Types of ForcesTypes of Forces

Terms, Symbols, and Terms, Symbols, and DefinitionsDefinitions

Force - a push or pull upon an object resulting from the Force - a push or pull upon an object resulting from the object’s interaction with another object. Forces only exist object’s interaction with another object. Forces only exist as a result of an interaction.as a result of an interaction.

Contact Forces - those types of forces which result when Contact Forces - those types of forces which result when the two interacting objects are perceived to be physically the two interacting objects are perceived to be physically contacting each other. contacting each other.

Action-at-a-Distance Forces - those types of forces Action-at-a-Distance Forces - those types of forces which result even when the two interacting objects are which result even when the two interacting objects are not in physical contact with each other, yet are able to not in physical contact with each other, yet are able to exert a push or pull, despite their physical separation. exert a push or pull, despite their physical separation.

Examples of ForcesExamples of Forces

Contact ForcesContact Forces

Frictional Force Frictional Force

Tension ForceTension Force

Normal ForceNormal Force

Air Resistance ForceAir Resistance Force

Applied ForceApplied Force

Spring ForceSpring Force

Action-at-a-Distance ForcesAction-at-a-Distance Forces

Gravitational ForceGravitational Force

Electrical ForceElectrical Force

Magnetic ForceMagnetic Force

UnitsUnits

Force is a quantity which is measured using the standard Force is a quantity which is measured using the standard metric unit known as the Newton metric unit known as the Newton

A Newton is abbreviated by a “N.” A Newton is abbreviated by a “N.”

To say “10.0 N” means 10.0 Newtons of force. One To say “10.0 N” means 10.0 Newtons of force. One Newton is the amount of force required to give a 1-kg Newton is the amount of force required to give a 1-kg mass an acceleration of 1m/s/s. Thus, the following unit mass an acceleration of 1m/s/s. Thus, the following unit equivalency can be stated:equivalency can be stated:

VectorsVectors

A force is a vector quantity A force is a vector quantity A quantity which has both magnitude and direction A quantity which has both magnitude and direction To fully describe the force acting upon an object, you To fully describe the force acting upon an object, you must describe both the magnitude (size or numerical must describe both the magnitude (size or numerical value) and the direction. value) and the direction.

For Example: For Example: – 10 Newtons is not a full description of the force acting upon an 10 Newtons is not a full description of the force acting upon an

object. In contrast, 10 Newtons, downwards is a complete object. In contrast, 10 Newtons, downwards is a complete description of the force acting upon an object; both the description of the force acting upon an object; both the magnitude (10 Newtons) and the direction (downwards) are magnitude (10 Newtons) and the direction (downwards) are given.given.

Type of Type of Force Force

Description of forceDescription of force

Applied Applied ForceForceFFappapp

An applied force is a force which is applied to an object by a An applied force is a force which is applied to an object by a person or another object.person or another object.

Gravity Gravity ForceForce(also known (also known as Weight)as Weight)FFgravgrav

The force of gravity is the force with which the earth, moon, or The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. other massively large object attracts another object towards itself. By definition, this is the weight of the object. The force of gravity on By definition, this is the weight of the object. The force of gravity on earth is always equal to the weight of the object as found by the earth is always equal to the weight of the object as found by the equation:equation:

Fgrav = m * gFgrav = m * gwhere g = 9.8 m/swhere g = 9.8 m/s22 (on Earth) and m = mass (in kg) (on Earth) and m = mass (in kg)

Normal Normal ForceForceFFnormnorm

The normal force is the support force exerted upon an object which The normal force is the support force exerted upon an object which is in contact with another stable object.is in contact with another stable object.

Type of Type of Force Force

Description of forceDescription of force

Friction Friction ForceForceFFfrictfrict

The friction force is the force exerted by a surface as an object The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. moves across it or makes an effort to move across it.

Air Air Resistance Resistance ForceForceFFairair

The air resistance is a special type of frictional force which acts The air resistance is a special type of frictional force which acts upon objects as they travel through the air. The force of air upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object.resistance is often observed to oppose the motion of an object.

Spring ForceSpring ForceFFspringspring

The spring force is the force exerted by a compressed or stretched The spring force is the force exerted by a compressed or stretched spring upon any object which is attached to it.spring upon any object which is attached to it.

Tension Tension ForceForceFFtenstens

The tension force is a force which is transmitted through a string, The tension force is a force which is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from rope, cable or wire when it is pulled tight by forces acting from opposite endsopposite ends

Mass VS WeightMass VS WeightWeightWeight

the weight of an object is the force of the weight of an object is the force of gravity acting upon that objectgravity acting upon that objectweight is related to the pull of the Earth weight is related to the pull of the Earth (or any other planet) upon that (or any other planet) upon that stuffstuffthe weight of an object (measured in the weight of an object (measured in Newtons) will vary according to where in Newtons) will vary according to where in the universe the object isthe universe the object isWeight depends upon which planet is Weight depends upon which planet is exerting the force and the distance the exerting the force and the distance the object is from the planetobject is from the planetWeight, being equivalent to the force of Weight, being equivalent to the force of gravity, is dependent upon the value of gravity, is dependent upon the value of g. On earth's surface g is 9.8 m/sg. On earth's surface g is 9.8 m/s22 Furthermore, the g value is inversely Furthermore, the g value is inversely proportional to the distance from the proportional to the distance from the center of the planet. So if we were to center of the planet. So if we were to measure g at a distance of 400 km measure g at a distance of 400 km above the earth's surface, then we would above the earth's surface, then we would find the g value to be less than 9.8 m/sfind the g value to be less than 9.8 m/s22

MassMassThe mass of an object refers to the The mass of an object refers to the amount of matter that is contained by amount of matter that is contained by the objectthe objectMass is related to how much Mass is related to how much stuffstuff is is therethereThe mass of an object (measured in The mass of an object (measured in kg) will be the same no matter where kg) will be the same no matter where in the universe that object is locatedin the universe that object is locatedMass is never altered by location, the Mass is never altered by location, the pull of gravity, speed or even the pull of gravity, speed or even the existence of other forcesexistence of other forcesFor example, a 2-kg object will have a For example, a 2-kg object will have a mass of 2 kg whether it is located on mass of 2 kg whether it is located on Earth, the moon, or Jupiter; its mass Earth, the moon, or Jupiter; its mass will be 2 kg whether it is moving or not will be 2 kg whether it is moving or not (at least for purposes of our study); (at least for purposes of our study); and its mass will be 2 kg whether it is and its mass will be 2 kg whether it is being pushed upon or not.being pushed upon or not.

Sliding VS Kinetic FrictionSliding VS Kinetic Friction

As mentioned above, the friction force is the force exerted by a 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 surface as an object moves across it or makes an effort to move across it. There are two types of friction force - static friction and across it. There are two types of friction force - static friction and sliding frictionsliding friction

Sliding friction Sliding friction results when an object slides across a surface.results when an object slides across a surface.Sliding FSliding Ffrictfrict = = μμslidingsliding• F• Fnormnorm

The symbol The symbol μμslidingsliding represents the represents the coefficient of sliding frictioncoefficient of sliding friction

Static Friction Static Friction results when the surfaces of two objects are at rest results when the surfaces of two objects are at rest relative to one another and a force exists on one of the objects to relative to one another and a force exists on one of the objects to set it into motion relative to the other object.set it into motion relative to the other object.

Sliding FSliding Ffrictfrict ≤ ≤ μμslidingsliding• F• Fnormnorm

The symbol The symbol μμslidingsliding represents the represents the coefficient of static friction coefficient of static friction between the between the two surfaces.two surfaces.

Balanced ForcesBalanced Forces Because a force is a vector which has a

direction, it is common to represent forces using diagrams in which a force is represented by an arrow

The size of the arrow is reflective of the magnitude of the force and the direction of the arrow reveals the direction which the force is acting

Furthermore, because forces are vectors, the affect of an individual force upon an object is often canceled by the affect of another force

For example, the affect of a 20-Newton upward force acting upon a book is canceled by the affect of a 20-Newton downward force acting upon the book. In such instances, it is said that the two individual forces balance each other; there would be no unbalanced force acting upon the book.

Unbalanced ForcesUnbalanced Forces

Other situations could be imagined in Other situations could be imagined in which two of the individual vector which two of the individual vector forces cancel each other ("balance"), forces cancel each other ("balance"), yet a third individual force exists that is yet a third individual force exists that is not balanced by another force.not balanced by another force.

For example, imagine a book sliding For example, imagine a book sliding across the rough surface of a table across the rough surface of a table from left to right. The downward force from left to right. The downward force of gravity and the upward force of the of gravity and the upward force of the table supporting the book act in table supporting the book act in opposite directions and thus balance opposite directions and thus balance each other. However, the force of each other. However, the force of friction acts leftwards, and there is no friction acts leftwards, and there is no rightward force to balance it. In this rightward force to balance it. In this case, an unbalanced force acts upon case, an unbalanced force acts upon the book to change its state of motion.the book to change its state of motion.

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

ObjectObject Mass (kg)Mass (kg) Weight (N)Weight (N)

MelonMelon 1 kg1 kg ??

AppleApple ?? 0.98 N0.98 N

TurtleTurtle 25 kg25 kg ??

The RickThe Rick ?? 980 N980 N

AnswersAnswers

ObjectObject Mass (kg)Mass (kg) Weight (N)Weight (N)

MelonMelon 1 kg1 kg 9.8 N9.8 N

AppleApple 0.1 kg0.1 kg 0.98 N0.98 N

TurtleTurtle 25 kg25 kg 245 N245 N

The RickThe Rick 100 kg100 kg 980 N980 N

PracticePractice

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

The force exerted by gravity on 1 kg = 9.8 N. The force exerted by gravity on 1 kg = 9.8 N.

The force exerted by gravity on 5 kg = The force exerted by gravity on 5 kg = ? ? N. N.

The force exerted by gravity on The force exerted by gravity on ? ? kg = 98 N. kg = 98 N.

The force exerted by gravity on 70 kg = The force exerted by gravity on 70 kg = ? ? N. N.

  

AnswersAnswers

The force exerted by gravity on 1 kg = 9.8 N. The force exerted by gravity on 1 kg = 9.8 N.

The force exerted by gravity on 5 kg = The force exerted by gravity on 5 kg = 49 49 N. N.

The force exerted by gravity on The force exerted by gravity on 10 10 kg = 98 N. kg = 98 N.

The force exerted by gravity on 70 kg = The force exerted by gravity on 70 kg = 686 686 N. N.

  

PracticePractice

When a person diets, is their goal to lose When a person diets, is their goal to lose mass or to lose weight? Explain.mass or to lose weight? Explain.

AnswersAnswers

When a person diets, is their goal to When a person diets, is their goal to lose mass or to lose weight? Explain.lose mass or to lose weight? Explain.

Generally, people diet because they wish to Generally, people diet because they wish to reduce the amount of matter on their body - they reduce the amount of matter on their body - they wish to remove the wish to remove the blubberblubber. So people diet . So people diet to to lose masslose mass..

PracticePracticeAre these situations balanced or unbalanced?Are these situations balanced or unbalanced?

Situation A: Situation A: ? ?

Situation B: Situation B: ? ?

Situation C: Situation C: ? ?

Situation D: Situation D: ??

AnswersAnswers

Situation A: Situation A: BalancedBalanced

Situation B: Situation B: UnbalancedUnbalanced

Situation C: Situation C: BalancedBalanced

Situation D: Situation D: UnbalancedUnbalanced

Are these situations balanced or unbalanced?Are these situations balanced or unbalanced?