Lesson 1: Newton's First Law of Motion

Newton's First Law

Inertia and Mass

State of Motion

Balanced and Unbalanced Forces


Lesson 2: Force and Its Representation

The Meaning of Force

Types of Forces

Free-Body Diagrams

Determining the Net Force


Lesson 3 : Newton's Second Law of Motion

Newton's Second Law

The Big Misconception

Finding Acceleration

Finding Individual Forces

Free Fall and Air Resistance


Lesson 4 : Newton's Third Law of Motion

Newton's Third Law

Action and Reaction Force Pairs


 

Lesson 2: Force and Its Representation

Types of Forces

A force is a push or pull acting upon an object as a result of its interaction with another object. There are a variety of types of forces. Previously in this lesson, a variety of force types were placed into two broad category headings on the basis of whether the force resulted from the contact or non-contact of the two interacting objects.

Contact Forces

Action-at-a-Distance Forces

Frictional Force
Gravitational Force
Tensional Force
Electrical Force
Normal Force
Magnetic Force
Air Resistance Force

Applied Force

Spring Force

 

These types of individual forces will now be discussed in more detail. To read about each force listed above, continue scrolling through this page. Or to read about an individual force, click on its name from the list below.

 

 

Type of Force

(and Symbol)

Description of Force

Applied Force

Fapp

An applied force is a force which is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

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Gravity Force

(also known as Weight)

Fgrav

The force of gravity is the force at which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity which is directed "downward" towards the center of the earth. The force of gravity on earth is always equal to the weight of the object as found by the equation:

Fgrav = m * g

where g = 9.8 m/s2 (on Earth)

and m = mass (in kg)

(Caution: do not confuse weight with mass.)

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Normal Force

Fnorm

The normal force is the support force exerted upon an object which is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects which are in contact with each other.

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Friction Force

Ffrict

The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. The friction force opposes the motion of the object. For example, if a book moves across the surface of a desk, then the desk exerts a friction force in the opposite direction of its motion. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. The friction force can be calculated using the equation:

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Air Resistance Force

Fair

The air resistance is a special type of frictional force which acts upon objects as they travel through the air. Like all frictional forces, the force of air resistance always opposes the motion of the object. This force will frequently be neglected due to its negligible magnitude. It is most noticeable for objects which travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.

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Tensional Force

Ftens

The tension is the force which is transmitted through a string, rope, or wire when it is pulled tight by forces acting from each end. The tensional force is directed along the wire and pulls equally on the objects on either end of the wire.

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Spring Force

Fspring

The spring force is the force exerted by a compressed or stretched spring upon any object which is attached to it. An object which compresses or stretches a spring is always acted upon by a force which restores the object to its rest or equilibrium position. For most springs (specifically, for those which are said to obey "Hooke's Law"), the magnitude of the force is directly proportional to the amount of stretch or compression.

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A few further comments should be added about the single force which is a source of much confusion to many students of physics - the force of gravity. As mentioned above, the force of gravity acting upon an object is sometimes referred to as the weight of an object. Many students of physics confuse weight with mass. The mass of an object refers to the amount of matter that is contained by the object; the weight of an object is the force of gravity acting upon that object. Mass is related to "how much stuff is there" and weight is related to the pull of the Earth (or any other planet) upon that stuff. The mass of an object (measured in kg) will be the same no matter where in the universe that object is located. Mass is never altered by location, the pull of gravity, speed or even the existence of other forces. For example, a 2-kg object will have a mass of 2 kg whether it is located on Earth, the moon, or Jupiter; its mass will be 2 kg whether it is moving or not (at least for purposes of our study); and its mass will be 2 kg whether it is being pushed or not.

On the other hand, the weight of an object (measured in Newtons) will vary according to where in the universe the object is. Weight depends upon which planet is exerting the force and the distance the object is from the planet. Weight, being equivalent to the force of gravity, is dependent upon the value of g. On earth's surface g is 9.8 m/s2 (often approximated as 10 m/s2). On the moon's surface, g is 1.7 m/s2. Go to another planet, and there will be another g value. Furthermore, the g value is inversely proportional to the distance from the center of the planet. So if we were to measure g at a distance of 400 km above the earth's surface, then we would find the g value to be less than 9.8 m/s2. (The nature of the force of gravity will be discussed in more detail in a later unit of The Physics Classroom.) Always be cautious of the distinction between mass and weight. It is the source of much confusion for many students of physics.

The meaning of each of these forces will have to be thoroughly understood to successfully proceed through this unit. Ultimately, you must be capable of reading a verbal description of a physical situation and know enough about these forces to recognize their presence (or absence) and to construct a free-body diagram which illustrates their relative magnitude and direction.

(Sample problems which you should be able to do are shown elsewhere at our GBS physics site. Also consider the Free Body Diagrams interactive exercise at the Shockwave Physics Studios.)

 

Check Your Understanding

1. Complete the following table showing the relationship between mass and weight.

Object

Mass (kg)

Weight (N)

Melon
1 kg
Apple
0.98 N
Pat Eatladee
25 kg
Fred
980 N

2. Different masses are hung on a spring scale calibrated in Newtons.

  1. The force exerted by gravity on 1 kg = 9.8 N.
  2. The force exerted by gravity on 5 kg = ______ N.
  3. The force exerted by gravity on _______ kg = 98 N.
  4. The force exerted by gravity on 70 kg = ________ N.

 

 

 

3. Does a person diet to lose mass or to lose weight? Explain.

 

 




Lesson 2: Force and Its Representation

Go to Lesson 3

 


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