Friction is a force between two solid surfaces which are in contact with each other and which opposes the relative motion between the two.
The friction arises because of two reasons.
One is, that the surfaces are not smooth on a microscopic level and that leads to rubbing of the two Producing a force opposing the motion.
The second is the interaction between molecules of the two surfaces.
If the interaction is attractive, the motion between the two surfaces is opposed.
The frictional force can never be eliminated but can be reduced by Making the surfaces smoother or by inserting some smooth material (like power or liquid) between two surfaces.
The force is considered to be independent of the relative velocity, Between two surfaces but this is an approximation.
What is described above is sliding friction.
There is another concept of rolling friction where when two bodies are moving relative to each other, the surfaces in contact is stationary.
This happens when a wheel is rolling on a plane.
Here the part of the wheel which is in contact with the plane is not moving and therefore there is no sliding of the two surfaces.
But there is still some frictional force because when the wheel rolls the part of the wheel which is in contact with the plane has to be separated and that requires some force and this force opposes the motion.
Rolling friction is much smaller than sliding friction.
Finally, when a solid is moving in fluid (liquid or gas), there is a force opposing the motion of the solid.
This force is not called friction but viscosity.
Types Of Friction Force
Static Friction
Static means stationary, So, an object will stay-in-place until it experiences a great enough force to overcome the static friction force (Which is the coefficient of static friction multiplied by the normal force).
The coefficient of static friction depends on the surface the object is resting upon and the normal force is the electromagnetic force that keeps all of us from falling into the hot, melting core of the Earth!
Sliding Friction
The friction force acting between two relatively sliding and is measured as the force required to just move the body over the other.
As the name suggests, this friction arises when the object slides over the surface.
This friction is weak in strength from static friction.
You can easily slide the heavy objects from one position to another.
Do you know that without sliding friction you won’t be able to write on paper?
The tip of the pencil slides over the surface and allows you to write perfectly.
Another notable example is the braking system in the bikes.
It is the sliding friction between the brake pads and the bike rim which slows down your bike.
Fluid Friction
Viscosity is also known as fluid friction.
Viscosity is defined as the resistance offered by liquids to motion.
It arises from the internal resistance between layers of moving fluid.
A viscous liquid is one that moves slowly example honey, glycerin, tar, and others.
The viscosity of a liquid determines the terminal velocity of objects moving through them.
The effect of viscosity is taking into consideration in the design of ships, submarine, aircraft and other vehicles to reduce energy wasting to a minimum when these bodies pass through fluids.
Rolling Friction
It is the force resisting the motion of a rolling ball or wheel (a curved surface).
This type of friction is typically a combination of several friction forces at the point of contact between the wheel and the ground or other surfaces.
It is the weakest type of friction (compared to static and sliding friction).
This is the reason that wheels and ball bearings facilitate motion.
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