it moves the two pads, ome on each side of rim. These pads attached to break leather press against the rim, causing the wheel to slow down due to friction as shown in figure 4.16.
Friction is required in many situations, for example
Friction can sometime be a hindrance, for example
There are many ways to reduce unwanted friction, few are discussed below.
If we set a heavy spherical ball, ring or cylinder rolling, it experiences an opposing force callled rolling friction. When a body rolls over a surface, the force of friction is called rolling friction, For the same weight, rolling friction is much smaller (even by 2 or 3 orders of magnitude) than static or sliding friction.
This is the reason why discovery of the wheel has been a major milestone in human history. It is rolling friction that helps a heavy deep freezer with wheels to easily move as shown in figure 4.18.
A fluid is a collection of molecules that are held together by weak cohesive forces and the forces exerted by the walls of container. Both liquids and gases are fluids as they can flow and can exert force on the walls of their container.
When an object moves through a fluid, the fluid exerts a retarding force that tends to reduce the speed of the object. The moving body exerts a force on the fluid to push it out of the way. By Newton's third law, the fluid pushes back on the body with an equal and opposite force. This retarding force experienced by an object moving through a fluid is called the drag force, which is the result of fluid friction.
Dissipative force decreases the mechanical energy in a system. Dissipative forces acting on an object always oppose the motion of the object, For example in case of the sky diver, when the parachute opened some energy is dissipated into the air thereby increasing its temperature. The sky diver safety depends on air resistance as a dissipative force.
In winter when we rub our hands together we feel the sensation of warmth as shown in figure 4.21 (a). It is because friction causes the increase in the temperature our hands, which makes our hands warm. Similarly you would have noticed shooting stars (a small piece of rock or dust that hits Earth's atmosphere from space) as shown in figure 4.21 (b). When they plow through the atmosphere, meteors are heated, and they glow. A meteor compresses air in front of it. The air heats up, in turn heating the meteor. The intense heat vaporizes most meteors, creating what we call shooting stars.