Download School of Modern Skills Science Department Name: Grade: 8

School of Modern Skills
Name:
CHAPTER : 1
LESSON
Science Department
Grade: 8
1 : MEASURING MOTION
 Observing Motion by Using a Reference Point
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Motion is an object’s change in position relative to another object, or reference point. The
object that appears to stay in place is called a reference point.
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The direction of an object’s motion can be described with a reference direction, such as north,
south, east, west, up, or down.
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Common Reference Points The Earth’s surface is a common reference point for
determining motion.
 Speed Depends on Distance and Time
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Speed is the distance traveled by an object divided by the time taken to travel that distance.
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The SI unit for speed is meters per second (m/s). Kilometers per hour (km/h), feet per second
(ft/s), and miles per hour (mi/h) are other units commonly used to express speed.
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Determining Average Speed Average speed equals the total distance divided by the total
time.
average speed = total distance
total time
- Speed can be shown on a graph of distance versus time.
 Velocity: Direction Matters
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The speed of an object in a particular direction is called velocity.
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Speed and velocity are two different terms with two different meanings. Velocity must
include a reference direction.
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You can think of velocity as the rate of change of an object’s position. An object’s velocity is
constant only if its speed and direction don’t change.
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You can combine different velocities to find the resultant velocity.
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School of Modern Skills
Name:
Science Department
Grade: 8
1- when you combine two velocities that are in the same direction, add them together to
find the resultant velocity.
2- when you combine two velocities that are in opposite directions, subtract the smaller
velocity from the larger velocity to find the resultant velocity. The resultant velocity is in
the direction of larger velocity.
 Acceleration:
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The rate at which velocity changes over time is called acceleration.
An object accelerates if its speed, or direction, or both change.
An increase in velocity is commonly called positive acceleration.
A decrease in velocity is commonly called negative acceleration, or deceleration.
average acceleration = final velocity – starting velocity
time it takes to change velocity
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Velocity is expressed in meters per second (m/s), and time is expressed in seconds (s). So
acceleration is expressed in meters per second per second, or (m/s)/s, which equals m/s2.
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Acceleration can be shown on a graph of velocity versus time.
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The Circular Motion :Continuous Acceleration An object traveling in a circular motion is
always changing its direction. Therefore, its velocity is always changing, so it is
accelerating.
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The acceleration that occurs in circular motion is known as centripetal acceleration.
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School of Modern Skills
Name:
CHAPTER : 1
LESSON
Science Department
Grade: 8
1 : MEASURING MOTION
Part A: Circle the letter of the best answer for each question. / 5 marks
1. What can be shown on a graph of distance versus time?
a. acceleration
b. speed
c. time
d. motion
2. What is the unit of acceleration ?
a. seconds
b. meters per second
c. meters per second per second
d. seconds per meter
3. If velocity increase, acceleration is ?
a. positive
b. negative
c. 0
d. none of the above
4. Velocity Depends on
a. Direction
b. Speed
c. Both a and b
d. None of the above
5. A reference point is
a. Stable
b. Moving
c. Both a and b
d. None of the above
6. What can be shown on a graph of distance versus time?
a. acceleration
b. speed
c. time
d. motion
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School of Modern Skills
Name:
Science Department
Grade: 8
7. What is the unit of acceleration ?
a. in seconds
b. in meters per second
c. in meters per second per second
d. in seconds per meter
8. What is the acceleration that occurs in circular motion called?
a. centripetal acceleration
b. negative acceleration
c. positive acceleration
d. velocity
9. Why is an object traveling in a circular motion accelerating?
a. Its direction is always changing.
b. Earth is rotating.
c. Both a and b
d. None of the above
10. The difference between speed and velocity is that
a. velocity has direction, while speed does not.
b. speed has direction, while velocity does not.
c. velocity involves time, while speed does not.
d. speed involves time, while velocity does not.
Part B: Match the correct definition with the correct term
/ 2 marks
11. Distance traveled in a period of time
a. acceleration
12. speed of an object in a particular direction
b. speed
13. a measure that does not change when an object’s location changes
change
c. velocity
14. a change in a moving object’s speed or direction
____
Part C: Answer the following questions
d. motion
15. What is the average acceleration of a train that speeds up from 2 m/s to 12 m/s in 2 s on a
straight section of track? Show your work below.( write equation and solve)
_____________________________________________________________________________
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_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
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School of Modern Skills
Name:
CHAPTER : 1
LESSON
Science Department
Grade: 8
2 : WHAT IS FORCE ?
 Forces Acting on Objects
- Force is a push or pull that is exerted on an object in order to change the motion of an object
.
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All forces have both size and direction.
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A force can change the acceleration of an object. This acceleration can be a change in the
speed or direction of the object.
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Scientists express force using a unit called the Newton (N).
 Unseen Sources and Receivers of Forces
- It is not always easy to tell what is exerting a force or what is receiving a force.
- Example: 1- You cannot see what exerts the force that pulls magnets to refrigerators.
2- You cannot see that the air around you is held near Earth’s surface by a force
called gravity.
 Determining net force
- The net force is the combination all of the forces acting on an object.
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Usually, more than one force is acting on an object.
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Determining net force depends on the directions of the forces.
1- Forces in the Same Direction Two forces are added to determine the net force
if the forces act in the same direction. The net force will be in the same
direction as the individual forces.
2- Forces in Different Directions If forces are acting in opposite directions, the
net force can be found by subtracting the smaller force from the larger one.
 Balanced and Unbalanced Forces
- Balanced Forces: When the forces on an object produce a net force of 0 N, the forces are
balanced.
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Balanced forces will not cause a change in the motion of a moving object. Balanced forces
do not cause a nonmoving object to start moving.
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Unbalanced Forces: When the net force on an object is not 0 N, the forces on the object are
unbalanced.
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Unbalanced forces produce a change in motion, such as a change in speed or a change in
direction. Unbalanced forces are necessary to cause a non-moving object to start moving,
or to change the motion of moving objects.
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School of Modern Skills
Name:
Newton
Balanced forces
Science Department
Grade: 8
Force
Net Force
Unbalanced Forces
Zero
Part A. Fill each blank with the word that best completes the sentence.
1. _____________ a push or a pull exerted on an object in order to change the object’s
motion.
2. The SI unit for Force is ___________.
3. ____________ is the combination of all forces acting on an object.
4.______________ is the combined force that causes something to move
5. ______________ is the combined force that does not cause movement
6. Balanced Forces results in ____________ net force.
Part B: Circle the letter of the best answer for each question. / 4 marks
7. Which of the following choices best represents force?
a. push or a pull always causing motion
b. push or a pull always causing acceleration
c. push or a pull acting without an object
d. push or a pull acting on an object
8. When the net forces equal 0 N, they are which of the following?
a. balanced
b. unbalanced
c. push
d. pull
9. Which of the following always causes change in speed, direction, or both?
a. balanced forces
b. unbalanced forces
c. either balanced or unbalanced forces
d. any combination of forces
10. What is the net force on an object when you combine a force of 10 N north with a force
of 5N south?
a. 5N South
b. 15 N North
c. 50 N North
d. 5 N North
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School of Modern Skills
Name:
Science Department
Grade: 8
Part C: Answer the following questions
/ 3 marks
Look at the diagrams, and calculate the net force in each diagram. Predict the direction each
object will move.
a.
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b.
_______________________________________________________________
c.
_____________________________________________________________________
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School of Modern Skills
Name:
CHAPTER : 1
LESSON
Science Department
Grade: 8
3 : FRICTION A FORCE THAT OPPOSES MOTION
 The Source of Friction
- Friction is a force that opposes motion between two surfaces that are in contact.
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What causes Friction?
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Friction occurs because the surface of any object is rough. Even surfaces that feel smooth are
covered with microscopic hills and valleys.
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When two surfaces are in contact, the microscopic hills and valleys of one surface stick to the
tiny hills and valleys of the other surface. This contact causes friction.
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The Effect of Force on Friction
- Rough surfaces have more microscopic hills and valleys than smooth surfaces do.
So, the rougher the surface is, the greater the friction is.
 Types of Friction
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Kinetic Friction is friction between moving surfaces.
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The amount of kinetic friction between two surfaces depends in part on how the surfaces
move.
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Surfaces can slide past each other, or a surface can roll over another surface.
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Usually, the force of sliding kinetic friction is greater than the force of rolling kinetic
friction.
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It is usually easier to move objects on wheels than to slide the objects along the floor.
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Static Friction occurs when a force is applied to an object but does not cause the object to
move.
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The object does not move because the force of static friction balances the force applied.
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Static friction disappears as soon as an object starts moving, and then kinetic friction
immediately occurs.
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School of Modern Skills
Name:
■
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Science Department
Grade: 8
Friction: Harmful and Helpful
Without friction,
1- a car’s tires could not push against the ground to move the car forward
2- the brakes could not stop the car.
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However, friction can also cause problems in a car.
1- Friction between moving engine parts increases their temperature
2- causes the parts to wear down.
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Friction can be both harmful and helpful, so it may be necessary to decrease or increase
friction.
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Some ways to reduce friction
1 - One way to reduce friction is to use lubricants.
- Lubricants are substances that are applied to surfaces to reduce the friction between the
surfaces.
- Some examples of common lubricants are motor oil, wax, and grease.
- Lubricants are usually liquids, but they can be solids or gases.
2 - Another way to reduce friction by switching from sliding kinetic friction to rolling kinetic
friction.
- Ball bearings can be placed between wheels and axels to make it easier for wheels to turn by
reducing friction.
3 - Another way to reduce friction is to make surfaces that rub against each other smoother.
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Some ways to increase friction
1 - Making surfaces rougher is one way to increase friction. For example, sand scattered on
icy roads keeps cars from skidding.
2- Another way to increase friction is to increase the force pushing the surfaces together.
- For example, if you are sanding a piece of wood, you can sand the wood faster by pressing
harder on the sandpaper.
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School of Modern Skills
Name:
CHAPTER : 1
LESSON
Science Department
Grade: 8
3 : FRICTION A FORCE THAT OPPOSES MOTION
Write the letter of the correct answer in the space provided.
____ 1. What is a force that opposes motion between two surfaces that are in contact?
a. friction
c. velocity
b. motion
d. acceleration
____ 2. Which are the two factors that determine the amount of friction?
a. the force and the roughness of the surfaces
b. hills and valleys and the roughness of the surfaces
c. the force and how much surfaces come in contact
d. the force and the weight exerted on the surfaces
____ 3. Which of these is opposed by kinetic friction?
a. a book sitting on a table
c. a box sliding on a floor
b. a cat standing in a yard
d. a child leaning on a building
____ 4. Which of these is opposed by static friction?
a. a pencil rolling on a table
c. a chair sliding on a floor
b. a dog running in a yard
d. a worker pushing on a nonmoving box
____ 5. In which of the following activities is friction helpful?
a. engine parts wearing out
c. soil erosion
b. car tires moving a car forward d. holes developing in your socks
____ 6. In which of the following activities is friction harmful?
a. brakes stopping a car
c. an eraser erasing
b. a person walking
d. car engine parts wearing out
____ 7. Which would NOT be a way to reduce friction?
a. using ball bearings
c. pouring sand on ice
b. using sandpaper
d. lubricating with oil
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School of Modern Skills
Name:
CHAPTER : 1
LESSON
Science Department
Grade: 8
4 : GRAVITY : A FORCE OF ATTRACTION
 The Effects of Gravity on Matter
- Gravity is a force of attraction between objects that is due to their masses.
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Gravity can change the motion of an object by change its speed or direction or both.
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All matter is affected by gravity and all objects experience an attraction toward all other
objects because all matter has mass, and gravity is a result of mass.
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The mass of most objects is too small to cause a force large enough to move objects toward
each other.
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The Size of Earth’s Gravitational Force
Compared with all objects around you, Earth has a huge mass. Therefore, Earth’s
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gravitational force is very large.
You must apply forces to overcome the Earth’s gravitational force any time you lift objects
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or even parts of your body.
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Newton and the Study of Gravity
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The Core of an Idea
Why do objects fall toward the Earth? What keeps the planets moving in the sky?
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In 1665, British scientist Sir Isaac Newton made the connection between these two questions
when, as legend has it, he saw an apple falling from a tree.
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Newton knew that unbalanced forces are needed to change the motion of objects. He
concluded that an unbalanced force on the apple made the apple fall.
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He also reasoned that an unbalanced force on the moon kept the moon moving around the
Earth.
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He proposed that these two forces are actually the same force––gravity.
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The birth of a law
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Newton summarized his ideas about gravity in a law known as the law of universal
gravitation. This law describes the relationships between gravitational force, mass, and
distance.
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The law is called universal because it applies to all objects in the universe.
Newton and the Study of Gravity
- Gravitational Force Increases as Mass Increases
- Gravitational force is small between objects that have small masses.
- Gravitational force is large when the mass of one or both objects is large.
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School of Modern Skills
Name:
Science Department
Grade: 8
- Gravitational Force Decreases as Distance Increases
- Gravitational force is strong when distance between two objects is small.
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If the distance between two objects increases, the gravitational force pulling them together
decreases rapidly.
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Weight as a Measure of Gravitational Force
- Weight is a measure of the gravitational force on an object.
- Weight changes whenever gravitational force changes.
- Mass is the amount of matter in an object. An object’s mass does not change if
gravitational force changes.
- Weight is related to mass, but they are not the same.
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Units of Weight and Mass
- Weight is a measure of the gravitational force on an object.
- Gravity is a force, and weight is a measure of gravity. So, weight is also measured in Newtons.
- The SI unit of mass is the kilogram (kg). Mass is often measured in grams (g) and milligrams
(mg) as well.
- The SI unit of mass is the kilogram (kg). Mass is often measured in grams (g) and milligrams
(mg) as well.
- On Earth, a 100 g object weighs about 1 N.
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School of Modern Skills
Name:
CHAPTER : 1
LESSON
Science Department
Grade: 8
4 : GRAVITY : A FORCE OF ATTRACTION
Part A. Fill each blank with the word that best completes the sentence. / 3 marks
1. ______________is the measure of the gravitational force exerted on an object
2. _____________is the force of attraction between objects that is due to their masses and the
distance between the objects
3.___ _____________ is the tendency of all objects is to resist any change in motion.
4.______________ law states that an object at rest remains at rest, and an object in motion remains
in motion at constant speed and in a straight line unless acted on by an unbalanced force
5. _____________ law states that the acceleration of an object depends on its mass and the amount
of force applied.
Part B: Circle the letter of the best answer for each question. / 4 marks
6. As the distance between two objects increases, the force of gravity between them
a.decreases.
b.increases only slightly.
c.does not change.
d.increases greatly.
7. Which of the following is the equation for Newton’s second law of motion?
a.
F= m × a
b.
m=F × a
c.
a=m ÷ F
d.
F=m ÷ a
8. Which is a common unbalanced force acting on objects in motion?
a. inertia
b. acceleration
c. friction
d. speed
Part D: Answer the following questions
/ 3 marks
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11. What force is necessary to accelerate a 70 kg object at rate of 5 m/s ? ( write the equation)
_____________________________________________________________________________
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12. Which object will accelerate more if the same force is applied to both: a pickup truck or a
tractor-trailer truck? why?
_____________________________________________________________________________
_____________________________________________________________________________
13. How does the gravitational force between two objects that are close together compare to the
gravitational force between two objects as they move farther apart?
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_____________________________________________________________________________
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School of Modern Skills
Name:
Science Department
Grade: 8
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