Bab 4 Hukum-hukum gerakan

1 / 105

# Bab 4 Hukum-hukum gerakan - PowerPoint PPT Presentation

Bab 4 Hukum-hukum gerakan. A tugboat, left, is a small but powerful ship used primarily to tow larger ships in harbors or inland waterways. How could such a small boat moves a large object?. Major Concepts. Konsep daya Newton's First Law of Motion Jisim (intersia, graviti)

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about 'Bab 4 Hukum-hukum gerakan' - KeelyKia

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

Bab 4

Hukum-hukum gerakan

A tugboat, left, is a small but powerful ship used primarily to tow larger ships in harbors or inland waterways. How could such a small boat moves a large object?

Major Concepts
• Konsep daya
• Newton's First Law of Motion
• Jisim (intersia, graviti)
• Newton's Second Law of Motion
• Newton's Third Law of Motion
• Applikasi Hukum Newton
• Daya normal
• Daya geseran

Daya

• Daya adalah agen yang menyebabkan perubahan dalam halaju sesuatu objek
• Dalam kata lain, daya adalah agen yang menyebabkan pecutan kepada sesuatu objek
Contoh-contoh daya yang bertindak.

Objek dalam kota adalah objek yang ditindak oleh daya. Daya yang bertindak itu pula berasal daripada agent luar daripada kotak itu (persekitaran)

Daya bersih
• Daya bersih ialah hasiltambah vektor kesemua daya yang bertindak pada sesuatu objek
• Ia juga dikenali daya jumlah (total force), daya hasil (resultant force) atau daya tak terseimbangkan (unbalanced force)

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

Contoh bergambah

Fxdan Fy adalah komponen-konponen leraian daya F yang selari dan berserenjang dengan satah condong

Daya bersih sifar
• Bila daya bersih ialah sifar:
• pecutan sifar
• Halaju malar
• Keseimbangan berkalu jika daya bersih ialah sifar
• Jika objek dalam keadaan rehat akan kekal rehat
• Jika objek bergerak, it kekal bergerak pada halaju malar
Contoh: Blok di atas meja yang pegun

Fup

Fbersih = 0

kekal tak bergerak

Fdown

Kelas daya
• Daya kontak (misalnya, tendangan bola) melibatkan kontak secara fizikal anta dua objek untuk interaksi berlaku
• Daya medan bertindak melalui ruang (misalnya: daya graviti)
• tak payah ada kontak fizikal
Cara sukat daya
• Neraca spring boleh digunakan untuk menentukurkan magnitud sesuatu daya
• Unit daya ialah NewtonN = kg.m/s2
Rangka inersia
• Apa-apa objek atau proses fizikal boleh diperhatikan daripada mana-mana rangka rujukan yang dipilih
• Misalnya, dari kamar kamu ke, dari atas motosikal (laju malar) ke, atau dari kapalterbang yang sedang takeoff
• Misalnya, memerhatikan seorang gadis cantik yang sedang tidur nyenyak dari rangka-rangka rujukan yang berlainan
• rangka-rangka seperti kamar, motosikal laju malar adalah rangka yang berbeza berbanding dengan kapalterbang yang sedang take off
• Dalam rangka rujukan kapalterbang, gadis tidur itu tidak kelihatan halaju malar tapi memecut relatif kepada rangka rujukan
• Dalam rangka rujukan kamar, motosikal laju malar, gadis tidur itu kelihatan berhalaju malar relatif kepada rangka rujukan
• Jadi, ada bezanya di antara rangka yang memecut berbanding dengan rangka yang tidak memecut
• Rangka yang tidak memecut (relatif kepada rangka yang memecut) dipanggil rangka inersia
Apa-apa rangka inersia yang bergerak dengan halaju malar relatif kepada suatu rangka inersia yang diketahui, mereka juga merupakan rangka-rangka inersia.
• Secara praktiknya tiada rangka rujukan inersia yang mutlak
• Rangka rujukan yang bergerak dengan halaju malar relatif kepada bintang jauh adalah penghampiran rangka inersia terbaik
• Bumi dianggap suatu rangka inersia yang baik walaupun terdapat suatu pecutan memusat hasil daripada gerakan kisaran di sekitar paksinya
Quick quiz
• (a) Fikirkan suatu rangka bukan inertial yang you pernah ‘berehat’ di dalam
• (b) Apakah pemerhatian dalam rangka tersebut yang membawa anda kepada kesimpulan bahawa rangka itu bukan inersial?
Hukum Newton pertama
• Dalam ketidakhadiran daya luar, jika diperhatikan dari suatu rangka inersia, sesuatu objek dalam keadaan rehat tetap akan berehat dan objek dalam gerakan tetap akan bergerak pada halaju malar
• Hukum pertama memerihalkan apa yang berlaku dalam ketidakhadiran daya bersih
• Ia juga mengatakan bahawa jika tiada daya bersih bertidak pada suatu objek pecutannya mestilah sifar
• Nota: hukum ini hanya beraplikasi dalam rangka inersia saje, tidak dalam rangka bukan inersia

daya luar hampir sifar, gerak halaju malar

daya luar kurang tapi masih ada

Quick Quiz 5.1

Which of the following statements is most correct?

(a) It is possible for an object to have motion in the absence of forces on the object.

(b) It is possible to have forces on an object in the absence of motion of the object.

(c) Neither (a) nor (b) is correct.

(d) Both (a) and (b) are correct.

Quick Quiz 5.1

Answer: (d). Choice (a) is true. Newton’s first law tells us that motion requires no force: an object in motion continues to move at constant velocity in the absence of external forces. Choice (b) is also true. A stationary object can have several forces acting on it, but if the vector sum of all these external forces is zero, there is no net force and the object remains stationary.

Jisim dan inersia
• Kecenderungan suatu objek untuk menentang usaha mengubah halajunya dikenali inersia
• Jisim ialah sifat sesuatu jasad/objek yang menentukan berapa banyak penentangan objek itu terhadap perubahan dalam halajunya
• Lebih jisim lebih enggan ia berubah halajunya terhadap daya luar
Nota tambahan tentang jisim
• Jisim ialah sifat hakiki sesuatu jasad
• Jisim tidak bergantung kepada cara ia disukat
• Jisim suatu kuantiti skalar
• Unit SI jisim ialah kg
Jisim vs. berat
• Jisim dan berat adalah dua jenis kuantiti yang berlainan
• Berat adalah bersamaan dengan magnitud daya graviti bertindak ke atas sesuatu objek
• Berat berubah-ubah mengikut lokasi, tapi jisim tidak
Hukum Newton kedua
• Daya ialah sebab perubahan dalam pergerakan, yang diukur oleh pecutannya
• Secara algebra, SF = m a

a1 = F/m1

a2 = F/m2

a3 = F/(m1+m2)

Superbike dengan Newton II
• rekaan superbike mengaplikasikan hukum newton kedua: utk memaksimumkan pecutan ke depan motosikal dicipata seringan yang mingkin (supaya m kecil) dan menggunakan engin seberkuasa yang mungkin (supaya daya memecut ke depan lebih besar)
Hukum Newton dalam sebutan komponen
• Hukum Newton juga terexpres dalam sebutan komponennya:
• SFx= m ax
• SFy = m ay
• SFz = m az

Quick Quiz 5.2

An object experiences no acceleration. Which of the following cannot be true for the object?

(a) A single force acts on the object.

(b) No forces act on the object.

(c) Forces act on the object, but the forces cancel.

Quick Quiz 5.2

Answer: (a). If a single force acts, this force constitutes the net force and there is an acceleration according to Newton’s second law.

Quick Quiz 5.3

An object experiences a net force and exhibits an acceleration in response. Which of the following statements is always true?

(a) The object moves in the direction of the force.

(b) The acceleration is in the same direction as the velocity.

(c) The acceleration is in the same direction as the force.

(d) The velocity of the object increases.

Quick Quiz 5.3

Answer: (c). Newton’s second law relates only the force and the acceleration. Direction of motion is part of an object’s velocity, and force determines the direction of acceleration, not that of velocity.

Quick Quiz 5.4

You push an object, initially at rest, across a frictionless floor with a constant force for a time interval Δt, resulting in a final speed of v for the object. You repeat the experiment, but with a force that is twice as large. What time interval is now required to reach the same final speed v?

(a) 4Δt

(b) 2Δt

(c) Δt

(d) Δt/2

(e) Δt/4

Quick Quiz 5.4

Answer: (d). With twice the force, the object will experience twice the acceleration. Because the force is constant, the acceleration is constant, and the speed of the object (starting from rest) is given by v = at. With twice the acceleration, the object will arrive at speed v at half the time.

Daya graviti
• Daya graviti, Fg, adalah daya yang dikenakan ke atas suatu objek oleh bumi
• Daya itu berarah ke bawah dan menuju ke pusat bumi
• Magnitudnya dikenali sebagai berat objek itu
• Berat = |Fg|= mg
• pecutan yang terhasil akibat tindakan graviti ke atas apa-apa objek adalah sama, g
Nota tambahan berkenaan berat
• Disebabkan kesandarannya pada g, berat berubah dengan lokasi
• g, dan seterusnya berat, menjadi makin kurang pada altitud yang lebih tinggi
• Berat bukannya sifat hahiki sesuatu objek
Jisim graviti vs. jisim inersia
• Jisim memainkan dua peranan yang berasing dalam mekanik
• Jisim dalam hukum Newton ialah jisim inersia yang mengukur rintangan
• Jisim inersia ditakrifkan sebagai pemalar kadar (constant of proportionality) antara pecutan dengan daya yang menyebabkannya.
• Inilah apa yang dimaksudkan oleh m dalam F = ma
Jisim graviti vs. jisim inersia, samb
• Manakala, dalam daya tarikan graviti ke atas suatu jasad oleh bumi,

Fg = mgg

• Jisim mg ialah ialah “pemalar kadar” yang menentukan berapa kuatnya daya graviti bertindak di antara objek dengan Bumi
• Lebih besar mg lebih kuatlah tarikan graviti oleh bumi ke atas jasad itu
Jisim graviti diukur
• Cummings 3-9, 3-10
Jisim inersial diukur
• Fig 3.11 cumming
Kesetaraan antara jisim inersia dan jisim graviti
• Eksperimen yang paling jitu telah menentukan, setakat yang dibenarkan oleh teknologi hari ini, bahawa jisim graviti bagi suatu objek adalah sama nilai dengan jisim inersia objek itu
• Penting kerana inilah titik tolak Einstein mengemukakan teori kerelatifan amnya

Quick Quiz 5.5

A baseball of mass m is thrown upward with some initial speed. A gravitational force is exerted on the ball

(a) at all points in its motion

(b) at all points in its motion except at the highest point

(c) at no points in its motion

Quick Quiz 5.5

Answer: (a). The gravitational force acts on the ball at all points in its trajectory.

Quick Quiz 5.6

Suppose you are talking by interplanetary telephone to your friend, who lives on the Moon. He tells you that he has just won a newton of gold in a contest. Excitedly, you tell him that you entered the Earth version of the same contest and also won a newton of gold! Who is richer?

(a) You

(c) You are equally rich

Quick Quiz 5.6

Answer: (b). Because the value of g is smaller on the Moon than on the Earth, more mass of gold would be required to represent 1 newton of weight on the Moon. Thus, your friend on the Moon is richer, by about a factor of 6!

Newton’s Third Law
• If two objects interact, the force F12 exerted by object 1 on object 2 is equal in magnitude and opposite in direction to the force F21 exerted by object 2 on object 1
• F12 = - F21
• Note on notation: FAB is the force exerted by A on B
Newton’s Third Law, Alternative Statements
• Forces always occur in pairs
• A single isolated force cannot exist
• The action force is equal in magnitude to the reaction force and opposite in direction
• One of the forces is the action force, the other is the reaction force
• It doesn’t matter which is considered the action and which the reaction
• The action and reaction forces must act on different objects and be of the same type
Action-Reaction Examples, 1
• The force F12 exerted by object 1 on object 2 is equal in magnitude and opposite in direction to F21 exerted by object 2 on object 1
• F12 = - F21
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
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

Quick Quiz 5.7

If a fly collides with the windshield of a fast-moving bus, which object experiences an impact force with a larger magnitude?

(a) the fly

(b) the bus

(c) the same force is experienced by both

Quick Quiz 5.7

Answer: (c). In accordance with Newton’s third law, the fly and bus experience forces that are equal in magnitude but opposite in direction.

Quick Quiz 5.8

If a fly collides with the windshield of a fast-moving bus, which object experiences the greater acceleration?

(a) the fly

(b) the bus

(c) the same acceleration is experienced by both

Quick Quiz 5.8

Answer: (a). Because the fly has such a small mass, Newton’s second law tells us that it undergoes a very large acceleration. The huge mass of the bus means that it more effectively resists any change in its motion and exhibits a small acceleration.

Quick Quiz 5.9

Which of the following is the reaction force to the gravitational force acting on your body as you sit in your desk chair?

(a) The normal force exerted by the chair

(b) The force you exert downward on the seat of the chair

(c) Neither of these forces

Quick Quiz 5.9

Answer: (c). The reaction force to your weight is an upward gravitational force on the Earth due to you.

Quick Quiz 5.10

In a free-body diagram for a single object, you draw

(a) the forces acting on the object and the forces the object exerts on other objects

(b) only the forces acting on the object

Quick Quiz 5.10

Answer: (b). Remember the phrase “free-body.” You draw one body (one object), free of all the others that may be interacting, and draw only the forces exerted on that object.

Applications of Newton’s Law
• Assumptions
• Objects can be modeled as particles
• Masses of strings or ropes are negligible
• When a rope attached to an object is pulling it, the magnitude of that force, T, is the tension in the rope
• Interested only in the external forces acting on the object
• can neglect reaction forces
• Initially dealing with frictionless surfaces
Objects in Equilibrium
• If the acceleration of an object that can be modeled as a particle is zero, the object is said to be in equilibrium
• Mathematically, the net force acting on the object is zero
Equilibrium, Example 1a
• A lamp is suspended from a chain of negligible mass
• The forces acting on the lamp are
• the force of gravity (Fg)
• the tension in the chain (T)
• Equilibrium gives
Equilibrium, Example 1b
• The forces acting on the chain are T’ and T”
• T” is the force exerted by the ceiling
• T’ is the force exerted by the lamp
• T’ is the reaction force to T
• Only T is in the free body diagram of the lamp, since T’ and T” do not act on the lamp
Equilibrium, Example 2a
• Example 5.4
• Conceptualize the traffic light
• Categorize as an equilibrium problem
• No movement, so acceleration is zero
Equilibrium, Example 2b
• Analyze
• Need two free-body diagrams
• Apply equilibrium equation to the light and find T3
• Apply equilibrium equations to the knot and find T1 and T2
Objects Experiencing a Net Force
• If an object that can be modeled as a particle experiences an acceleration, there must be a nonzero net force acting on it.
• Draw a free-body diagram
• Apply Newton’s Second Law in component form
Newton’s Second Law, Example 1a
• Forces acting on the crate:
• A tension, the magnitude of force T
• The gravitational force, Fg
• The normal force, n, exerted by the floor
Newton’s Second Law, Example 1b
• Apply Newton’s Second Law in component form:
• Solve for the unknown(s)
• If T is constant, then a is constant and the kinematic equations can be used to more fully describe the motion of the crate
• The normal force is not always equal to the gravitational force of the object
• For example, in this case
• n may also be less than Fg
Inclined Planes
• Forces acting on the object:
• The normal force, n, acts perpendicular to the plane
• The gravitational force, Fg, acts straight down
• Choose the coordinate system with x along the incline and y perpendicular to the incline
• Replace the force of gravity with its components
Multiple Objects
• When two or more objects are connected or in contact, Newton’s laws may be applied to the system as a whole and/or to each individual object
• Whichever you use to solve the problem, the other approach can be used as a check
Multiple Objects, Example 1
• First treat the system as a whole:
• Apply Newton’s Laws to the individual blocks
• Solve for unknown(s)
• Check: |P21| = |P12|
Multiple Objects, Example 2
• Forces acting on the objects:
• Tension (same for both objects, one string)
• Gravitational force
• Each object has the same acceleration since they are connected
• Draw the free-body diagrams
• Apply Newton’s Laws
• Solve for the unknown(s)
Multiple Objects, Example 3
• Draw the free-body diagram for each object
• One cord, so tension is the same for both objects
• Connected, so acceleration is the same for both objects
• Apply Newton’s Laws
• Solve for the unknown(s)
Problem-Solving Hints Newton’s Laws
• Conceptualize the problem – draw a diagram
• Categorize the problem
• Equilibrium (SF = 0) or Newton’s Second Law (SF = m a)
• Analyze
• Draw free-body diagrams for each object
• Include only forces acting on the object
Problem-Solving Hints Newton’s Laws, cont
• Analyze, cont.
• Establish coordinate system
• Be sure units are consistent
• Apply the appropriate equation(s) in component form
• Solve for the unknown(s)
• Finalize
• Check extreme values
Forces of Friction
• When an object is in motion on a surface or through a viscous medium, there will be a resistance to the motion
• This is due to the interactions between the object and its environment
• This resistance is called the force of friction
Forces of Friction, cont.
• Friction is proportional to the normal force
• ƒs£µsn and ƒk= µk n
• These equations relate the magnitudes of the forces, they are not vector equations
• The force of static friction is generally greater than the force of kinetic friction
• The coefficient of friction (µ) depends on the surfaces in contact
Forces of Friction, final
• The direction of the frictional force is opposite the direction of motion and parallel to the surfaces in contact
• The coefficients of friction are nearly independent of the area of contact
Static Friction
• Static friction acts to keep the object from moving
• If F increases, so does ƒs
• If F decreases, so does ƒs
• ƒs µs n where the equality holds when the surfaces are on the verge of slipping
• Called impending motion
Kinetic Friction
• The force of kinetic friction acts when the object is in motion
• Although µk can vary with speed, we shall neglect any such variations
• ƒk =µk n

Quick Quiz 5.11

You press your physics textbook flat against a vertical wall with your hand. What is the direction of the friction force exerted by the wall on the book?

(a) downward

(b) upward

(c) out from the wall

(d) into the wall

Quick Quiz 5.11

Answer: (b). The friction force acts opposite to the gravitational force on the book to keep the book in equilibrium. Because the gravitational force is downward, the friction force must be upward.

Quick Quiz 5.12

A crate is located in the center of a flatbed truck. The truck accelerates to the east, and the crate moves with it, not sliding at all. What is the direction of the friction force exerted by the truck on the crate?

(a) to the west

(b) to the east

(c) No friction force exists because the crate is not sliding.

Quick Quiz 5.12

Answer: (b). The crate accelerates to the east. Because the only horizontal force acting on it is the force of static friction between its bottom surface and the truck bed, that force must also be directed to the east.

Quick Quiz 5.13

You place your physics book on a wooden board. You raise one end of the board so that the angle of the incline increases. Eventually, the book starts sliding on the board. If you maintain the angle of the board at this value, the book

(a) moves at constant speed

(b) speeds up

(c) slows down

(d) none of these

Quick Quiz 5.13

Answer: (b). At the angle at which the book breaks free, the component of the gravitational force parallel to the board is approximately equal to the maximum static friction force. Because the kinetic coefficient of friction is smaller than the static coefficient, at this angle, the component of the gravitational force parallel to the board is larger than the kinetic friction force. Thus, there is a net downhill force parallel to the board and the book speeds up.

Quick Quiz 5.14

You are playing with your daughter in the snow. She sits on a sled and asks you to slide her across a flat, horizontal field. You have a choice of (1) pushing her from behind, by applying a force downward on her shoulders at 30° below the horizontal (part a below), or (2) attaching a rope to the front of the sled and pulling with a force at 30° above the horizontal (part b below). Which would be easier for you and why?

(a) #1, because the normal force between the sled and the snow is increased

(b) #1, because the friction force between the sled and the snow is decreased

(c) #2, because the normal force between the sled and the snow is increased

(d) #2, because the friction force between the sled and the snow is decreased

Quick Quiz 5.14

Answer: (b). When pulling with the rope, there is a component of your applied force that is upward. This reduces the normal force between the sled and the snow. In turn, this reduces the friction force between the sled and the snow, making it easier to move. If you push from behind, with a force with a downward component, the normal force is larger, the friction force is larger, and the sled is harder to move.

Friction in Newton’s Laws Problems
• Friction is a force, so it simply is included in the SF in Newton’s Laws
• The rules of friction allow you to determine the direction and magnitude of the force of friction
Friction Example, 1
• The block is sliding down the plane, so friction acts up the plane
• This setup can be used to experimentally determine the coefficient of friction
• µ = tan q
• For µs, use the angle where the block just slips
• For µk, use the angle where the block slides down at a constant speed
Friction, Example 2
• Draw the free-body diagram, including the force of kinetic friction
• Opposes the motion
• Is parallel to the surfaces in contact
• Continue with the solution as with any Newton’s Law problem
Friction, Example 3
• Friction acts only on the object in contact with another surface
• Draw the free-body diagrams
• Apply Newton’s Laws as in any other multiple object system problem

Ringkasan

• students should understand each of the following and be able to demonstrate their understanding in problem applications as well as in conceptual situations.
• Force
• Vector nature of force
• Weight
• Normal force
• Mass
• Newton's laws
• First law (law of inertia)
• Second law (F = ma)
• Third law (action-reaction force pairs)

buatlah soalan-soalan tutorial

untuk latihan dan kefahaman