Engineering Mechanics: Contact Forces

The design and analysis problems dealt in engineering mechanics have normal forces whenever any two bodies come in contact. If observed at atomic level bodies actually never touch each other. It is the electromagnetic repulsion between the electrons of the outermost shell of atoms of external layers of the two bodies coming closer which holds one body against the other. But for the purpose of engineering mechanics analysis the force of interaction between bodies in contact is treated as a mechanical force and is called as normal force.

Normal force act, as the name is, normal to the contact surface. Both the surface in contact experience equal magnitude forces in opposite directions. The magnitude of the normal force is determined by the balance of forces acting on the system of bodies. To determine the normal force acting at any contact, make the free body diagram of the body under consideration. Assume a variable for the magnitude of the normal force, the direction of the normal force is perpendicular to the contact surface and towards the body on which calculation is being done. The magnitude of the normal force can be calculated from the usual force balance equations.

When the bodies in contact tend to move along the contact surface, an opposing force between the bodies develops to resist the motion along the contact surface. The contact force developed this way between the bodies is called as Frictional force because it resists the tendency of motion. Frictional force acts parallel to the contact surface and its direction is opposite to the direction in which the body is tending to move. At molecular level the frictional force is developed by the electromagnetic interaction between the outer shell electrons of the atoms of the surfaces in contact. For statics frictional force is as good as any mechanical force.

The magnitude of the frictional force is zero when there is no tendency of motion along the contact surface. The magnitude of the frictional force increases, as the tendency to motion increases and is equal to the component of force, along the contact surface, causing the tendency to motion. The frictional force increases until it reaches its maximum value, which is proportional to the normal force. This maximum value of frictional force is called as Limiting Friction and is equal to the magnitude of the normal force multiplied by a constant called as coefficient of static friction, as there is no relative motion at the contact surface. The coefficient of static friction is characteristic to the pair of surfaces in contact.

There are two forces acting at any contact between two bodies, normal force and frictional force. Net force at the contact is the vector summation of the two forces and is called as the Contact Force. In the next article we will move from statics to dynamics.