You lift a book from the ground and place it on to a table, in this you are applying force. This is a simple yet effective illustration of what force is and how we see and use it everyday. This article will discuss and explain force from a classical mechanics perspective.
Issac Newton explained force as something that causes an object's velocity to change. This means that an object or body that moves with zero velocity (no movement) or a constant velocity (constant motion with no increase or decrease in speed) requires no applied force for this motion to continue. In fact, since acceleration by its very definition is a change in velocity, force is an action required to cause an object to accelerate or decelerate.
In the real world, an object or body, will usually not have one but multiple forces acting on it, we will therefore consider the net force applied. The net force is the summation of all forces acting on the object or body and is also referred to as the total or resultant force. When the total net force or induced acceleration on a body is zero, this is known as the state of equilibrium. In equilibrium, the net force on the object is such that the object is not compelled to move in any manner other than how it currently is, if it is not at rest.
On the other hand, when the net force on an object is not zero, then the object is not in equilibrium and will be moving with some acceleration. The forces acting on an object may be gravitational, frictional or simply applied as external loading.
Gravitational force is the force which attracts objects to the Earths surface (or any other heavenly body with gravitational pull). The gravitational force of an object is determined from its mass and the gravitational constant of the environment it's in (for Earth the gravitational constant, g = 32.2 ft/s2 = 9.80 m/s2).
Frictional forces can be applied to an object in motion when traveling through highly viscous (viscosity referring to the measure of internal friction) liquids or gases or when traveling along rough surfaces.
External force can refer to loading that is applied to the object or body such as a pitched baseball being hit by a bat and being moved as a projectile in a direction opposite of where it was thrown.
A conceptual example will clarify the concept of net force on an object.
Consider a soccer ball at rest is being kicked along a grassy surface. An external force is being applied to the ball as it is being kicked, and so it quickly goes from rest to accelerating.
The soccer ball although not very heavy does have mass and therefore a gravitational force pulling it to the Earth. As the soccer ball is moving along the grassy surface, it is resisted by the blades of grass as well as the atmospheric air.
Altogether, the soccer ball undergoes an external, gravitational, and two frictional forces. From this situation, we realize that the net force will allow the ball to travel some distance as it is being pulled to the Earth while being slowed by the resisting force of the grassy surface and atmospheric air before coming to a complete stop.
Serway, R. A., Beichner, R. J., & Jewett, J. W. (2000). Physics For Scientists and Engineers, Volume 1 (5th Ed.). Orlando, FL: Saunders College Publishing.