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Physics Grade 10 Advanced level

Mechanics and kinematics

UNIT 2: Mechanics and Kinematics

Grade 10 Unit AP.2 - Mechanics and Kinematics

Core Standards…………………………….…….. 3

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Contents


Objectives…………………………………....…….. 4

Motion in one dimension………........………….... 5

Deriving equations of motion ................................ 19

Forces and their effects……..............................….. 25

Resultant force ……...........................................….. 32

Friction ……….……..........................................….. 37

Static and dynamic friction...............................….. 40

Questions………………………………..............….. 46

References………………………..…………...…...... 49

10A.26.1 Understand the concepts of displacement, speed, velocity and acceleration, represent them graphically and interpret graphs that represent them.

10A.26.2 Derive, from the definitions of velocity and acceleration, equations that represent uniformly accelerated motion in a straight line and

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26.1

26.2

26.3

26.4

26.5

26.6


use them to solve problems relating to the motion of objects under uniform acceleration.

10A.26.3 Know that a force acting on an object can cause deformation or velocity change

10A.26.4 Identify forces acting on a body, determine resultants, resolve forces into components and use the vector triangle to represent forces in equilibrium

10A.26.5 Show a qualitative knowledge of frictional forces and viscous forces, including air and water resistance, and distinguish between static and dynamic friction.

10A.26.6 Identify factors affecting friction and use the concepts of static and dynamic coefficients of friction.

By the end of the unit, students understand, manipulate and represent graphically the concepts of displacement, speed, velocity and acceleration to solve problems related to moving objects. They know that a force can cause a change in velocity or shape of an object, resolve multiple forces acting on an object and distinguish between dynamic and static friction.

Students who progress further derive equations that represent uniformly accelerated linear motion and use them to solve problems. They know that the

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horizontal and vertical components of projectile motion are independent of one another. They identify factors affecting friction and use the static and dynamic coefficients of friction.

Wherever we look in the universe we find things that are moving. Animals, including humans, are clearly able to move and we can build machines such as cars, boats and aero planes for transport. However, we also find motion in less obvious places.

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26.1Motion in one dimension

واحد بُعد في الحركة


► Displacement measures the change in position هي اإلزاحةالموقع في التغير قياس

Δx=x f−x iThe following figure gives examples of determining displacements for a variety of situations.

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1. DISPLACEMENT اإلزاحة

Represented as Dx (if horizontal) or Dy (if vertical)

بالرمز لها ( Dxنرمز أو ) أفقية رأسية ) Dyلو لو)

Vector quantity (i.e. needs directional information)

) اتجاهها ) لتعريف تحتاج متجهه كمية + or - is generally sufficient to indicate

direction for one-dimensional motion + – بعد في االتجاه لتحديد أو إشارة وضع يكفي


Average velocity is the ratio of the change of position to the time interval during which that changes occurred.

المتوسطة المتجهة ( هيالسرعة الذي ) الزمن في التغير إلى الموقع تغير معدل نسبةالتغيير هذا فيه تم

Consider the car in the figure. The car is moving along a highway in a straight line (the x-axis). Suppose that the positions of

the car are x i at time

t i and x f at time

t f . In

the time intervalΔt=t f−t i , the

displacement of the car is Δx=x f−x i .

Thevavg , is defined as the displacement

divided by the time interval during which the displacement occurred.

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2. VELOCITY المتجهة السرعة

mmm

xxx if

701080

1

D

mmm

xxx if

608020

2

D


) تمت : ) الذي الزمن في التغير إلى اإلزاحة معدل نسبة المتوسطة المتجهة السرعةاإلزاحة فيه

In SI, the unit of velocity is meters per second, abbreviated as m/s.The average velocity of an object can be positive or negative, depending on the sign of the displacement. (The time interval is always positive)

اإلزاحة إشارة حسب سالبة او موجبة تكون أن ممكن المتوسطة المتجهة السرعةExample:

Problem:

8

v⃗1 average=Δ x⃗1

Δt=+70m

10 s=+7m /s

v⃗2 average=Δ x⃗2

Δt=−60m

10 s=−6m /s


During a race on level ground, Hamad runs with an average velocity of 6.02 m/s to the east. What is Hamad’s displacement after 137 s?

. والزمن المتوسطة السرعة أعطيت إذا اإلزاحة احسب

Velocity is not the same as speed

العددية السرعة مثل ليست المتجهة السرعة Velocity describes motion with both a

direction and a numerical value (a magnitude).

واتجاه • مقدار لها المتجهة السرعة Speed has no direction, only magnitude.

فقط • مقدار لها العددية السرعة Average speed is equal to the total distance traveled divided

by the time interval. العددية • على المتوسطةالسرعة مقسومة المقطوعة الكلية المسافة تساوي

الكلي الزمن

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Apparatus: األدوات

Ticker timer and tape, suitable low-voltage A c. power supply, trolley, runway, laboratory jack or stand.

Procedure: الخطوات

1. Set up the apparatus as in the diagram.

2. Connect the ticker timer to a low-voltage power supply.

3. Give the trolley a small push to start it moving.

4. Adjust the angle of inclination of the runway until the trolley moves with constant velocity – the spots on the tape are all equidistant.

5. Most ticker timers make 50 spots per second. Therefore the time interval between two adjacent spots is 0.02 s.

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Practice Lab:

MEASUREMENT OF VELOCITY السرعة قياس


6. Measure the length s of t en adjacent spaces.

7. The time t is 8. As the trolley was travelling at constant velocity we can say that v= st .

9. Repeat using pushes of varying strengths.

10. Tabulate results as shown.

الرسم - في كما األدوات تثبت

بين - المسافات نالحظ حيث ، منتظمة لسرعة حركتها وصول ومالحظة العربة تدفعالزمن ان وحيث ، نقاط عشرة بين المسافة تقاس ثم متساوية، النقاط

السرعة حساب يمكن عندها ، الجهاز يحددها نقطتين كل .بين

Results: النتائج s (m) t (s) v (m/s)

Notes : مالحظات Ignore the initial five or six dots on the tape as this shows

the initial acceleration due to the push.

Ticker timers that use pre carbonated tape are recommended because the friction due to paper drag is reduced.

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Ensure that the voltage rating of the timer is not exceeded.

Some timers make one hundred dots in one second.

The quantity that describes the rate of change of velocity in a given time interval is called acceleration.

للزمن بالنسبة السرعة تغير معدل هي► Acceleration has direction and magnitude. Thus, acceleration is

a vector quantity.متجهة . كمية العجلة لذلك مقدار ولها اتجاه لها العجلة

► Acceleration is the rate of change of velocity with respect to time

► The magnitude of the average acceleration is calculated by dividing the total change in an object’s velocity by the time interval in which the change occurs.

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3.Acceleration العجلة


► The units of acceleration in SI are meters per second per second,

which is written as meters per second squared (m/s2

).

► When measured in these units, acceleration describes how much the velocity changes in each second.

Solved example:A shuttle bus slows down with an average

acceleration of −1 .8 m/s2

. How long does it take the bus to slow from 9.0 m/s to a complete stop?

LٍSolution:

Given: v i= 9 .0 m/s, v f= 0 m/s, aavg=−1 . 8 m/s2

Unknown: Δt

aavg=ΔvΔt

Δt=Δvaavg

=v f−v iaavg

=0−9 .0−1. 8

=5 .0 s

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On a distance-time graph, the slope of the line is numerically equal to the speed.

- السرعة يساوي الخط ميل الزمن، المسافة منحنى فيSlope of a line Average speed

slope=change in vertical coordinateschange in horizantal coordinates vavg=

ΔxΔt

=x f−xit f−ti

If an object is not moving, a horizontal line is shown on a distance-time graph.

الزمن - المسافة منحنى على أفقي بخط يمثل ساكن الجسم كان إذا

Time is increasing to the right, but its distance does not change. It is not moving. We say it is At Rest.

ساكن الجسم أن يقال يمينا، اتجهنا كلما الزمن بزيادة تتغير ال المسافة كانت .إذاIf an object is moving at a constant speed, it means it has the same increase in distance in a given time:

أزمنة في متساوية مسافات يقطع فانه ، منتظمة بسرعة متحرك الجسم كان إذامتساوية

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Motion graphsللحركة البياني التمثيل

Distance-time graphs- الزمن المسافة منحنى


Time is increasing to the right, and distance is increasing constantly with time. The object moves at a constant speed.

. منتظمة بسرعة يتحرك الجسم ، الزمن بازدياد تزداد المسافةA steeper line indicates a larger distance moved in a given time. In other words, higher speed.

. أي المحدد الزمن في اكبر المقطوعة المسافة كانت اكبر الخط ميل كان كلما. سرعة أكثر يكون الجسم

The following figure represents straight-line graphs of position-versus-time for three different objects.

أجسام ثالث حركة يمثل التالي البياني . الرسم

15


Object 1 has a constant positive speed because its position increases uniformly with time. Thus, the slope of this line is positive.Object 2 has zero speed because its position does not change (the object is at rest). Hence, the slope of this line is zero.Object 3 has a constant negative speed because its position decreases with time. As a result, the slope of this line is negative.

الثالث والجسم ساكن، الثاني والجسم موجبة، ثابتة بسرعة يتحرك األول الجسمسالبة ثابتة بسرعة .يتحرك

On a speed-time graph, the slope of the line is numerically equal to the acceleration.

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Speed-time graphs - الزمن السرعة منحنى


- العجلة يساوي الخط ميل الزمن، السرعة منحنى فيSlope of a line Acceleration

slope=change in vertical coordinateschange in horizantal coordinates a= Δv

Δt=v f−v it f−ti

► A straight horizontal line on a speed-time graph means that speed is constant. It is not changing over time.

. بمرور – تتغير ال منتظمة السرعة أن يعني الزمن السرعة منحنى في األفقي الخطالزمن.

This graph shows increasing speed. The moving object is accelerating.

بعجلة . يتحرك الجسم إن الزمن بمرور السرعة في زيادة يمثل البياني .الرسم

17


A steeper line indicates a larger speed moved in a given time. In other words, higher acceleration. . يكون الجسم أي المحدد الزمن في اكبر السرعة كانت اكبر الخط ميل كان كلما) عجلة ) تسارعا .أكثر

► On a speed-time graph, the area under the line is numerically equal to the distance travelled.. المقطوعة – المسافة تمثل المنحنى تحت المساحة الزمن السرعة منحنى في

Problem:

The table below gives some data for a car:

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Speed (m/s) 0 10 20 30 40 50 60

Time (s) 0 1 2 3 4 5 6

Plot a speed-time graph for the car.

- زمن سرعة لمنحني بيانيا مثل

Solution:

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When velocity changes by the same amount during each time interval, acceleration is constant.

تكون • العجلة الزمنية، نفسالفترات خالل بنفسالمقدار السرعة تتغير عندمامنتظمة

The relationships between displacement, time, velocity, and constant acceleration are expressed by the equations shown next. These equations apply to any object moving with constant acceleration.

والعجلة • السرعة ، الزمن ، اإلزاحة بين العالقات عن للتعبير المعادالت تستخدم

These equations use the following symbols:Dx = displacementvi = initial velocityvf = final velocity

Dt = time interval

Equations for Constantly Accelerated Straight line motion

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26.2Deriving equations of motion

الحركة معادالت اشتقاق


Solved example:A racing car reaches a speed of 42 m/s. It then begins a uniform negative acceleration, using its parachute and braking system, and comes to rest 5.0 s later. Find the distance that the car travels during braking.

سيارة لتوقف الالزمة المسافة سرعتها ،حساب بمعرفة التوقف وزمن .االبتدائية

SOLUTION:Given: vi= 42 m/s, v f= 0 m/s, Δt=5 sUnknown: Δx

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Δx=12

( vi+v f )Δt

Δx=12

( 42+0)(5 )

Δx=105 m

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Acceleration due to gravityاألرضية الجاذبية عجلة


Free fall is the motion of a body when only the force due to gravity is acting on the body.

: هي فقط واحدة قوة تأثير تحت الجسم سقوط هو الحر الجاذبية قوة السقوطاألرضية

The acceleration on an object in free fall is called the acceleration due to gravity, or free-fall acceleration.

السقوط عجلة أو األرضية الجاذبية عجلة تسمى الحر السقوط في الجسم عجلةالحر.

When there is no air resistance, all objects fall with the same acceleration regardless of their masses.

اختلفت مهما العجلة بنفس تتحرك األجسام جميع الهواء، مقاومة وجود عدم حالة فيكتلهم

Acceleration due to gravity is constant during upward and downward motion.

األسفل أو لألعلى الحركة كانت سواء ثابتة األرضية الجاذبية عجلة

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Free-fall acceleration on Earth’s surface is (−9 .81 m/s2≈−10 m/s2

) at all points in the object’s motion.

Consider a ball thrown up into the air: Moving upward: velocity is decreasing,

acceleration is −9 .81 m/s2

Top of path: velocity is zero, acceleration is −9 .81 m/s2

Moving downward: velocity is increasing, acceleration is −9 .81 m/s2

Solved example:A ball is thrown vertically upward with a speed of 25.0 m/s from a height of 2.0 m. How long does it take the ball to reach its highest point?

SOLUTION:

Given: vi= 25 . 0 m/s, v f= 0 m/s, g=−10 m/s2

Unknown: Δt

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A Freely falling bodies undergo constant acceleration


v f=vi+g×Δt0=25+−10×Δt

Δt=2510

=2. 50 s

Apparatus:

Millisecond timer, metal ball, trapdoor and electromagnet.

25

Practice Lab:

MEASUREMENT THE ACCELERATION OF FREE FALL, g


Procedure:

1. Set up the apparatus. The millisecond timer starts when the ball is released and stops when the ball hits the trapdoor.

2. Measure the height h as shown, using a meter stick.

3. Release the ball and record the time t from the millisecond timer.

4. Repeat three times for this height h and take the smallest time as the correct value for t.

5. Repeat for different values of h.

6. Calculate the values for g using the equation h=1

2gt2

. Obtain an average value for g. Alternatively draw a graph of h against t

2

and use the slope to find the value of g.

الرسم في كما األدوات تجهيز( االرتفاع ( hنقيس , وكذلك الخطوات تعاد السقوط زمن ونحسب تسقط المعدنية الكرة نجعل

االرتفاع 26


-ومربع االرتفاع بين العالقة تمثيل ويتم ، المرفق الجدول في النتائج تسجل. الجاذبية عجلة تحسب الناتج الميل خالل ومن بيانيا الزمن

Results:h(m) t 1( s ) t 2( s ) t3( s ) t (s ) g( m/s2 )

Notes: Place a piece of paper between the ball bearing and

the electromagnet to ensure a quick release.

In some models of this apparatus, a pressure pad is used in place of the trapdoor; a manually operated

spring-release mechanism may also be used in place of the electromagnet.

What is a force? ؟ القوة هي ما

What effects do forces have on materials and moving objects?

؟ المتحركة واألجسام المواد على تأثيرها ما

27

26.3Forces and their effects

وتأثيرها القوى


How do objects balance? متزنة؟ األجسام تكون كيف

A force is a 'push' or a 'pull'. We need to understand forces because they affect all the objects around us. Some forces only act when two objects are touching.These are called contact force acts on an object only by touching it. (Your hand and the desk exert forces only when they touch the book).Some forces act at a distance (long-range force). For example the gravitational pull of the Earth holds the Moon in orbit at a distance of over 350 000 km!

All the pieces of metal and concrete in this bridge are acted upon by forces. This changes their shapes. Some pieces are in tension: the forces are stretching them. Other pieces are being compressed: the forces are squashing them. The weight of the bridge makes it bend in the middle.

و دفع هي تالمس. سحبالقوة قوى تكون القوى بعض بنا، يحيط ما لدراسة نحتاجهاالجاذبية كقوى بعد عن .وبعضها

وكذلك ضغط أو شد تكون قد بقوى تتأثر المرفق الشكل في الجسر مكونات جميعالمنتصف في ينحني يجعله الذي .وزنة

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Forces change the speed of objectsاألجسام سرعة تغير القوى


Forces make things start to move. They also increase or decrease the object's speed. For example, a plane at rest on the runway will not start to move until its engines exert a force. Once it is moving, the force makes the plane accelerate. On landing, the planehas to exert a force in the opposite direction to make it slow down and stop.

. ساكنة تكون كالطائرة سرعتها من تقلل أو تزيد أو الحركة تبدأ األشياء تجعل فالقوى . الحركة عكس تعمل فالقوى الهبوط في أما تتسارع ثم فتحركها بقوة آالتها تؤثر حتى

. تقف حتى السرعة لتقلل

Moving objects tend to move in straight lines. They only change direction when a force acts. For example, a snooker ball moves in a straight line untilit hits the 'cushion' at the edge of the table. When the ball meets the cushion, a force makes it bounceoff in a different direction.The cue hitting the ball and the ball hitting the cushion are examples of contact forces.

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Forces change the direction of moving objectsاألجسام حركة اتجاه تغير القوى


مستقيم خط في الحركة األجسام بقوة تحاول تتأثر عندما حركتها اتجاه تغير ولكنهابالوسادة تصطدم حتى مستقيم بخط تتحرك التي السنوكر كرة ذلك على وكمثال

حركتها اتجاه من فتغير .الجانبية

1- Place any object-a book, for example- in a student's outstretched hand and ask hat forces are acting on the book and the origin of each force. Downward-gravity; upward student's hand pushing up.

راحة- 1 على جسما الطالب يضع المؤثرة أن القوى تحديد منهم والطلب كالكتاب يدهواتجاهها.

2- Student work in small groups to explore how an unbalanced force can produce acceleration. change the velocity using dynamics trolleys with ticker- timers or light gates.

سرعة- 2 تغير لدراسة الضوئية البوابات أو الزمني الدقاق واستخدام مجموعات عمل. متحرك جسم

We measure force in units called Newton (N).These are named after the famous physicist Sir Isaac Newton.

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Activities أنشطة

The unit of force القوة وحدة


(1N resultant force acting on 1kg produces an acceleration of 1m/s2).We can measure a force using a spring balance or Newton meter.

ورمزه ) النيوتن بوحدة القوة (Nتقاس

عجلة وتكسبه كيلوجرام واحد كتلته جسم على المؤثرة القوة هو الواحد والنيوتن1m/s2مقدارها

Forces can cause materials to stretch. When the forces are removed, the material may spring back to its original length. We call this elasticity. For example, a rubber band can be stretched to several times its length but it will still go back to its original size. Rubber is a very elastic material.

Metal wires will also stretch and spring back into shape if the forces are small. However, if the forces are too large, the wire will be left permanently

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Forces cause deformation on objectsلألجسام تشوه تسبب القوى


Stretched. This happens when we go beyond the material's elastic limit.

المواد استطالة على القوى تعمل القوة حيث بزوال األصلي لوضعه يعود ما ومنهاعلى بقوة التأثير ذلك على وكمثال ، بالمرونة الخاصية هذه تسمى حيث المؤثرة

. معين حد إلى تستطيل حيث والنابض المعدنية األسالك وكذلك المطاط، من شريطدون دائم بشكل يستطيل الحد هذا عن المؤثرة القوة زادت وان المرونة حد يسمى

. المؤثرة القوة زوال عند األصلي للطول الرجوع

Purpose: To determine the relationship between the forces applied to a spring and the vertical distance of its bend.

المؤثرة الهدف: القوة بين العالقة النابضواستطالته دراسة على

Equipments : 1-Meter stick 2-spring scale 3- Ruler 4- loads

- ......األدوات: – - أثقال مسطرة نابض

Theory: The amount of bend or stretch in an elastic material depends on the applied force. This relationship was developed by the British scientist Sir Robert Hook (1635-1703). Hooke's law can be applied to elastic bands, springs, wooden meter sticks

32

Practice Lab: Forces on a spring

: نابض على المؤثرة القوى عملي تقرير


- anything that can bend or stretch. If we bend or stretch too far so called 'elastic limit' is exceeded and the object either breaks or stretches out of shape.

Hook’s Law F = K DxProcedure:

1. Clamp one end of the meter stick to the desk.

2. Adjust the spring scale so that the pointer is on 0 Newton’s.

3. Place the loop around the free end of the stick and hook the spring scale around the loop.

4. Pull with a force of 1 Newton and have a partner record the height of the end of the meter stick above the desk.

5. Repeat step 4. For 2 Newton’s up to 7 Newton’s. (don’t exceed the elastic limit of the spring)

6. Record you data in the table provided.

:الخطوات1 . -، المتوفر حسب أثقال وضع يتم أثقال بدون الصفر عالمة النابضووضع تثبيت بعد

. االستطالة ومقدار المؤثرة القوة مقدار المرفق الجدول في يسجل مرة كل وفي2 -ً بيانيا واالستطالة القوة بين العالقة تمثيل يتم

Data and Data Analysis: 1- Which variable is the independent variable?

33


2- Which variable is the dependent variable?

3- Draw a graph of your data on the grid provided. Draw a line of best fit through the points.

4- List the sources of error for this lab.

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Force (N) Bending (mm)1234

_____ _______67

_____ _______


Question:

1- A spring is 80mm long. It stretches 12mm when a load of 1 N is applied. How long will the spring be when a load of 2N hangs from it?

35


Force is a vector quantity.

1- Two forces are equal when the two forces and direction are the same.

36

26.4 Resultant force (The sum of two or more forces (

أو لقوتين االتجاهي الجمع هي المحصلة القوة


For example the two forces F1 and F2 shown in the figure, are equal. Even though they don’t begin or end at the same point, they have the same length and direction.

كانت- 1 إن حتى واالتجاه المقدار نفس لهما كان إن متساويتين أنهما قوتين عن نقول. الشكل في كما مختلفة البداية نقطة

2- Forces of F1and F2 acting in the same direction,add up to give a resultant force of F =F1 +F2

كانت -2 إذاجمعهما حاصل هي المحصلة فان االتجاه بنفس الجسم على تؤثران .القوتان

3- Forces of F and F acting in opposite directions cancel out to give no resultant force at all: These forces are balanced.

فالمحصلة -3 متعاكسين اتجاهين في وتؤثران مقدارا متساويتان القوتان كانت إذامتزنة القوى أن ونقول صفرا .تساوي

4- Forces of F1 › F2 acting in the opposite direction, a resultant force of F =F1 - F2

37


اتجاه- 4 وفي طرحهما حاصل فالمحصلة األخرى من اكبر القوى إحدى كانت إذااألكبر.

► Note: When the two forces are at right angle the resultant force can determine by:F =√F1

2+F22

نظرية مالحظة:► باستخدام المحصلة فتحسب متعامدتان القوتان كانت إنفيثاغورس.

Suppose two forces are exerted on an object and the sum is not zero how could you find a third force that, when added to the other two would add up to zero? Such a force, one that produces equilibrium, is called the equilibrant.To find the equilibrant, first find the sum of the two forces exerted on the object. This sum is the resultant force, F, the single force that would produce the same effect as the two individual forces added together. The equilibrant is thus a force with a magnitude equal to the resultant, butthe opposite direction. Figure illustrates this procedure for twovectors, but any number of vectors could be used.

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Equilibrium and the Equilibrantاالتزان تحقق التي والقوة االتزان


فإننا اتزان حالة في القوى تكون أن وأردنا صفرا تساوي ال القوى محصلة كانت إذافي الجسم فيكون االتجاه في وتعاكسها للمحصلة مقدارا مساوية ثالثة قوة نضيف

الشكل في كما اتزان حالة في الحالة .هذه

Question:

1- Each set of force vectors below is drawn to the same scale. For each situation, write its letter next to the correct number of the expression.

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a. Forces in set have same direction. ( )

b. Forces in set have same magnitude.( )

c. Forces in set are equal. ( )

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Question:

2- Forces of 12 N and 5 N both act at the same point, but their directions can be varied.

a) What is their greatest possible resultant?

b) What is their least possible resultant?

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c) If the two forces are at right angles, find the resultant.

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26.5 Friction

االحتكاك

Friction

the resistive force that occurs when two surfaces contact each other.


Friction makes it possible: من االحتكاك قوة :تمكنك

To walk. السير To use wheeled vehicles. للسيارات إطارات استخدام To sit. الجلوس To hold books. بكتبك اإلمساك

To know further information about friction you may run this experiment:

التجربة إجراء يمكنك وتأثيراتها االحتكاك عن المزيد لمعرفةاآلتية

Put an object on any ramp ( e.g. wooden beam)

What do you think the forces acting on the object?

What do you think the force that prevents the object to slide?

Gradually try to increase the angle between the ramp and the floor.

Expect when the object will start to move.

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Activity نشاط


What is Friction? االحتكاك قوة هي ما

It’s the force that tries to stop materials sliding across each other.

فوق تتحرك التي المواد إيقاف تحاول التي القوة تلك هيالبعض بعضها

There is friction between your hands when you rub them together and friction between your shoes and the

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ground when you walk along. Friction prevents machinery from moving freely and heats up its moving parts.

عندما السير عند تلحظها وكذلك ببعض، دلكهما عند يديك بين االحتكاك قوة تلحظعن األرض السير د يالمسحذائك

Friction is not always a nuisance. It gives shoes and tyres grip on the ground, and it is used in most braking systems. In cars, for example, rubber pads are pressed against the wheels to slow them down.

الثبات سيارتك وإطارات حذائك تعطي التي فهي مزعجه ً دائما االحتكاك قوة تعتبر الالسيارات كابح نظام في كذلك وتستخدم األرض، على

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Static and dynamic frictionوالحركية الساكنة االحتكاك قوة


To initiate motion of the box the man must overcome the Force of Static Friction

Upon sliding, the baseball player will come to a complete stop due to the Force of Kinetic Friction

When the block below is pulled gently, friction stops it moving. As the force is increased, the friction rises until the block is about to slip. This is the starting or static friction.

حركته تقاوم وتظل تحركه تمنع االحتكاك قوة فإن بلطف الخشبي المكعب جذب عند . االحتكاك بقوة االبتدائية القوة هذه تسمى الحركة في ويبدأ أكبر بقوة تجذبه حتى

الساكنة

With a greater downward force on the block, the static friction is higher. Once the block starts to slide, the friction drops: moving (i.e. dynamic friction is less than static friction).

مقاومة تحاول تظل االحتكاك قوة فإن الحركة في الخشبي المكعب يبدأ وعندماقوة ) من أقل تكون الحركية االحتكاك بقوة عندئذ وتسمى أقل بقوة ولكن الحركة

الساكنة ( االحتكاك

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Dynamic friction heats materials up. When something is moved against the force of friction, its energy of motion (called kinetic energy) is changed into thermal energy (heat ). Brakes and other machinery must be designed so that they get rid of this thermal energy. Otherwise their moving parts may become so hot that they seize up

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Static friction is greater than ….. … dynamic friction


We can compare between the static and dynamic friction by doing the following activity. النشاط هذا خالل من

والحركية الساكنة االحتكاك قوة مقارنة يمكننا

Put a wooden block on a wooden plane. Gradually pull the block by a spring balance using ascending

magnitudes of force ( 2N, 4N, ….. )( keep increasing the force unless the block start to slide)

Record the results in the table below ( static friction). When the block start to slide try to keep its velocity at constant

value ( dynamic friction ) and record the results in the table. Draw a graph between the applied forces and friction force.

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Activity 2 نشاط

Activity 3 نشاط

Factors affect frictionاالحتكاك قوة على المؤثرة العوامل

force friction


In the diagram up use different blocks with different weights. Record the maximum static friction in each case in the table

below (1). From the results you recorded, what do you observe? Now change the plane with different planes with different

materials like what in the table below (2). What is the relation between the kind of material and the friction

produced? مختلفة بأوزان مكعبات استخدم( الجدول في حالة كل في احتكاك قوة أقصى (1سجل تالحظ؟ ماذا ، المسجلة النتائج خالل من وسجل مختلفة مواد من بأسطح واستبدله المستوي السطح غير اآلن

الجدول ) في (2نتائجك الناتجة؟ االحتكاك وقوة المواد نوع بين العالقة هي ما

Now we know that the resistance due to contact between two solid surfaces is called friction.

But when the resistance arises from air or water we describe it as viscous force.

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Table (2)Table (1) Viscous force اللزوجة قوة

Weight friction Material friction


الناتجة االحتكاك اللزوجة عن قوة بقوة توصف الماء أو الهواء

Example: مثال

When a Parachutist jump from the aero plane he starts to fall freely. As the velocity increases, so does the upward viscous force until it equals the downward force of gravity, giving a resultant force of zero so that no further acceleration takes place.

زادت وكلما ، الحر بالسقوط أجسامهم تبدأ الطائرة من المظليون يقفز عندماقوة ) ( تتساوى أن إلى الهواء احتكاك لزوجة قوة معها زادت السقوط سرعة

عندها للعجلة قيمة ال وعندها ً صفرا القوى محصلة وتصبح االحتكاك وقوة .الجاذبية

Questions:

1- Describe and explain the motion of a skydiver

falling from rest.

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2- Name 2 sports where friction is helpful and needed.

Explain

Name 2 sports where friction is reduced for better performance. Explain

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1- Simpson drives his car with an average velocity of 48.0 km/h to the east.How long will it take him to drive 144 km on a straight highway?

السرعة بمعرفة معلومة مسافة قطع زمن .حساب2- What is the average speed of a car which travels 400 m in 20

s?

والزمن المسافة بمعرفة السرعة حساب

3- A car traveling at 7.0 m/s accelerates uniformly at 2 .5 m/s2

to reach a speed of 12.0 m/s. How long does it take for this acceleration to occur?

والنهائية االبتدائية والسرعتين العجلة عرفت إذا المسافة .حساب

4- Omar accelerates his skateboard uniformly along a straight path from

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Questions


rest to 12.5 m/s in 2.5 s. What is Omar’s acceleration? معلوم زمن خالل نهائية لسرعة سكون من الجسم تحرك إذا العجلة حساب

5- A person pushing a stroller starts from rest, uniformly accelerating

at a rate of 0 .500 m/s2

. What is the velocity of the stroller after it has traveled 4.75m?

لج الحركة ومسافة العجلة بمعرفة النهائية السرعة سكون سحساب من بدا م .

6- Use the graphs below to answer questions A-C.ال األسئلة عن واجب البيانية الرسوم تاليةاستخدم

a. Which graph represents an object moving with a constant positive velocity?

b. Which graph represents an object at rest?

c. Which graph represents an object moving with constant positive acceleration?

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7- A plane starting at rest at one end of a runway undergoes a uniform

acceleration of 5 m/s2

for 15 s before takeoff.

a. What is its speed at takeoff?b. How long must the runway be for the plane to be able to take off?

المناسب المطار مدرج طول وكذلك طائرة اقالع سرعة حساب8- A car accelerates uniformly from rest to a speed of 6.6 m/s in 6.5 s.

Find the distance the car travels during this time.

النهائية والسرعة الزمن بمعرفة سكون من تحرك لجسم المسافة .حساب9- A car with an initial speed of 6.5 m/s accelerates at a uniform rate of

0 . 92 m/s2

for 3.6 s. Find the final speed and the displacement of the car during this time.

والزمن والعجلة االبتدائية السرعة بمعرفة المقطوعة والمسافة النهائية السرعة .حساب10- A car accelerates uniformly in a straight line from rest at the rate of

2 .3 m/s2

a- What is the speed of the car after it has traveled 55 m?

b- How long does it take the car to travel 55 m?

أن علما المقطوعة والمسافة العجلة بمعرفة لها الوصل وزمن النهائية السرعة حسابسكون من تحرك الجسم

11- A worker drops a wrench from the top of a tower 80.0 m tall. What is the velocity when the wrench strikes the ground?

معلوم ارتفاع من األرضسقط لسطح جسم وصل سرعة .حساب12-How far above the floor would you need to drop a pencil to

have it land in 1 s?واحدة ثانية خالل لألرضويصل جسم ليسقط االرتفاع .ما

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13- A spring stretches by 10cm when a force of 20N is applied. When a force of 22N is used, the spring stretches by 11 cm.

What would be the extension for forces of ?

1) 10N 2) 1N3) 15N

مختلفة قوى تأثير نابضتحت استطالة حساب

14- The 225-N force is exerted on the crate toward the north and the 165-N force is exerted toward the east. Find the magnitude of the resultant force.

متعامدتين لقوتين محصلة .حساب

15- What are the components of a vector of magnitude 1.5 N at an angle of 35° from the positive x-axis?

البياني بالرسم ممثلة لقوة والعمودية األفقية المركبات .حساب

Resources:Books:

1-Holt Physics )Raymond A. Serway, Ph.D., Jerry S. Faughn, Ph.D.(

2- Complete physics )Stephen Pople(

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3- Physics first )George Bethell & David Coppock(

3- Glencoe Physics )Paul W .Zitzewitz(

Websites:1. http://physics.slss.ie 2.http://science-class.net 3. http://ffden-

2.phys.uaf.edu/211_fall2002.web.dir/ben_townsend/staticandkineticfriction.htm

4. http://www.westminster.edu/acad/sim/pdf/SSTATICANDKINETICFRICTION.pdf

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http://www.westminster.edu/acad/sim/pdf/SSTATICANDKINETICFRICTION.pdf

http://science-class.net/

http://physics.slss.ie/

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