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Overview of Study of Force

by Ron Kurtus (revised 30 September 2014)

In simple terms, a force is a push, a pull, or a drag on an object. There are three main types of force:

An applied force is an interaction of one object on another that causes the second object to change its velocity.

A resistive force passively resists motion and works in a direction opposite to that motion.

An inertial force resists a change in velocity. It is equal to and in an opposite direction of the other two forces.

There is no such thing as a unidirectional force or a force that acts on only one object. There must always be two objects involved, acting on each other. One object acts on the other, while the second resists the action of the first.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

Applied force

An applied force is an interaction that causes the second object to change its velocity.

Force equation

The force required to overcome the inertia of an object is according to the equation:

F = ma


Types of applied force

There are several types of applied force:

The most common form of force is a push through physical contact. For example, you can push on a door to open it. An object can also collide with another object, exerting a force and causing the second object to accelerate. This is another type of push and can be called an impulse force, since the time interval is very short.

You can pull on an object to change its velocity. Gravitation, magnetism, and static electricity are some of the pulling forces that act at a distance with no physical contact required to move objects.

Finally, if two objects or materials are in contact, one can drag the other along by friction or other means.

Resistive force

A resistive force passively inhibits or resists the motion of an object. It is a form of push-back. It is considered passive, since it only responds to actions on the object. Friction and fluid resistance are the major resistive forces.


When an object is being pushed along the surface of another object or material, the resistive force of friction pushes back on the first object to resist its motion.

Fluid resistance

Fluid resistance pushes back on the moving object, which is basically trying to plow through the fluid. It also included friction on the surface of the object.

Air resistance and water resistance are common forms of fluid resistance.

Inertial force

An inertial force works against a change in velocity, caused by an applied force, as well as a resistive force.

Against applied force

According to Newton's Third Law of Motion or the Action-Reaction Law:


This is often stated as: "For every action there is an equal and opposite reaction."

When you push on an object, an equal inertial force pushes back. This is the resistance to acceleration.

Likewise, when swinging an object on a rope around you in a circle, you pull on the rope to change the direction of motion. In turn, you can feel a pull on the rope.

Against resistive force

When a resistive force like friction, slows down the motion of an object, the inertial force will push in the opposite direction and tend to keep the object moving.


The main type of force is an applied force, which is an interaction of one object on another that causes the second object to change its velocity. Other types of forces include a resistive force that passively resists motion and an inertial force that resists a change in velocity.

There must always be two objects involved in a force, acting on each other.

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Ron Kurtus' Credentials


Forces - Physics Hyperbook

Force - Wikipedia

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Resistive force

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