Kuliah 4 Hukum Newton I

Geological Engineering DepartmentFaculty of Engineering

NEWTON LAW


Isaac Newton (1642 –1727)

• Published Principia• 1687• At age 45

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Newton’s First law (Law of inertia)

In the absence of a net external force, an object at rest remains at rest, and an object in motion continues in motion with constant velocity (i.e., constant speed and direction).

A body at rest will remain at rest A body at rest will remain at rest and a body in motion will remain and a body in motion will remain in the same state of motion unless in the same state of motion unless acted upon by an external acted upon by an external unbalanced force.unbalanced force.

That is: That is: force causes accelerationforce causes acceleration..

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Second Law:

ΣFon object = m aof object

where Fnet is the vector sum of all external forces

on the object considered

• m = (inertial) mass

Newton’s Second Law

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2nd Law of Motion: Implications

The Newton [N] is the unit of force in SI units.

[F] = [m] [ a ][N] = [kg] [m/s/s]

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Forces

• are interactions between two objects (i.e., a push or pull of one object on another)

• can be broadly categorized as contact or non-contact forces

• have a direction and a magnitude -- vectors• can be used to predict and explain the motion

of objects• described by Newton’s Laws of Motion

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Examples

• Pushing table– contact, magnitude, motion…

• Magnets on document camera– non-contact

• Pulling heavy object with two ropes – force is vector …

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Contact Forces Action-at-a-Distance Forces

Frictional Force Gravitational Force

Tension Force Electrical Force

Normal Force Magnetic Force

Air Resistance Force

Applied Force

Spring Force

Types of forces

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Objek dalam kota adalah objek yang ditindak oleh gaya. Gaya yang bertindak itu pula berasal daripada agent luar daripada kotak itu.

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Oleh kerana gaya bersifat vektor, dua gaya yang bertindak secara simultaneously (F1,F2) setara dengan daya bersih R (dan vice versa)

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Penjabaran komponen-kompenen gaya

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a1 = F/m1

a2 = F/m2

a3 = F/(m1+m2)

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Common forces 1. Weight• Gravitational force (weight)

– Universal force of attraction between 2 massive bodies

– For object near earth’s surface directed “downward” with magnitude mg

– Notation: FG

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Gaya Karena Adanya Gravitasi: Berat

• Gaya yang paling umum adl gaya tarikan gravitasi bumi atau dinamakan berat (w).

• Benda dipercepat ke bumi dengan percepatan 9,81 m/s2.

• Dari hukum kedua newton

merumuskan bahwa Fg = m.a atau w = m.g


2nd Law of Motion: Implications

When a sheet of paper and a book are dropped at the same time, what will happen?

Will (1) the book accelerate faster that the sheet; (2) vice versa or (3) will they each accelerate at the same rate?

Why do you think so?

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Common forces 2. Normal forces

• Two objects A, B touch

• exert a force at 90° to surface of contact

• Notation: nB on A is normal force on A due to B

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Gaya NormalTegak lurus bidang kontak.

N

N

N

W

W

W

F

Fx

Fy

W sin W cos

N = W

N = W - Fy

N = W cos

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3. Tension forces• contact force when a string or rope pulls on an object

• exerted on an object

• exerted in direction of string or rope

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4. Friction forces•Kinetic friction

• appears when object slides across surface

• points in direction opposite to velocity

•Static friction

• keeps an object ‘stuck’ on surface (prevents motion)

• points in direction opposite to ‘velocity’

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Gaya GesekSifat-sifat :• Berlawanan dengan arah gerak

atau kearah mana benda akan bergerak.

• Makin kasar makin besar gaya geseknya.

W

W sin

W cos

fN

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Gaya Gesek

• Gaya gesek statis : gaya gesek yang bekerja pada dua permukaan benda yang bersinggungan yang masih dalam keadaan diam relatip satu dengan lainnya

fs = s N• Gaya gesek kinetis : gaya gesek yang

bekerja pada dua permukaan benda yang saling bergerak relatip

fk = k N

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Kinetic Friction

The kinetic friction force is proportional to the magnitude of the normal force.

where the proportionality constant μk is called the coefficient of kinetic friction.

PHY211 Fall 2010 Lecture 6-2

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Static Friction

The box is in static equilibrium, so the static friction must exactly balance the pushing force:

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Newton’s 3rd Law

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Action-Reaction Examples, 2

• The normal force (table on monitor) is the reaction of the force the monitor exerts on the table– Normal means

perpendicular, in this case

• The action (Fg, Earth on monitor) force is equal in magnitude and opposite in direction to the reaction force, the force the monitor exerts on the Earth

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Free Body Diagram

• In a free body diagram, you want the forces acting on a particular object

• The normal force and the force of gravity are the forces that act on the monitor

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GAYA SENTRIPETAL• Dalam gerak melingkar ada percepatan

sentripetal, sehingga berdasarkan hukum Newton II, maka besarnya gaya sentripetal adalah :

rF

2vm

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GAYA Fiktif• Jika kita sedang duduk di Kereta Api (KA),

kemudian KA bergerak maju maka kita akan merasa ada yang mendorong ke belakang bagian atas badan kita.

• Gaya demikian ini disebut gaya fiktif.

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QUESTION:


Newton’s Laws of Motion are:Newton’s Laws of Motion are:

Acceleration (or deceleration) occurs if and Acceleration (or deceleration) occurs if and only if there is a net external only if there is a net external forceforce..

a = F/m [Note this is a vector eqn.]a = F/m [Note this is a vector eqn.]

The force exerted by a first object on a The force exerted by a first object on a second is always equal and opposite second is always equal and opposite

the the the force exerted by the second on the the force exerted by the second on the first. Ffirst. F1212 = - F = - F2121


33rdrd Law in Action—Tug of War Law in Action—Tug of War

??

It’s the Law!It’s the Law!FF1212 = - F = - F2121

#1#1#2#2

FF1212FF2121


Third Law of Motion:Third Law of Motion:

If two objects interact, If two objects interact, the force of the 1the force of the 1stst on the 2 on the 2ndnd is is equal equal and and oppositeopposite to the force of the 2to the force of the 2ndnd acting on the 1st body. acting on the 1st body.

FF1212 = - = - FF2121

FF1212 FF2121 #1#1#2#2


22ndnd Law of Motion: Law of Motion: ExampleExampleBroken Atwood MachineBroken Atwood Machine

F = g (mF = g (m2 2 – m– m11) )

– – μ g μ g (m(m2 2 +m+m11), ),

if mif m2 2 > m> m11

a = F/(ma = F/(m2 2 +m+m11))

aa

aag mg m11

g mg m22

a = g(ma = g(m2 2 – m– m11)/(m)/(m2 2 +m+m11))

- - μ g μ g

FrictionFriction

- μ g (- μ g (mm2 2 +m+m11))


Laws of MotionLaws of Motion

11stst Law: Law:An acceleration is caused by a net An acceleration is caused by a net external force.external force.

Draw Free Body diagram!Draw Free Body diagram!

- mgmg

½ D ½ D ρA v ρA v 22


Laws of Motion in Laws of Motion in Action:Action:

Sledding on a HillSledding on a Hill

Draw “free body diagram”Draw “free body diagram”

Physics 1710Physics 1710 Unit 1—Review Unit 1—Review

ΘΘmgmg

N = mg cos N = mg cos ΘΘ

F = mg sin F = mg sin ΘΘ

FFff = = μNμN

ΣΣF = ma = F – FF = ma = F – Fff = mg sin = mg sin Θ – μ mg cos ΘΘ – μ mg cos Θ


• What will make the biggest difference in What will make the biggest difference in winning a sledding race?winning a sledding race?

1.1. The mass of the sledder.The mass of the sledder.

2.2. The weight of the sled. The weight of the sled.

3.3. The friction of the sled skids or The friction of the sled skids or runners.runners.

4.4. The slope of the hill.The slope of the hill.

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I think that the use of the Response Card I think that the use of the Response Card system contributes to the quality of my system contributes to the quality of my instruction in this course.instruction in this course.

Answer Now !

81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120

1.1. Strongly agree. Strongly agree.

2.2. Agree.Agree.

3.3. Disagree.Disagree.

4.4. Strongly disagreeStrongly disagree


Summary:Summary:

• Newton’s Laws of Motion are:Newton’s Laws of Motion are:

(1) Acceleration (or deceleration) occurs if (1) Acceleration (or deceleration) occurs if and only if there is a net external and only if there is a net external forceforce..

(2)(2) a a = = FF//m m [Note this is a vector eqn.][Note this is a vector eqn.]

(3) The force exerted by a first object on a (3) The force exerted by a first object on a second is second is always always equal and opposite the the equal and opposite the the force exerted by the second on the first. force exerted by the second on the first. FF1212 = = - F- F2121


Summary (cont’d.) :Summary (cont’d.) :

• Weight is the force of gravity equal to Weight is the force of gravity equal to gg times times the mass of the object.the mass of the object.

• gg =9.80 =9.80 N/kgN/kg

• FFff = = μ Nμ N

• Free body diagrams are sketches of all the Free body diagrams are sketches of all the forces acting on a bodyforces acting on a body..



• In a moving or accelerating Frame of In a moving or accelerating Frame of ReferenceReference

• v v ′ ′ = = v – v v – vframe of referenceframe of reference

• a a ′ ′ = = a – a a – aframe of referenceframe of reference

• The The llCentripetal Centripetal acceleration is acceleration is

• a = - a = - ω2 rr

or or ||aa| | = v = v 22/ / |r|,|r|, toward the center. toward the center.



• v v = = vvinitial initial + + aa tt

• r = rr = rinitialinitial + v+ vintitial intitial t t + ½ + ½ aa t t 22

• vvx x 2 2 = v = v x,initialx,initial 22+ 2 a+ 2 axx ΔxΔx

v vy y 2 2 = v = v y,initialy,initial 22+ 2 a+ 2 ayy ΔyΔy

• Mass density ρ = M/VMass density ρ = M/V

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Tugas/Assignment

• Hal.116, No.11• Hal.117, No.15• Hal.117, No.25

Kuliah 4 Hukum Newton I

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