grade 10 advanced level - science curriculum office ... 10 unit ap.2 ... ticker timer and tape, ......

Physics

Grade 10 Advanced level

Mechanics and kinematics

2

Grade 10 Unit AP.2 - Mechanics and Kinematics

UNIT 2: Mechanics and Kinematics

Core Standards…………………………….…….. 3

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

26.1

26.2

26.3

Contents

26.4

26.5

26.6

3


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 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.

4


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

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.

5


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.

► Displacement measures the change in position ارغير في املعي لياش اإلزاحح

if xxx

26.1 Motion in one dimension

احذ احروح في تعذ

1. DISPLACEMENT

اإلزاحح

Represented as x (if horizontal) or y (if vertical)

ورمس ا تارمس x (أفميح ) أy ( رأسيح )

Vector quantity (i.e. needs directional information)

(ذحراج رعريف اذداا) وميح مرد

+ or - is generally sufficient to indicate direction for one-dimensional motion

رحذيذ االذداي في تعذ احذ + أ –يىفي ضع إشارج

6


The following figure gives examples of determining displacements for a variety of situations.

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

رغ١ش الغ إ ازغ١ش ف اض از ر ف١ زا ازغ١١ش( ؼذي)غجخ ازعطخ ازغخ اغشػخ

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 ix at

time it and fx at time ft . In the time interval

if ttt , the displacement of the car is

if xxx .

2. VELOCITY امردح اسرعح

7


The avgv , is defined as the displacement divided by the time interval during

which the displacement occurred.

اإلصاحخ إ ازغ١ش ف اض از رذ ف١ اإلصاحخ( ؼذي)غجخ : ازعطخ ازغخ اغشػخ

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)

اإلصاحخ إشبسحرى عجخ ا عبجخ حغت أازعطخ ى ازغخ اغشػخ

Example:

sm

s

m

t

xv average

7

10

7011

sm

s

m

t

xv average

6

10

6022

8


Problem:

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?

. اسرعح امرسطح اسمه أعطيد إرا اإلزاحححسة ا

Velocity is not the same as speed

اعذديحاسرعح يسد مث امردح اسرعح

Velocity describes motion with both a direction and a numerical value (a magnitude).

اغشػخ ازغخ ب مذاس ارغب •

Speed has no direction, only magnitude.

اغشػخ اؼذد٠خ ب مذاس فمػ •

Average speed is equal to the total distance traveled divided by the time interval.

رغب اغبفخ اى١خ امطػخ مغخ ػ اض اى ازعطخ اغشػخ اؼذد٠خ •

9


Apparatus: األداخ

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

Procedure: اخطاخ

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.

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

Practice Lab:

MEASUREMENT OF VELOCITY اسرعحلياش

10


7. The time t is 10 × 0.02 = 0.2 s.

8. As the trolley was travelling at constant velocity we can say that t

sv .

9. Repeat using pushes of varying strengths.

10. Tabulate results as shown.

وب ف اشع األدادرضجذ -

٠خ، ص زغب مبغ ح ، ح١ش الحظ اغبفبد ث١ اياؼشثخ الحظخ صي حشوزب غشػخ زظرذفغ -

ر١ ٠حذدب اغبص ، ػذب ٠ى حغبة مػ، ح١ش ا اض ث١ و مبغرمبط اغبفخ ث١ ػششح

.اغشػخ

Results: اىرائح

s (m) t (s) v (m/s)

Notes: مالحظاخ

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.

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

Some timers make one hundred dots in one second.

11


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

ؼذي رغ١ش اغشػخ ثبغجخ ض

► Acceleration has direction and magnitude. Thus, acceleration is a vector

quantity.

زه اؼغخ و١خ زغخ. اؼغخ ب ارغب ب مذاس

► 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.

3. Acceleration

حعحاي

12


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

written as meters per second squared ( 2m/s ).

► 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 2m/s 8.1 . How long does it take the bus to slow from 9.0 m/s

to a complete stop?

Solution:

t

avv

:Unknown

m/s 1.8 m/s, 0 m/s, 9.0 :Given 2

avgfi

s 0.58.1

0.90

avg

if

avg

avg

a

vv

a

vt

t

va

13


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

اض، ١ اخػ ٠غب اغشػخ -ف ح اغبفخ

Slope of a line Average speed

scoordinate horizantalin change

scoordinate lin vertica changeslope

if

if

avgtt

xx

t

xv

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

اض -ػ ح اغبفخ أفموب اغغ عبو ٠ض ثخػ إرا

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

Motion graphs

ارمثي اثياوي حروح

Distance-time graphs

اض-ح اغبفخ

14


. اغغ عبو أاض وب ارغب ١٠ب، ٠مبي ص٠بدحوبذ اغبفخ ال رزغ١ش ة إرا

If an object is moving at a constant speed, it means it has the same increase in distance in a given time:

زغب٠خ أصخ٠مطغ غبفبد زغب٠خ ف فباغغ زحشن ثغشػخ زظخ ، وب إرا

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

.اغغ ٠زحشن ثغشػخ زظخ، اض ثبصد٠بداغبفخ رضداد

A steeper line indicates a larger distance moved in a given time. In other words, higher speed.

.ػخعش أوضشاغغ ٠ى أ. وب وب ١ اخػ اوجش وبذ اغبفخ امطػخ اوجش ف اض احذد

15


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

. أعغباشع اج١ب ازب ٠ض حشوخ صالس

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.

. صبثزخ عبجخ ثغشػخ٠زحشن ثغشػخ صبثزخ عجخ، اغغ اضب عبو، اغغ اضبش ٠زحشن األياغغ

16


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

اض، ١ اخػ ٠غب اؼغخ -ف ح اغشػخ

Slope of a line Acceleration

scoordinate horizantalin change

scoordinate lin vertica changeslope

if

if

tt

vv

t

va

► A straight horizontal line on a speed-time graph means that speed is

constant. It is not changing over time.

.رزغ١ش ثشس اض ال. اغشػخ زظخ أاض ٠ؼ –ف ح اغشػخ األفماخػ

Speed-time graphs

اسمه -مىحى اسرعح

17


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

.اغغ ٠زحشن ثؼغخ إ. اشع اج١ب ٠ض ص٠بدح ف اغشػخ ثشس اض

A steeper line indicates a larger speed moved in a given time. In other words, higher acceleration.

(.ػغخ) رغبسػب أوضشاغغ ٠ى أ. وب وب ١ اخػ اوجش وبذ اغشػخ اوجش ف اض احذد

► On a speed-time graph, the area under the line is numerically equal to

the distance travelled.

.اض اغبحخ رحذ اح رض اغبفخ امطػخ –ف ح اغشػخ

18


Problem:

Solution:

The table below gives some data for a car:

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.

ص -ض ث١ب١ب ح عشػخ

19


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

خالي فظ افزشاد اض١خ، اؼغخ رى زظخ ػذب رزغ١ش اغشػخ ثفظ امذاس •

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.

، اض ، اغشػخ اؼغخ اإلصاحخرغزخذ اؼبدالد زؼج١ش ػ اؼاللبد ث١ •

These equations use the following symbols:

x = displacement

vi = initial velocity

vf = final velocity

t = time interval

Equations for Constantly Accelerated Straight line motion

26.2 Deriving equations of motion

اشرماق معادالخ احروح

20


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.

.تمعرفح سرعرا االترذائيح زمه ارلف ،حساب امسافح االزمح رلف سيارج

SOLUTION:

x :Unknown

s 5 m/s, 0 m/s, 42 :Given fi

tvv

m 105

)5)(042(2

1

)(2

1

x

x

tvvx fi

21


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

األسظ١خاغبرث١خ جق لح احذح فمػ رأص١شعغ رحذ عمغ اي: اغمغ احش

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

.ػغخ اغمغ احش أ األسظ١خػغخ اغغ ف اغمغ احش رغ ػغخ اغبرث١خ

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

رزحشن ثفظ اؼغخ ب اخزفذ وز األعغبف حبخ ػذ عد مبخ ااء، ع١غ

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

األعف أ ألػصبثزخ عاء وبذ احشوخ األسظ١خػغخ اغبرث١خ

Acceleration due to gravity

األرضيحعدح ادارتيح

22


Free-fall acceleration on Earth’s surface is ( 22 m/s 10m/s 81.9 ) at all points in the object’s motion.

Consider a ball thrown up into the air:

Moving upward: velocity is decreasing, acceleration is 2m/s 81.9

Top of path: velocity is zero, acceleration is 2m/s 81.9

Moving downward: velocity is increasing, acceleration is 2m/s 81.9

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:

:Unknown

m/s 01 m/s, 0 m/s, 25.0 :Given 2

fi

t

gvv

s 50.210

25

10250

t

t

tgvv if

A Freely falling bodies undergo constant acceleration

سمط األخسا احر ذحد ذأثير عدح ثاترح

23


Apparatus:

Millisecond timer, metal ball, trapdoor and electromagnet.

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.

Practice Lab:

MEASUREMENT THE ACCELERATION OF FREE FALL, g

24


6. Calculate the values for g using the equation 2

2

1gth .

Obtain an average value for g. Alternatively draw a graph of h against 2t and use the slope to find the value of g.

وب ف اشع األدادرغ١ض

م١ظ االسرفبع(h)

رؼبد اخطاد وزه االسرفبع , غؼ اىشح اؼذ١خ رغمػ حغت ص اغمغ

- رغغ ازبئظ ف اغذي اشفك ، ٠ز رض١ اؼاللخ ث١ االسرفبع شثغ اض ث١ب١ب خالي

.ا١ ابرظ رحغت ػغخ اغبرث١خ

Results:

(m) h (s) 1t (s) 2t (s) 3t (s) t )(m/s 2g

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.

25


What is a force? امح ؟ب

What effects do forces have on materials and moving objects?

؟ ب رأص١شب ػ ااد األعغب ازحشوخ

How do objects balance? و١ف رى األعغب زضخ؟

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.

ػعب ػ ثؼذ وم ةحزبعب ذساعخ ب ٠ح١ػ ثب، ثؼط ام رى ل رالظ . عحتامح دفغ

.اغبرث١خ

ع١غ ىبد اغغش ف اشى اشفك رزأصش ثم لذ رى شذ أ ظغػ وزه صخ از ٠غؼ ٠ح ف

.ازصف

26.3 Forces and their effects

ام ذأثيرا

26


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 plane has to exert a force in the opposite direction to make it slow down and stop.

وبطبئشح رى عبوخ حز رؤصش آالرب ثمح . فبم رغؼ األش١بء رجذأ احشوخ أ رض٠ذ أ رم عشػزب

. أب ف اجغ فبم رؼ ػىظ احشوخ زم اغشػخ حز رمف. فزحشوب ص رزغبسع

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 until it hits the 'cushion' at the edge of the table. When the ball meets the cushion, a force makes it bounce off in a different direction. The cue hitting the ball and the ball hitting the cushion are examples of contact forces.

س ثمح وضبي ػ ره وشح سىب رغ١ش ارغب حشوزب ػذب رزأ ي األعغب احشوخ ف خػ غزم١ارح

.اغوش از رزحشن ثخػ غزم١ حز رصطذ ثبعبدح اغبج١خ فزغ١ش ارغب حشوزب

Forces change the speed of objects

ام ذغير سرعح األخسا

Forces change the direction of moving objects

ام ذغير اذداي حروح األخسا

27


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.

.عبب٠ذ وبىزبة اطت رحذ٠ذ ام اؤصشح اد أ ٠عغ اطبت عغب ػ ساحخ -1

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.

.ع زحشنػ غػبد اعزخذا اذلبق اض أ اجاثبد اعئ١خ ذساعخ رغ١ش عشػخ ط -2

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

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

(N)رمبط امح ثحذح ا١ر سض

1m/s2ا١ر ااحذ امح اؤصشح ػ عغ وزز احذ و١عشا رىغج ػغخ مذاسب

Activities أوشطح

The unit of force حذج امج

28


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

ب ب ٠ؼد ظؼ األص ثضاي امح اؤصشح ح١ش رغ ز ح١ش رؼ ام ػ اعزطبخ ااد

اخبص١خ ثبشخ ، وضبي ػ ره ازأص١ش ثمح ػ شش٠ػ اطبغ، وزه األعالن اؼذ١خ

حذ ؼ١ ٠غ حذ اشخ ا صادد امح اؤصشح ػ زا احذ ٠غزط١ ثشى ح١ش رغزط١ إ. ابثط

.دائ د اشعع طي األص ػذ صاي امح اؤصشح

Forces cause deformation on objects

ذسثة ذشي ألخسا ام

29


Purpose: To determine the relationship between the forces applied to a

spring and the vertical distance of its bend.

ػ ابثط اعزطبز دساعخ اؼاللخ ث١ امح اؤصشح :اذف

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

...... -أصمبي –غطشح -بثط :األداخ

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 - 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 x

Procedure:

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

Practice Lab:

Forces on a spring

ام امؤثرج ع واتض: ذمرير عمي

30


the spring) 6. Record you data in the table provided.

: اخطاخ

ازفش ، ف و شح ٠غغ ٠ز ظغ أصمبي حغت . ثؼذ رضج١ذ ابثط ظغ ػالخ اصفش ثذ أصمبي -1

. ف اغذي اشفك مذاس امح اؤصشح مذاس االعزطبخ

٠ز رض١ اؼاللخ ث١ امح االعزطبخ ث١ب١با -2

Data and Data Analysis:

1- Which variable is the independent variable?

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.

Force (N) Bending (mm)

1

2

3

4

_____ _______

6

31


4- List the sources of error

for this lab.

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?

7

_____ _______

32


Force is a vector quantity.

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

امج امحصح ي ادمع االذداي مذيه أ أوثر

33


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

مي ػ لر١ أب زغب٠ز١ إ وب ب فظ امذاس االرغب حز إ وبذ مطخ اجذا٠خ خزفخ وب -1

.ف اشى

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

.إرا وبذ امرب رؤصشا ػ اغغ ثفظ االرغب فب احصخ حبص عؼب -2

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

إرا وبذ امرب زغب٠زب مذاسا رؤصشا ف ارغب١ زؼبوغ١ فبحصخ رغب صفشا مي أ -3

.ام زضخ

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

34


.إرا وبذ إحذ ام اوجش األخش فبحصخ حبص غشحب ف ارغب األوجش -4

► Note: When the two forces are at right angle the resultant force can

determine by:

F = 𝐹12 + 𝐹2

2

.إ وبذ امرب زؼبذرب فزحغت احصخ ثبعزخذا ظش٠خ ف١ضبغسط :مالحظح ►

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, but the opposite direction. Figure illustrates this procedure for two vectors, but any number of vectors could be used.

رغب صفشا أسدب أ رى ام ف حبخ ارضا فئب ع١ف لح صبضخ غب٠خ الوبذ حصخ ام إرا

.االرغب ف١ى اغغ ف ز احبخ ف حبخ ارضا وب ف اشىمذاسا حصخ رؼبوغب ف

Equilibrium and the Equilibrant

االذسان امج اري ذحمك االذسان

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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.

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?

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

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Friction makes it possible: :ذمىىه لج االحرىان مه

To walk. اغ١ش

To use wheeled vehicles. اعزخذا إغبساد غ١بساد

To sit. اغط

To hold books. اإلغبن ثىزجه

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

26.5 Friction

االحرىان

Friction

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

38


ؼشفخ اض٠ذ ػ االحزىبن رأص١شارب ٠ىه إعشاء ازغشثخ ا٢ر١خ

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.

Activity اطوش

39


What is Friction? لج االحرىان يما

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

ره امح از رحبي إ٠مبف ااد از رزحشن فق ثؼعب اجؼط

There is friction between your hands when you rub them together and friction between your shoes and the ground when you walk along. Friction prevents machinery from moving freely and heats up its moving parts.

ئه األسض رحظ لح االحزىبن ث١ ٠ذ٠ه ػذ دىب ثجؼط، وزه رحظب ػذ اغ١ش ػذب ٠الظ حزا

اغ١ش د ػ

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.

از رؼط حزائه إغبساد ع١بسره اضجبد ػ األسض، رغزخذ ف اا ضػظال رؼزجش لح االحزىبن دائ

اغ١بساد وبثحف ظب وزه

40


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.

حز رغزث ثمح أوجش ػذ عزة اىؼت اخشج ثطف فئ لح االحزىبن رغ رحشو رظ رمب حشوذ

ثمح االحزىبن اغبوخ االثزذائ١خرغ ز امح . ٠جذأ ف احشوخ

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).

ػذب ٠جذأ اىؼت اخشج ف احشوخ فئ لح االحزىبن رظ رحبي مبخ احشوخ ى ثمح أل رغ

( رى أل لح االحزىبن اغبوخ ) ػذئز ثمح االحزىبن احشو١خ

Static and dynamic friction

لج االحرىان اساوىح احرويح

41


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

Static friction is greater than ….. … dynamic friction

42


We can compare between the static and dynamic friction by doing the following activity. خالي زا اشبغ ٠ىب مبسخ لح االحزىبن اغبوخ احشو١خ

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.

force friction

Activity 2 2 اطوش

43


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?

خزفخ اعزخذ ىؼجبد ثأصا

(1)عغ ألص لح احزىبن ف و حبخ ف اغذي

خالي ازبئظ اغغخ ، برا رالحظ؟

(2)ثأعطح اد خزفخ عغ زبئغه ف اغذي اعزجذا٢ غ١ش اغطح اغز

اؼاللخ ث١ ع ااد لح االحزىبن ابرغخ؟ ب

Weight friction

Material friction

Table (2)

Activity 3 3 اطوش

Table (1)

Factors affect friction

اعام امؤثرج ع لج االحرىان

44


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.

ااء أ ابء رصف ثمح اضعخ ػ لح االحزىبن ابرغخ

Example: مثاي

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.

ػذب ٠مفض اظ١ اطبئشح رجذأ أعغب ثبغمغ احش ، وب صادد عشػخ اغمغ صادد ؼب

ااء إ أ رزغب لح اغبرث١خ لح االحزىبن رصجح حصخ ام صفشاا ( احزىبن ) لح ضعخ

. ػذبػغخ ل١خ ال ػذب

Viscous forceلج اسخح

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Questions:

1- Describe and explain the motion of a skydiver falling from rest.

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?

.حغبة ص لطغ غبفخ ؼخ ثؼشفخ اغشػخ

2- What is the average speed of a car which travels 400 m in 20 s?

حغبة اغشػخ ثؼشفخ اغبفخ اض

3- A car traveling at 7.0 m/s accelerates uniformly at 2m/s 5.2 to reach a speed of 12.0

m/s. How long does it take for this acceleration to occur?

.ػشفذ اؼغخ اغشػز١ االثزذائ١خ ابئ١خ إراحغبة اغبفخ

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

rest to 12.5 m/s in 2.5 s. What is Omar’s acceleration?

رحشن اغغ عى غشػخ بئ١خ خالي ص ؼ إراحغبة اؼغخ

5- A person pushing a stroller starts from rest, uniformly accelerating at a rate of 2m/s 500.0 . What is the velocity of the stroller after it has traveled 4.75m?

. ثذا عىطحغبة اغشػخ ابئ١خ ثؼشفخ اؼغخ غبفخ احشوخ ظ

Questions

47


6- Use the graphs below to answer questions A-C.

رب١خاي األعئخاعزخذ اشع اج١ب١خ اعت ػ

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?

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

of 2m/s 5 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?

حغبة عشػخ الالع غبئشح وزه غي ذسط اطبس ابعت

8- 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.

.ثؼشفخ اض اغشػخ ابئ١خحغبة اغبفخ غغ رحشن عى

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9- A car with an initial speed of 6.5 m/s accelerates at a uniform rate of 2m/s 92.0 for

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

.حغبة اغشػخ ابئ١خ اغبفخ امطػخ ثؼشفخ اغشػخ االثزذائ١خ اؼغخ اض

10- A car accelerates uniformly in a straight line from rest at the rate of 2m/s 3.2

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?

اغغ رحشن عى أاغبفخ امطػخ ػب حغبة اغشػخ ابئ١خ ص اص ب ثؼشفخ اؼغخ

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?

.عمػ اسرفبع ؼ األسضحغبة عشػخ ص عغ غطح

12- How far above the floor would you need to drop a pencil to have it land

in 1 s?

.٠ص خالي صب١خ احذح ألسضب االسرفبع ١غمػ عغ

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) 1N 3) 15N

ل خزفخ رأص١شحغبة اعزطبخ بثط رحذ

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.

.حغبة حصخ مر١ زؼبذر١

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15- What are the components of a vector of magnitude 1.5 N at an angle of 35°

from the positive x-axis?

.اؼد٠خ مح ضخ ثبشع اج١ب األفم١خحغبة اشوجبد

Resources:

Books:

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

2- Complete physics (Stephen Pople)

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

http://physics.slss.ie/

http://science-class.net/

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

grade 10 advanced level - science curriculum office ... 10 unit ap.2 ... ticker timer and tape, ......

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