Force vs Stress
 

Force and stress are concepts used in daily life for very commonplace situations, such as exercising power over something is usually being called force, and someone pressurized mentally being called stressed. Though their meanings are derived from the similarity they share in physical concepts, explicit analysis display significant differences from the common usage of the words.

Both concepts are extensively discussed in the field of classical mechanics and they play a pivotal role in explaining the mechanics of materials. However, the concept of force extends well beyond classical mechanics into realms of relativity and quantum mechanics.

What is Force?

Force is known as the factor which causes a change of momentum of a body, where the rate of change of momentum is proportional to the magnitude of the force. Though prevalent for centuries, the concept was mathematically defined and presented by Sir Isaac Newton in Philosophiæ Naturalis Principia Mathematica published in 1687.

From the above expression, which is known as Newton’s second law, the following expression can be derived. F = m.a = d(mv) / dt (Where F is force, m is mass, and a is acceleration). The first law states that an object retains its state (motion in a straight line with constant velocity or at rest) unless acted upon by an unbalanced external force and the third law states that there is an equal and opposite reaction to every force.  

What is Stress?

A force can be acting on a body on a larger area rather than a single line within the body. For example, when a force is applied to the ends of a wooden bar or a rod, the force is transferred equally across the area of the rod. The same situation occurs when the rod is pressed. Since the internal forces created involve both the magnitude and the area which it inflicts on the body, to evaluate the internal forces clearly, both has to be taken into account.

Stress is defined as the force acting on a unit area within a deformable body. Mathematically it is expressed as τ = F / A (where τ is the stress, F is force, and A is the area). Stresses acting on the cross sections are known as Normal stresses (where the net force is normal to the plane of the area considered) and stresses acting parallel to the area is known as Shear stresses (where the net force is tangential to the plane of the area considered).

What is the difference between Force and Stress?

• Stress is the force acting per unit area within a deformable body.

• Force is the factor causing the stress.