Types of Forces
Types of Forces
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
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.
 
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.
(also known as                   Weight)
Fgrav
The force of gravity is the                   force with 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)
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. For                   instance, if a person leans against a wall, the                   wall pushes horizontally on the person.
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.                   There are at least two types of friction force -                   sliding and static friction. Thought it is not                   always the cast, the friction force often opposes                   the motion of an object. For example, if a book                   slides 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                   maximum amount of friction force which a surface                   can exert upon an object can be calculated using                   the formula below:
Air Resistance                   Force
Fair
The air resistance is a special                   type of frictional force which acts upon objects as                   they travel through the air. The force of air                   resistance is often observed to oppose the motion                   of an object. This force will frequently be                   neglected due to its negligible magnitude (and due                   to the fact that it is mathematically difficult to                   predict its value). 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. Air resistance will                   be discussed in more detail in Lesson 3 .
Tension Force
Ftens
The tension force is the                   force which is transmitted through a string, rope,                   cable or wire when it is pulled tight by forces                   acting from opposite ends. The tension force is                   directed along the length of the wire and pulls                   equally on the objects on the opposite ends of the                   wire.
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 of the spring.
 
Confusion          of Mass and Weight
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 the 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 upon 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.
 
Sliding          versus Static Friction
As mentioned above , the friction          force is the force exerted by a surface as an object moves          across it or makes an effort to move across it. For the          purpose of our study of physics at The Physics Classroom,          there are two types of friction force - static friction and          sliding friction. Sliding          friction results when an object slides across a          surface. As an example, consider pushing a box across a          floor. The floor surface offers resistance to the movement          of the box. We often say that the floor exerts a friction          force upon the box. This is an example of a sliding friction          force since it results from the sliding motion of the box.          If a car slams on its brakes and skids to a stop (without          antilock brakes), there is a sliding friction force exerted          upon the car tires by the roadway surface. This friction          force is also a sliding friction force because the car is          sliding across the road surface. Sliding friction forces can          be calculated from knowledge of the coefficient of friction          and the normal force exerted upon the object by the surface          it is sliding across. The formula is:
Sliding Ffrict =
 Fnorm
The symbol
represents the coefficient of          sliding friction between the two surfaces. The          coefficient value is dependent primarily upon the nature of          the surfaces which are in contact with each other. For most          surface combinations, the friction coefficients show little          dependence upon other variables such as area of contact,          temperature, etc. Values of
have been experimentally determined for a variety of surface          combinations and are often tabulated in technical manuals          and handbooks. The values of
provide a measure of the relative amount of adhesion or          attraction of the two surfaces for each other. The more that          surface molecules tend to adhere to each other, the greater          the coefficient values and the greater the friction          force.
Friction forces can also exist when the two surfaces are          not sliding across each other. Such friction forces are          referred to as static friction. Static friction results          when the surfaces of two objects are at rest relative to one          another and a force exists on one of the objects to set it          into motion relative to the other object. Suppose you were          to push with 5-Newtons of force on a large box to move it          across the floor. The box might remain in place. A static          friction force exists between the surfaces of the floor and          the box to prevent the box from being set into motion. The          static friction force balances the force which you exert on          the box such that the stationary box remains at rest. When          exerting 5 Newtons of applied force on the box, the static          friction force has a magnitude of 5 Newtons. Suppose that          you were to push with 25 Newtons of force on the large box          and the box were to still remain in place. Static friction          now has a magnitude of 25 Newtons. Then suppose that you          were to increase the force to 26 Newtons and the box finally budged from its resting position and was set into          motion across the floor. The box-floor surfaces were able to          provide up to 25 Newtons of static friction force to match          your applied force. Yet the two surfaces were not able to          provide 26 Newtons of static friction force. The amount of          static friction resulting from the adhesion of any two          surfaces has an upper limit. In this case, the static          friction force spans the range from 0 Newtons (if there is          no force upon the box) to 25 Newtons (if you push on the box          with 2 5 Newtons of force). This relationship is often          expressed as follows:
Sliding Ffrict
 Fnorm
The symbol
represents the coefficient of          static friction between the two surfaces. Like          the coefficient of sliding friction, this coefficient is          dependent upon the types of surfaces which are attempting to          move across each other. In general, values of static          friction coefficients are greater than the values of sliding          friction coefficients for the same two surfaces. Thus, it          typically takes more force to budge an object into          motion than it does to maintain the motion once it has been          started.
 
 
The meaning of each of these forces listed          in the table above will have to be thoroughly understood to          be successful during this unit. Ultimately, you must be able          to read 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.