A body is considered to be in a state of equilibrium when every force that acts up on the body is balanced. The forces are said to be balanced when the right side forces are cancelled by the left side forces and vice versa, and the downward forces are cancelled by upward forces and vice versa. However, this doesn’t imply that every force is equal to one another. Take the case of two bodies; these are at equilibrium when the forces acting on them are balanced. On the other hand, individual forces need not be equal to one another.
In the field of thermodynamics, the concepts of equilibrium are extended to incorporate possible variations in a body’s internal state like its pressure, density, temperature and other quantities that are required to describe its state completely. In the case of uncompromising thermodynamic equilibrium, the system’s temperature is uniform throughout the structure. Any gradients among state functions like density or pressure are balanced by outside forces so that such quantities stay in a constant state. Take the case of an air column; the equilibrium pressure at the base of an air column is much larger than at the top. It is due to the gravitational force, and density gradients in a centrifuge state are stabilised by the centrifugal forces.
Equilibrium is crucial in the well-being of our nature. There are many types of equilibrium states. The fundamental types of mechanical equilibrium are static equilibrium and dynamic equilibrium. Static equilibrium is defined as an equilibrium state where the sum of every force and torque on all particles of the system is equal to zero. Dynamic equilibrium is a form of equilibrium where the result of all forces exerted on the system is zero. The total acceleration of the system is zero; on the other hand, the system’s velocity is not zero. This means that the system is in motion with a fixed velocity. If the total force exerted on the system is zero and the system is still in motion with a constant velocity, the system is considered to be in the state of dynamic equilibrium.
Now let’s explore some of the real-world scenarios of equilibrium.
Real-World Examples of Equilibrium
Equilibrium is attained if every force acting on a body is balanced. There are numerous variations of equilibrium conditions depending on the physical context, whether it is biological, chemical or physical. A common scenario of physical equilibrium is a bike at rest. The gravitational force pulling the bike toward the Earth is stabilised by the force of the ground pushing the bike up. A bike at a fixed velocity is in the state of equilibrium as well since the frictional force is balanced by force generated from the bike’s engine.
When hot tea cools down to an ambient temperature, it is said to reach thermal equilibrium with the ambient condition since the quantity of heat lost by the tea to the air balances the quantity of heat absorbed by the tea from the air. Chemical equilibrium happens in reversible reactions when the forward reaction rate (transforming reactants to products) is equivalent to the backward reaction rate (products to reactants). A case of biological equilibrium is when the speed at which a specific species reproduces balances the speed at which each individual of the exact species dies.
A stationary body or a group of bodies is in the state of static equilibrium (a special type of mechanical equilibrium). A book on a desk is a simple case of static equilibrium. Other scenarios include a stack of cards in the game of rummy and a rock balance sculpture as long as the stack of cards or the sculpture is not in a collapsing state. Bodies in movement can also be in the equilibrium state. A kid sliding down a slanted structure at a fixed speed is in mechanical equilibrium. However, it is not in the state of static equilibrium (relative to the slanted structure of the Earth).
Some of the other familiar examples of equilibrium include: a vehicle moving at a constant velocity, a vehicle at rest in a parking space, two bodies at equal temperature, two kids balancing on a see-saw, two bodies with identical charge density and weight block suspended by brick or spring lying on a flat surface.