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Characteristics of Force

by Ron Kurtus (revised 1 September 2014)

There are two forms of force: applied and resistive. There are various types of each.

An applied force is an interaction of one object on another that causes the second object to change its velocity or direction. It is also called a push or pull that causes a change in motion. This could include a collision, physical contact, a force at a distance, and sometimes friction.

A resistive force resists motion or a change in motion. Friction and inertia are examples of resistive 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 forces

Usually, when you think of a force, you are considering an applied force, which is an an action of one object on another to overcome its inertia and perhaps attempt to overcome any other resistive forces. The force can be a collision, continuous push or pull, or a force at a distance.

Force equation

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

F = ma

where:

Collision

When one object collides with another object, the velocity of both objects will change. In a perfectly elastic collision. the change in velocity is instantaneous. Although a "force" is applied, there is no acceleration. It is mainly a transfer of momentum and energy.

Most collisions are inelastic, meaning that some energy is lost and there is a time lag between the transfer of momentum. In such a case, the force is called an impulse force. Since there is a time lag, the force equation can hold.

Physical contact

The most common form of force is a push or pull through physical contact. Simple examples include:

This type of push or pull is usually the result of some complex process such as a chemical reaction.

Friction

When an object is freely moving along a surface at a constant velocity, the force of friction can be applied to slow or even stop the motion.

Force at a distance

Gravitation, magnetism, and static electricity are some of the forces that act at a distance with no physical contact required to move objects. Whereas many forces are created, these forces occur in nature.

(See The Mysterious Force at a Distance for more information.)

Resistive forces

When an object acts on another object with an applied force, that object will resist motion or a change in motion. This is a resistive force acting on the first object. It is a form of push-back.

Restive forces are basically passive, meaning that they are the result of an active or applied force. They are often called pseudo, virtual, or false forces, because they do not directly cause motion.

Inertia

The inertia of an object provides a resistive force that pushes back on the first object to resist any change in velocity of the second object.

Friction

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

Air resistance

Air resistance pushes back on the first object to resist any motion of the that object

Spring resistance

Spring resistance pushes back on the first object to resist any change in shape of the second object.

Action-Reaction

When considering a force, 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.

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

Whenever one body exerts force upon a second body, the second body exerts an equal and opposite force upon the first body.

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.

This can be seen when swinging an object 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.

Summary

There are two forms of force: applied and resistive. An applied force is a push or pull that causes an object to accelerate. A resistive force resists motion or a change in motion. Applied forces can be by direct contact or at a distance.

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


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Resources and references

Ron Kurtus' Credentials

Websites

Physics Resources

Books

Forces In Nature by Liz Sonneborn Rosen; Publishing Group (2004) $25.25 - Understanding gravitational, electrical and magnetic force

The Science of Forces by Steve Parker; Heinemann (2005) $29.29 - Projects with experiments with forces and machines

Glencoe Science: Motion, Forces, and Energy, by McGraw-Hill; Glencoe/McGraw-Hill (2001) $19.32 - Student edition (Hardcover)

Top-rated books on Physics of Force


Questions and comments

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Physics topics

Characteristics of Force



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Work and Force

Force of Gravity

Force of Friction



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