Class 8 Science
When one surface is moving over another surface, a force comes into play and opposes their relative motion. This force is called friction or force of friction. Friction always opposes motion because it acts in opposite direction to motion. Force of friction arises due to contact between two surfaces. If there is no contact between two surfaces there would be no friction.
Friction is created because of irregularities on surfaces. The irregularities on surfaces result in interlocking of two surfaces. The interlocking between two surfaces creates friction. Hence, the force of friction will be greater in case of rough surface than in case of smooth surface.
The force of friction increases with increase in pressure between two surfaces. You must have seen that moving a heavy box is difficult compared to moving a light box. Pressure is higher in case of a heavy box.
The minimum amount of force that is required to overcome the force of friction is called static friction. In other words, the minimum force that is required to make an object moving is called static friction.
Once an object starts moving on a surface, some force is required to keep the object moving. The minimum force that is required to keep a moving object in motion at a constant speed is called sliding friction. Sliding friction is always less than static friction. Due to this, it is easier to push a moving box compared to pushing a static box.
When an object rolls over a surface, the friction created is called rolling friction. Rolling friction is less than sliding friction.
Static friction > Sliding friction > Rolling friction
Friction is all pervasive. Even a simple task, like writing on a page cannot be done without friction. Walking on road won't be possible because friction. But friction causes wear and tear of materials, and wastage of energy. Thus, friction is beneficial as well as harmful. Hence, friction is called a necessary evil.
When an object moves through a fluid, the fluids create friction. Friction created by a fluid is called fluid friction or drag. Force of friction is directly proportional to the density of a fluid. Hence, friction in oil will be greater than in water. Similarly, friction in water will be greater than that in air.
A shape which is wide in middle and tapered at ends is called a streamlined shape. Such a shape reduces drag or fluid friction. Hence, a streamlined body can easily move through a fluid. Birds and fish are naturally endowed with streamlined body. Boats and airplanes are also made streamlined so that they can easily move through air or water.
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