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Can an Object in Equilibrium Be Stationary?

Published in Physics Equilibrium 2 mins read

Yes, absolutely. An object in equilibrium can be stationary.

Equilibrium is a state where the net force acting on an object is zero. This means the object is not accelerating. As defined in physics, there are two primary types of equilibrium:

Types of Equilibrium

The reference material highlights these two types:

  1. Static Equilibrium: An object in static equilibrium is not moving: there is no translational or rotational movement in our chosen frame of reference. This is the specific type of equilibrium where an object remains stationary.
  2. Dynamic Equilibrium: This means the object is moving with a constant velocity. While the net force is still zero, the object is in motion, just not accelerating.

Therefore, the state of being stationary falls under the category of static equilibrium.

Understanding Static Equilibrium

In static equilibrium, not only is the object's velocity zero (it's not moving), but its angular velocity is also zero (it's not rotating). The sum of all forces pushing or pulling on the object cancels out, resulting in a net force of zero. Similarly, the sum of all torques causing rotation also cancels out, resulting in a net torque of zero.

Key Characteristics of Static Equilibrium:

  • Net external force is zero ($\Sigma \mathbf{F} = 0$).
  • Net external torque is zero ($\Sigma \mathbf{\tau} = 0$).
  • Linear velocity is zero ($\mathbf{v} = 0$).
  • Angular velocity is zero ($\mathbf{\omega} = 0$).

Examples of Stationary Objects in Equilibrium

Many everyday objects are in a state of static equilibrium.

  • A book lying motionless on a table. The force of gravity pulling it down is balanced by the normal force from the table pushing it up.
  • A parked car on a level surface. Gravity is balanced by the normal force, and if on a slope, the brakes and friction would counteract the gravitational component along the slope.
  • A building standing still. Its weight is supported by the foundation and structural elements.

Comparing Static and Dynamic Equilibrium

While both types of equilibrium involve zero net force (and zero net torque), the key difference lies in motion:

Feature Static Equilibrium Dynamic Equilibrium
Motion State Stationary (Velocity = 0) Moving at Constant Velocity
Net Force Zero ($\Sigma \mathbf{F}=0$) Zero ($\Sigma \mathbf{F}=0$)
Net Torque Zero ($\Sigma \mathbf{\tau}=0$) Zero ($\Sigma \mathbf{\tau}=0$)
Acceleration Zero ($\mathbf{a}=0$) Zero ($\mathbf{a}=0$)

In conclusion, an object in equilibrium can be stationary specifically when it is in a state of static equilibrium.