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Question 8: What is mechanical equilibrium? Explain different types of (mechanical) equilibrium.

Answer

Mechanical Equilibrium

It is the state of a body in which under the action of several forces and torques acting together there is no change in the translational or rotational motion of the body.

Types of Equilibrium

Acceleration is zero when a body is at rest or it is moving with uniform speed. Therefore, there are two cases (types) of equilibrium.

  • Static Equilibrium: A stationary object is a special case of mechanical equilibrium and called to be in static equilibrium. Thus, it is defined as; “a body is said to be in static equilibrium if it is at rest under the action of several forces acting together on the body”.

Example: Consider a book resting on the table. Two forces are acting on the book; one its weight ‘w’ acting downwards and the normal reaction of the surface of the table. However, the book is at rest. Therefore, the two forces cancel one another and there is no unbalanced force on the book. No acceleration is produced in the book and it is in ‘static equilibrium’.

  • Dynamic Equilibrium: When a body is moving with uniform velocity under the action of several forces acting together, the body is said to be in dynamic equilibrium.

When a body is moving with uniform speed, the speed may be either translational (linear) or rotational (angular). Thus, corresponding to the kinds of speed there are two subtypes of Dynamic Equilibrium.

  • Dynamic Translational Equilibrium: When a body is moving with uniform linear velocity the body is said to be in dynamic translational equilibrium. One of its examples is the paratrooper falling down with constant velocity is in dynamic translational equilibrium.
  • Dynamic Rotational Equilibrium: When a body is moving with uniform angular velocity the body is said to be in dynamic rotational equilibrium. An example is the CD rotating in the CD player with constant angular velocity.

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