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HIGH SCHOOL SCIENCE
Physical
Science 3:
Forces
WILLMAR PUBLIC SCHOOL
2013-2014 EDITION
C HAPTER 3
Forces
In this chapter you will:
1.Describe force.
2.Explain how the motion of an object is
affected when balanced and unbalanced forces act on it.
3.Compare and contrast the four kinds of
friction.
4.Describe the path of a projectile.
5.Describe Newton’s Three Laws of Motion.
6.Describe Momentum.
S ECTION 3.1
Forces
L OREM
O
BJECTIVES
I PSUM
:
1. Define
Lorem ipsum
force, and
dolor
give
sitexamples
amet
of forces.
2. Consectetur
Describe howadipisicing
forces combine
elit, sed
and
doaffect
eiusmod
tempor incididunt ut labore et dolore magna
motion.
aliqua.
3. Ut enim ad minim veniam, quis exercitation
Vocabulary:
ullamco laboris nisi ut aliquip ex commodo
force
consequat.
spring
scale
4. Duis
aute irure dolor in in voluptate velit esse
cillum dolore eu fugiat nulla pariatur.
Newtons
balanced force
unbalanced force
net force
Suppose your teachers asked you to move a heavy desk in
your classroom. The desk will not move by itself. How would
you move it? You might get on one side of the desk and start
pushing. Or you might grab the legs and start pulling. Either
way, you would be using a force. A force is needed to put the
desk in motion.
A force is a push or a pull that acts on an object. A force can
cause a resting object to move, or it can accelerate a moving
object by changing the object's speed or direction. Forces
differ in size and direction. You could use a small amount of
force or a lot of force. A force can be left or right, or up or
down.
Forces are often easy to measure. In fact, if you've ever
shopped at a grocery store, you may have measured forces
using a spring scale. A spring scale is the tool used to
measure force. The stretch of the spring in the scale depends
on the amount of weight (a type of force) acting on it. As more
fruit is placed on the scale, the spring is stretched farther and
the scale reading increases. Force is measured in Newtons, abbreviated as N. One
Newton is the force that causes a 1-kilogram mass to
accelerate at a rate of 1 meter per second each second (1 m/
s2). In fact, 1 Newton is equal to 1 kilogram-meter per second
squared (1 N = 1 kg·m/s2).
Even when an object is not in motion, forces are acting on it.
The floor is actually pushing up on the desk, at the same time,
gravity is pulling down on it. The size of the forces are the
2
same, but in different directions. Whenever forces are the
same in size but opposite in direction, they are balanced
forces. An object is not in motion will never move if balance
forces act on it.
How can you move the desk? You can move it because you
unbalance the forces. The size of your force is greater than the
force of air pushing on the other side of the desk and the
friction. Unbalanced forces act in the opposite direction
but differ in size. If unbalanced forces act on an object, the
object will always more in the direction of the greater force.
Force is a vector and is combined using vector addition. The
net force is the overall force acting on an object after all the
forces are combined.
Section Review:
1. How do forces change the motion of an object?
2.Why does a heavy chair not move by itself?
3. What tool do you use to measure force?
4.What is the unit of force?
5. What is the difference between a balanced force and an
unbalanced force?
6.If an unbalanced force acts on a ball, the ball will always
move in which direction?
When the forces on an object are balanced, the net force is
zero and there is no change in the object's motion. Balanced
forces have zero net force and no motion. When the forces on
an object do not balance, the net force is unbalanced and the
object accelerates. Unbalanced forces have net force that are
not zero and accelerate.
3
S ECTION 3.2
Friction
L OREM
O
BJECTIVES
I PSUM
:
1. Describe
Lorem ipsum
how dolor
friction
sitopposes
amet motion.
2. Consectetur
Identify types
adipisicing
of friction.elit, sed do eiusmod
tempor incididunt ut labore et dolore magna
aliqua.
Vocabulary:
3. Ut enim ad minim veniam, quis exercitation
friction
ullamco laboris nisi ut aliquip ex commodo
consequat.
static
friction
4.
Duisfriction
aute irure dolor in in voluptate velit esse
sliding
cillum dolore eu fugiat nulla pariatur.
rolling friction
fluid friction
air resistance
Friction is the rubbing of one object against another.
Friction is a force that resists, or works against, motion. It
slows down moving objects and creates heat. Try rubbing
your hands together; you will feel them getting warmer and
warmer. Friction occurs because no surface is perfectly
smooth. Even surfaces that look smooth to the unaided eye
appear rough or bumpy when viewed under a microscope.
All moving objects are subject to friction, a force that opposes
the motion of objects that touch as they move past each other.
Friction acts at the surface where objects are in contact.
Friction can work for or against us. You could not walk
without friction to keep your feet from sliding on the ground.
Without friction, you could not pick things up and hold them.
Too much friction between moving parts in a car engine can
cause the parts to wear out.
You know that friction produces heat. That’s why rubbing
your hands together makes them warmer. But do you know
why the rubbing produces heat? Friction causes the molecules
on rubbing surfaces to move faster, so they have more heat
energy. Heat from friction can be useful. It not only warms
your hands. On the other hand, heat from friction can be a
problem inside a car engine. It can cause the car to overheat.
To reduce friction, oil is added to the engine. Oil coats the
surfaces of moving parts and makes them slippery so there is
less friction.
There are four main types of friction: static friction, sliding
friction, rolling friction, and fluid friction.
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Static friction is the friction force that acts on objects that
are not moving. Static friction always acts in the direction
opposite to that of the applied force. Without this static
friction, your feet would slip out from under you, making it
difficult to walk.
Sliding friction is a force that opposes the direction of
motion of an object as it slides over a surface. Because sliding
friction is less than static friction, less force is needed to keep
an object moving than to start it moving. That’s why it’s
easier to slide a piece of furniture over the floor after you start
it moving than it is to get it moving in the first place.
Rolling friction is the friction force that acts on rolling
objects.Rolling friction is much weaker than sliding friction or
static friction. This explains why it is much easier to move
boxes on a wheeled dolly than by carrying or sliding them.
Fluid friction is friction that acts on objects that are moving
through a fluid. A fluid is a substance that can flow and take
the shape of its container. Fluids include liquids and gases. If
you’ve ever tried to push your open hand through the water in
a tub or pool, then you’ve experienced fluid friction between
your hand and the water. When a skydiver is falling toward
Earth with a parachute, fluid friction between the parachute
and the air slows the descent. Fluid pressure with the air is
called air resistance. The faster or larger a moving object is,
the greater is the fluid friction resisting its motion. The very
large surface area of a parachute, for example, has greater air
resistance than a skydiver’s body.
Section Review:
1. Complete the sentences using: friction heat little
motion resists
a. The rubbing of one object against another is
b. Friction is a force that
motion.
c. Every
friction.
.
, or works against,
that takes place on Earth is affected by
d.Smooth surfaces such as ice provide very
friction.
e. Friction makes
.
2.How is heat create with friction?
3. What are the four main types of friction?
4.Compare the strengths of static, sliding and rolling
friction.
5
S ECTION 3.3
Projectile Motion
L OREM
O
BJECTIVES
I PSUM
:
1. State
LoremNewton’s
ipsum dolor
law of
situniversal
amet
gravitation.
2. Consectetur
Explain how adipisicing
gravity affects
elit,the
sedmotion
do eiusmod
of
tempor incididunt ut labore et dolore magna
objects.
aliqua.
3. Ut enim ad minim veniam, quis exercitation
Vocabulary:
ullamco laboris nisi ut aliquip ex commodo
gravity
consequat.
law
of universal
gravitation
4. Duis
aute irure
dolor in in voluptate velit esse
cillummotion
dolore eu fugiat nulla pariatur.
projectile
Gravity is a force that acts between any two masses. ALL
matter has gravity. Newton was the first one to suggest that
gravity is universal and affects all objects in the universe.
That’s why his law of gravity is called the law of universal
gravitation. Universal gravitation means that the force that
causes an apple to fall from a tree to the ground is the same
force that causes the moon to keep moving around Earth.
Universal gravitation also means that while Earth exerts a pull
on you, you exert a pull on Earth. Anything that has mass, no
matter how small, exerts gravity on other matter. Earth has a
great amount of mass so the gravity of Earth itself is stronger
than the gravity of anything on Earth. The force of Earth's
gravity holds you on the ground. It is the pull gravity that
gives weight to all matter on Earth. The moon has gravity.
Since the moon has less mass than Earth, it has less gravity.
Newton’s law also states that the strength of gravity between
any two objects depends on two factors: the masses of the
objects and the distance between them.
• Objects with greater mass have a stronger force of gravity.
For example, because Earth is so massive, it attracts you and
your desk more strongly than you and your desk attract each
other. That’s why you and the desk remain in place on the
floor rather than moving toward one another.
• Objects that are closer together have a stronger force of
gravity. For example, the moon is closer to Earth than it is to
the more massive sun, so the force of gravity is greater
between the moon and Earth than between the moon and
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the sun. That’s why the moon circles around Earth rather
than the sun.
When gravity pulls objects toward the ground, it causes them
to accelerate. Acceleration due to gravity equals 9.8 m/s2. You
might think that an object with greater mass would accelerate
faster than an object with less mass. After all, its greater mass
means that it is pulled by a stronger force of gravity. However,
a more massive object accelerates at the same rate as a less
massive object. The reason? The more massive object is
harder to move because of its greater mass. As a result, it ends
up moving at the same acceleration as the less massive object.
Earth’s gravity also affects the acceleration of objects that
start out moving horizontally, or parallel to the ground. A
cannon shoots a cannon ball straight ahead, giving the ball
horizontal motion. At the same time, gravity pulls the ball
down toward the ground. The combination of an initial
forward velocity and the downward vertical force of gravity
causes the ball to follow a curved path. In the picture, part A
shows how gravity acts on falling objects. Although their
masses are different, the large and small balls fall at the same
rate. Part B shows that the yellow ball is a projectile,
following a curved path.
Gravity causes objects to accelerate downward, whereas air
resistance acts in the opposite direction and reduces
acceleration. A curved path of a falling object is projectile
motion. Air resistance and gravity are the only forces acting
on a projectile.
7
Section Review:
1. What two factors affect gravity?
2.How is weight related to gravity?
3. Why does the moon have less gravity than Earth?
4.On Earth, how fast do object accelerate due to gravity?
5. Why don’t larger object fall faster?
6.Why does projectile motion follow a curved path?
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S ECTION 3.4
Newton’s Laws
L OREM
O
BJECTIVES
I PSUM
:
1. State
LoremNewton’s
ipsum dolor
first sit
lawamet
of motion.
2. Consectetur
Explain inertia's
adipisicing
relationship
elit, sed
to mass.
do eiusmod
tempor incididunt ut labore et dolore magna
3. State Newton’s second law of motion.
aliqua.
4. Identify the relationship between acceleration
3. Ut enim ad minim veniam, quis exercitation
and weight.
ullamco laboris nisi ut aliquip ex commodo
5. State
Newton’s third law of motion.
consequat.
4. Duis aute irure dolor in in voluptate velit esse
cillum dolore eu fugiat nulla pariatur.
Vocabulary:
inertia
About 300 years ago, a scientist name Isaac Newton had some
ideas about forces and motion. His ideas became known as
Newton’s Laws of Motion.
According to Newton's first law of motion, the state of
motion of an object does not change as long as the net force
acting on the object is zero. An object in motion stays in
motion. An object at rest stays at rest. Thus, unless an
unbalanced force acts, an object at rest remains at rest, and an
object in motion remains in motion with the same speed and
direction. Suppose you see a soccer ball on the grass. It is at
rest. The force of ground pushing up on its is balanced by the
force of gravity pulling down on it. The soccer ball with keep
its state of motion, which in this case is at rest. Suppose you
kick the ball. You have unbalanced the forces with the force of
your kick. The ball will be in motion. The ball will stay in
motion until some other force comes along. In this case, a
force called friction will stop the ball after a while.
Newton's first law of motion is sometimes called the law of
inertia (in ur shuh). Inertia is the tendency of an object to
resist a change in its motion. In other words, neither the
direction nor the speed of the object will change as long as the
net force acting on it is zero. The inertia of an object depends
on its mass. Objects with greater mass also have greater
inertia. Think how hard it would be to push a big box full of
books. Then think how easy it would be to push the box if it
was empty. The full box is harder to move because it has
greater mass and therefore greater inertia. To change the
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motion of an object, inertia must be overcome by an
unbalanced force acting on the object.
According to Newton's second law of motion, the
acceleration of an object is equal to the net force acting on it
divided by the object's mass.
You might think that actions and reactions would cancel each
other out like balanced forces do. Balanced forces, which are
also equal and opposite, cancel each other out because they
act on the same object. Action and reaction forces, in contrast,
act on different objects, so they don’t cancel each other out
and, in fact, often result in motion.
acceleration = Force / mass
or
Force = mass x acceleration
Do you sometimes talk about weight and mass as if they were
the same thing? Although related to each other, mass and
weight are not the same. Weight is the force of gravity acting
on an object. An object's weight is the product of the object's
mass and acceleration due to gravity acting on it.
According to Newton's third law of motion, whenever one
object exerts a force on a second object, the second object
exerts an equal and opposite force on the first object. This
means that forces always act in pairs. These two forces are
called action and reaction forces. For every action, there is an
equal and opposite reaction. Action and reaction forces do
not act on the same object. When a rocket is launched,
burning fuel makes hot gases come out of the bottom of the
rocket down against the Earth. The Earth pushes back on the
rocket. The rocket takes off, moving away from Earth. The
gases pushing down is the action. The Earth pushing up on
the rocket is the reaction.
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Section Review:
1. What is Newton’s first law of motion?
2.What is another name for the Newton’s first law of
motion?
3. What is Newton’s second law of motion?
4.What does the “F” stand for in Newton’s second law?
What does the “m” stand for in Newton’s second law?
What does the “a” stand for in Newton’s second law?
5. Tori applies a force of 20 newtons to move a bookcase
with a mass of 40 kg. What is the acceleration of the
bookcase?
6.What is Newton’s third law of motion?
7. How do action and reaction forces interact?
11
S ECTION 3.5
Momentum
L OREM
O
BJECTIVES
I PSUM
:
1. Lorem
Describe
ipsum
momentum
dolor sitand
amet
the conservation of
momentum.
2. Consectetur adipisicing elit, sed do eiusmod
tempor incididunt ut labore et dolore magna
aliqua.
Vocabulary:
3. Ut enim ad minim veniam, quis exercitation
momentum
ullamco laboris nisi ut aliquip ex commodo
consequat.
4. Duis aute irure dolor in in voluptate velit esse
cillum dolore eu fugiat nulla pariatur.
What if a friend asked you to play catch with a bowling ball?
Hopefully, you would refuse to play! A bowling ball would be
too heavy to catch without risk of injury — assuming you
could even throw it. That’s because a bowling ball has a lot of
mass. This gives it a great deal of momentum. Momentum is
a property of a moving object that makes the object hard to
stop. It equals the object’s mass times its velocity.
Momentum is the product of an object's mass and its velocity.
An object with large momentum is hard to stop. An object has
a large momentum if the product of its mass and velocity is
large. The momentum for any object at rest is zero.
You can calculate momentum by multiplying an object's mass
(in kilograms) and its velocity (in meters per second).
Momentum is measured in units of kilogram-meters per
second.
Momentum = mass x velocity
This equation shows that momentum is directly related to
both mass and velocity. An object has greater momentum if it
has greater mass, greater velocity, or both. For example, a
bowling ball has greater momentum than a softball when both
are moving at the same velocity because the bowling ball has
greater mass. However, a softball moving at a very high
velocity — say, 100 miles an hour — would have greater
momentum than a slow-rolling bowling ball. If an object isn’t
moving at all, it has no momentum. That’s because its velocity
is zero, and zero times anything is zero.
12
In a closed system, the loss of momentum of one object equals
the gain in momentum of another object. When an action and
reaction occur, momentum is transferred from one object to
the other. However, the combined momentum of the objects
remains the same. According to the law of conservation of
momentum, if no net force acts on a system, then the total
momentum of the system does not change.
Section Review:
1. What is the momentum formula?
2.What is needed for an object to have a large momentum?
3. If you double the velocity of a moving object, how is its
momentum affected?
4.What is the law of conservation of momentum?
5. Create a diagram to illustrate the transfer and
conservation of momentum when a moving object
collides with a stationary object.
13