Mechanics: Forces

Definition and Types of Forces

A force is any influence that changes the motion of a body, either from its state of rest or motion.
There are two main classifications of forces - contact forces and non-contact forces.
Contact forces require physical interaction between objects. They include tensional, compressive, frictional, and normal forces.
Non-contact forces, or action-at-a-distance forces, act on an object without physical contact. Examples include gravitational, electrical, and magnetic forces.

Objects at rest or moving at a constant velocity are in mechanical equilibrium.
This means that the net force acting on them is zero. This net force is often referred to as the resultant force.
The resultant force is the vector sum of all the forces acting on a body at a given time. If this force is zero, the body is said to be in equilibrium.

Newton’s First Law states that an object in rest or in uniform motion will continue in that state unless acted upon by a net force.
Newton’s Second Law states that the force experienced by a body is equal to the mass of the body multiplied by its acceleration (F = ma).
Newton’s Third Law states that for every action, there is an equal and opposite reaction.

Friction is a force that opposes the relative motion between two objects in contact with each other.
Friction depends on the types of surfaces involved and the force pressing them together.
Air resistance is a type of fluid friction on moving bodies in the air. It acts in the opposite direction to an object’s movement.

Understanding forces is crucial for a wide range of applications, such as predicting a car’s stopping distance, calculating the stability of structures, designing propellers and aeroplane wings, and estimating the effects of gravitational forces on satellites.

The SI unit of force is the Newton (N). Be sure to keep your units consistent when performing calculations.
Vector quantities such as force include both magnitude and direction. When calculating resultant forces, ensure you account for their direction as well as their size.