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Applied Force

by Ron Kurtus (29 September 2015)

Usually, when you think of a force, you are considering an applied force, which is an interaction of one object on another that causes the second object to accelerate or change velocity or direction.

The force can be a push, pull, or drag. The resulting direction of an object depends on the relative direction of the force on the object.

A force equation shows the relationship between force, mass, and acceleration. The object will accelerate as long as the force is being applied.

Questions you may have include:

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



Types of applied forces

A force can be applied to another object by a direct push, pull, or drag. A collision is a form of a pushing application. A force at a distance is a form of pulling force.

Push

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

A pushing force is usually the result of some complex process, such as a chemical reaction.

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.

Pull

You can apply a force by directly pulling on an object, such as pulling on a rope to move a box. A special form of pulling is a force at a distance.

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. They typically pull on the object.

Whereas many forces are created, these forces occur in nature.

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

Dragging

If two objects or materials are in contact, and both are allowed to move, the motion of one object can cause motion on the other due to the friction between the surfaces. This dragging effect is different from the resistive force of friction, where one fixed object or material passively restricts the motion of another object.

Results of the direction of the force

The results of the direction that the force is applied on an object vary with the motion of the object.

Stationary object

A force applied on a stationary object will move that object in the direction of the force unless there is a large enough resistive force holding it back.

Moving object

A force applied on a moving object in the same direction of motion will accelerate or increase the speed of that object. This can be seen when pushing on a rolling wagon.

A force applied on a moving object in the opposite direction of motion will reduce the speed of that object. For example, you might slow down when walking into a brisk wind.

At an angle

A force applied at an angle to the motion of an object can not only change its speed but also change its direction. This can be seen by the curved path of the planets due to the gravitational pull from the Sun.

Force equation

The acceleration caused by an applied force is:

a = F/m

This relationship is more often written as the force equation:

F = ma

where:

Note that the object will accelerate as long as the force is being applied. Once the force stops, the object will move freely at a constant speed unless held back by a resistive force.

Summary

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

The force can be a push, pull, or drag. The resulting direction of an object depends on the relative direction of the force on the object.

A force equation is F = ma. The object will accelerate as long as the force is being applied.


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Resources and references

Ron Kurtus' Credentials

Websites

Types of Forces - Physics Classroom

Forces - Physics Hyperbook

Force - Wikipedia

Finding Acceleration - Physics Classroom

Physics Resources

Books

Top-rated books on Physics of Force


Questions and comments

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