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Chapter 5
Section 2 What Is a Force?
Warm-up # 40-forces
Look around the room and think about the objects
you see in terms of force. A force is always
exerted by one object on another object.
Answer in complete sentences:
A. Where do you see a force happening in the room
right now?
B. Which object is exerting the force, and which is
receiving it?
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Chapter 5
Section 2 What Is a Force?
Objectives
• Describe forces, and explain how forces act on objects.
• Determine the net force when more than one force is
acting on an object.
• Compare balanced and unbalanced forces.
• Describe ways that unbalanced forces cause changes
in motion.
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Chapter 5
Section 2 What Is a Force?
Forces Acting on Objects
1. In science, a force is simply a push or a pull
exerted on an object. All forces have both size and
direction.
2. Any change in motion is caused by an unbalanced
force.
3. Scientists express force using a unit called the
newton (N) and is measured with a spring scale.
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Click below for animation on forces.
http://my.hrw.com/sh2/sh07_10/student/flash/visual_concepts/70310.htm
This is from the online textbook. If it doesn’t work, go
to the online edition, choose Ch. 5, choose the red
tab-eactivities, click on forces to open. Choose file
and save as. Save into the same folder as the power
point. In the power point, choose insert, object,
choose create from file, browse to find the file, and
insert motion.
You may need to hyperlink it back to the file.
Contact [email protected] for help
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4.Objects always have some kind of force acting on them
5. What are 2 examples of objects on which you
exert a force when you are doing your school
work?
6. What is an example of a force that does not
cause an object to move?
Sitting on a chair, or writing on paper
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Chapter 5
Section 2 What Is a Force?
7. What is an example of an unseen source exerting a force?
• Unseen Sources and Receivers of Forces It is not
always easy to tell what is exerting a force or what is
receiving a force.
•For example, you cannot see what exerts the force
that pulls magnets to refrigerators. (magnetism)
You cannot see that the air around you is held near
Earth’s surface by a force called gravity.
Air pressure also acts on objects and we can’t see it.
The force that makes the
cat’s hair move is called
electromagnetism.
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Chapter 5
Section 2 What Is a Force?
Determining Net Force
• Usually, more than one force is acting on an object.
8. The net force is the combination all of the forces
acting on an object.
• Determining net force depends on the directions of the
forces. To watch , double click on icon
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Chapter 5
Section 2 What Is a Force?
• 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.
• 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.
9. How is net force determined if 2 students moving a
piano exert force in the same direction?
Now do #10 on your own
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Chapter 5
Section 2 What Is a Force?
10. Use the picture to help answer the question.
11. What will knowing the net force on an object tell you
about the object?
Motion and Direction
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Chapter 5
Section 2 What Is a Force?
Balanced and Unbalanced Forces
12. Balanced Forces When the forces on an object
produce a net force of 0 N, the forces are balanced.
There will be no change in motion. If the net force is
0 N then an object will either remain at rest or in
motion.
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Chapter 5
Section 2 What Is a Force?
Balanced and Unbalanced Forces, continued
13. Unbalanced Forces When the net force on an
object is not 0 N, the forces on the object are
unbalanced.
• 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 nonmoving object to start moving, or to change the
motion of moving objects.
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Forces and motion 3min
Go to discovery school and download video clips. Put them in the same
folder as the power point and choose insert, then choose video from file.
Drag the corners to the size you’d like. Contact me if help is needed
[email protected]
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Vectors show magnitude and direction, now complete the back of the
notes page, finish for homework
http://www.youtube.com/watch?v=A05n32Bl0aY
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Chapter 5-3
Section 3 Friction: A Force That Opposes
Motion
H.W.- H.W.-Study for your test which will be Wed.,
11/14, Look at “eactivities” in online textbook.
Warm-up # 41
Suppose you and a younger sister or brother are at a
neighborhood pool. Your sister or brother asks why there
are signs that say “NO RUNNING.” What would be your
answer?
Don’t copy the question, write a complete sentence for
your answer. The sign is there because...
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
Objectives
• Explain why friction occurs.
• List the two types of friction,and give examples of
each type.
• Explain how friction can be both harmful and helpful.
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
The Source of Friction
1. Friction is a force that opposes motion between
two surfaces that are in contact.
2. A
3. 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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click on icon.
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
• How does the roughness of surfaces that are
touching affect the friction between them?
4. the rougher the surface, the greater the friction
How does the amount of force exerted on a surface
affect the motion?
5. the greater the force, the greater friction
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Friction
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
The Source of Friction, continued
• The Effect of Force on Friction The amount of
friction depends on the force pushing the surfaces
together. If this force increases, the hills and valleys
of the surfaces can come into closer contact.
• The close contact increases the friction between the
surfaces. Objects that weigh less exert less downward
force than objects that weigh more, as shown on the
next slide.
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
The Source of Friction, continued
6. What are 2 ways friction can be increased?
-increasing the force on an object while moving it
(push down on sandpaper while moving it)
-adding rubber to the surface (bottom of shoes)
-make the surface rougher (add sand to an icy road)
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
Types of Friction
Kinetic Friction The word kinetic means “moving.” So,
kinetic friction is friction between moving surfaces.
• The amount of kinetic friction between two surfaces
depends in part on how the surfaces move. Surfaces
can slide past each other, or a surface can roll over
another surface.
•Static Friction- friction at rest
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
Types of Friction, continued
• Usually, the force of sliding kinetic friction is greater
than the force of rolling kinetic friction. It is usually
easier to move objects on wheels than to slide the
objects along the floor, as shown below.
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
Types of Friction, continued
Static Friction When a force is applied to an object
but does not cause the object to move, static friction
occurs.
• The word static means “not moving.” The object does
not move because the force of static friction balances
the force applied.
• Static friction disappears as soon as an object starts
moving, and then kinetic friction immediately occurs.
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
7. Kinetic friction-moving with force applied
(ex. pushing a cart on wheels)
Static Friction-not moving with force applied
(ex. A box that won’t move when you push it)
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
Friction: Harmful and Helpful
• Without friction, a car’s tires could not push against
the ground to move the car forward, and the brakes
could not stop the car. Without friction, a car is useless.
• However, friction can also cause problems in a car.
Friction between moving engine parts increases their
temperature and causes the parts to wear down.
• Friction can be both harmful and helpful, so it may be
necessary to decrease or increase friction.
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Helpful friction
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
Friction: Harmful and Helpful, continued
• Some Ways to Reduce Friction 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.
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Chapter 5
Section 3 Friction: A Force That Opposes
Motion
Friction: Harmful and Helpful, continued
• Friction can be reduced 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.
• Another way to reduce friction is to make surfaces
that rub against each other smoother.
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H.W.-Study for your test which will be Wed, 11/14, Work on
the Chapter 5 Review, Due Tues, 11/13.
Warm-up # 42 & 43-Complete the K & W on your handout.
K
W
What do you know
about gravity?
What do you want to
know about gravity?
L
What did you
learn?
E.Q. How are Mass and distance related to gravitational force between objects?
GPS- S8P4a. Recognize that every object exerts gravitational force on every other
object and that the force depends on the mass and distance.
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What is gravity? 2min
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1. Astronauts bounce on the moon when they
walk because the moon has less gravity than the
Earth.
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The Effects of Gravity on Matter
• 2. The force of attraction between objects that is due to
their masses and distances is gravity.
•Gravity can change the motion of an object by changing
its speed, direction, or both.
• All matter has mass. Therefore, all matter is affected by
gravity and all objects experience an attraction toward all
other objects.
• The mass of most objects is too small to cause a force
large enough to move objects toward each other.
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Chapter 5
Section 4 Gravity: A Force of Attraction
The Effects of Gravity on Matter, continued
• The Size of Earth’s Gravitational Force Compared
with all objects around you, Earth has a huge mass.
Therefore, Earth’s gravitational force is very large.
• You must apply forces to overcome the Earth’s
gravitational force any time you lift objects or even
parts of your body.
•3. If the earth’s mass doubled without changing in size,
your weight would:
•Would your mass change?
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Chapter 5
Section 4 Gravity: A Force of Attraction
Newton and the Study of Gravity
• The Core of an Idea Why do objects fall toward the
Earth? What keeps the planets moving in the sky?
• 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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Chapter 5
Section 4 Gravity: A Force of Attraction
Newton and the Study of Gravity, continued
• 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.
• He also reasoned that an unbalanced force on the
moon kept the moon moving around the Earth.
• He proposed that these two forces are actually the
same force––gravity.
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Newton and the Study of
Gravity, continued
•Law of universal gravitation describes the relationships
between gravitational force, mass, and distance.
• The law is called universal because it applies to all objects
in the universe.
•4. all objects are attracted to each other by gravitational
forces which depend on the mass and distance between
them
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• 5. The greater the mass of objects, the greater
the force between them.
Gravitational force is greater when two objects
are closer together.
6. As the distance between the objects
increases gravitational force decreases
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Chapter 5
Section 4 Gravity: A Force of Attraction
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Chapter 5
Section 4 Gravity: A Force of Attraction
Weight as a Measure of Gravitational Force
• The Differences Between Weight and Mass Weight
is related to mass, but they are not the same.
• 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.
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Chapter 5
Section 4 Gravity: A Force of Attraction
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Chapter 5
Section 4 Gravity: A Force of Attraction
Weight as a Measure of Gravity, continued
• Units of Weight and Mass The SI unit of force is a
newton (N). 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.
• On Earth, a 100 g object weighs about 1 N.
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7. Why are mass and weight often confused?
because we call mass weight and they are
the same number with different units
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Weight
Mass
spring scale
balance
measure of gravitational force
exerted on an object
amount of matter in an object
measured in kilograms
expressed in newtons
constant on Earth because
the mass of the earth does not change
never changes,
constant on earth
changes when gravitational
force changes
remains the same when
gravitational force changes
on the moon, is 1/6 of that on
Earth
•
••
•
•
••••
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Chapter 5
Matter in Motion
Concept Mapping
Use the terms below to complete the Concept
Mapping on the next slide.
speed
time
velocity
motion
acceleration
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H.W.-Study for your test which will be Wed, 11/14, Finish
the Chapter 5 Review, Due Tues, 11/13.
Warm-up # 43-Complete the L on the warmup.
K
W
What do you
know about
gravity?
What do you
want to know
about gravity?
L
What did you
learn?
C.W.- finish Ch. Review, Concept map on page 762
E.Q. How are Mass and distance related to gravitational force between objects?
GPS- S8P4a. Recognize that every object exerts gravitational force on every other
object and that the force depends on the mass and distance.
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H.W.-Test tomorrow, study!!!!!!
Add to Ch. Review Draw and Complete the concept map.
Use these words: speed, acceleration, force, velocity,
direction, motion
Motion
is due to a
force
which causes
acceleration
which is a change in
velocity
speed
dependent on
direction
C.W.- check Ch. Review,
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Chapter 5
Matter in Motion
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Chapter 5
Matter in Motion
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