Types of Mechanical Forces

Types of Mechanical Forces

A force exerted on a body can cause a change in either the shape or the motion of the body. The unit of force in SI system is the newton (N) and CGS system is dyne. No solid body is perfectly rigid and when forces are applied to it, changes in dimensions occur. Such changes are not always perceptible to the human eye since they are negligible. For example, the span of a bridge will sag under the weight of a vehicle and a spanner will bend slightly when tightening a nut. Civil engineers and designers need to appreciate the effects of forces on materials, together with the mechanical properties of materials.

Three main types of mechanical forces can act on a body.

They are:
  1. Tensile force
  2. Compressive force and
  3. Shear force

1. Tensile force

Tensile force that tends to stretch a material, as shown in Figure 1 below.

For example,
  1. Rubber bands, when stretched, are in tension.
  2. The rope or cable of a crane carrying a load is in tension.
  3. When a nut is tightened, a bolt is under tension
.
1. Tensile force

A tensile force will increase the length of the material on which it acts.

2. Compressive force

Compressive force that tends to squeeze or crush a material, as shown in Figure 2 below.
Compressive force
2. Compressive force



For example,
  1. A pillar supporting a bridge is in compression.
  2. The sole of a shoe is in compression.
  3. The jib of a crane is in compression.
A compressive force will decrease the length of the material on which it acts.

3. Shear force

Shear force that tends to slide one face of the material over an adjacent face.


Shear force
Figure 3: Shear force


For example,
  1. A rivet holding two plates together is in shear if a tensile force is applied between the plates, as shown in Figure 3.
  2. A guillotine cutting sheet metal, or garden shears, each provides a shear force.
  3. A horizontal beam is subject to shear force.
  4. Transmission joints on cars are subject to shear forces.

A shear force can cause a material to bend, slide or twist.

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