Equilibrium

Equilibrium state of balance. When a body or a system is in equilibrium, there is no net tendency to change. In mechanics, equilibrium has to do with the forces acting on a body. When no force is acting to make a body move in a line, the body is in translational equilibrium; when no force is acting to make the body turn, the body is in rotational equilibrium. A body in equilibrium at rest is said to be in static equilibrium. However, a state of equilibrium does not mean that no forces act on the body, but only that the forces are balanced. For example, when a lever is being used to hold up a raised object, forces are being exerted downward on each end of the lever and upward on its fulcrum, but the upward and downward forces balance to maintain translational equilibrium, and the clockwise and counterclockwise moments of the forces on either end balance to maintain rotational equilibrium. The stability of a body is a measure of its ability to return to a position of equilibrium after being disturbed. It depends on the shape of the body and the location of its center of gravity (see center of mass). A body with a large flat base and a low center of gravity will be very stable, returning quickly to its position of equilibrium after being tipped. However, a body with a small base and high center of gravity will tend to topple if tipped and is thus less stable than the first body. A body balanced precariously on a point is in unstable equilibrium. Some bodies, such as a ball or a cone lying on its side, do not return to their original position of equilibrium when pushed, assuming instead a new position of equilibrium; these are said to be in neutral equilibrium. In thermodynamics, two bodies placed in contact with each other are said to be in thermal equilibrium when, after a sufficient length of time, their temperatures are equal. Chemical equilibrium refers to reversible chemical reactions in which the reactions involved are occurring in opposite directions at equal rates, so that no net change is observed.


Reaction:
Equilibrium is achieved when the forward rate of a reaction is equal to the reverse rate of a reaction. This very simple principle can be observed in a closed container of liquid. In the container the liquid has vapor pressure that is influenced by the pressure above the liquid. In the closed container the pressure above the liquid will raise until it reaches equilibrium vapor pressure. At that time the amount of molecules leaving the liquid is equal to the amount of molecules entering the solution. This equilibrium reaction displays the quality of a reversible reaction.

water D water vapor

Rolling Friction

Rolling friction is the resistive force that slows down the motion of a rolling ball or wheel. This frictional force is typically a combination of several friction forces and is at the point of contact with the ground or other surface. When the materials are both hard, static friction and molecular friction slow down the rolling. When the wheel or tire is soft, its distortion slows down the motion. When the other surface is soft, the plowing effect is a major force in slowing the motion. The coefficient of rolling friction is determined experimentally.

static friction
The surface of the wheel and what it is rolling on are not perfectly smooth. They have irregularities. In sliding friction, this surface roughness is the reason for the static and kinetic resistance to motion. Although the wheel is not sliding, the surface roughness causes a "jiggle" when the wheel is rolling. The resistance from this movement is close to the point where static friction transitions to kinetic friction.
Treads
If the wheel or tire has treads or grooves with sharp edges, those edges add to the static friction when they come into contact with the ground or other surface. Treads can help to prevent spinning the tire when the force from the torque becomes larger than the static friction. They also help prevent skids when braking.
Molecular friction
Molecular friction is caused by the molecular attraction or adhesion of the materials. It is like a "stickiness" factor. When materials are pushed together, molecular forces try to prevent them from being pulled apart. This can be seen in highly polished metals and certain materials such as rubber. As an extreme example, you could put double-sided tape on the rim of a wheel and see the resistance to rolling from the sticky tape.
Wheel is soft
When a wheel or tire is relatively soft and can be easily deformed, the resulting friction is a form of plowing friction. The deformation of the tire takes up energy that would be used to roll the wheel. Deformation is the greatest factor in rolling friction of tires or wheels made of soft materials.
Increasing tire pressure is a way to reduce rolling friction in an automobile or bicycle.
Surface is soft
When the ground or other surface is relatively soft, the major source of friction comes from the plowing effect. The wheel sinks into the soft material and must push or plow its way through. Although rolling is more effective than sliding an object in a soft material, it still requires a substantial effort.
Trying to ride a bicycle through soft dirt is an example of the effect of rolling friction on a soft surface.
Although treads on the wheel or tire will help to move the tire through the soft material, they do not contribute much rolling friction force in resisting the motion.
Both are soft
Interestingly, you can drive a vehicle through soft dirt easier if its tires have less air. The deformation of the tire improves traction.

Reaction:
Rolling friction is the force that slows down the motion of a rolling wheel. This frictional force is typically a combination of several friction forces at the point of contact with the ground. When the materials are both hard, static friction and molecular friction slow down the rolling. When the wheel or tire is soft, its distortion slows down the motion. When the other surface is soft, the plowing effect is a major force in slowing the motion. draft