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


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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Types of forces

•Contact forces

– normal (?)

– frictional

– tension

– spring

Non-contact forces

– gravitational

– electric

– magnetic

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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 cos

W sin

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:

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Tugas/Assignment

• Hal.116, No.10• Hal.117, No.14• Hal.117, No.26

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Kuliah 4 Hukum Newton I

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