Laws of Force and Motion

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This pdf contains a brief introduction to the force and motion chapter.
1. Force and Motion
A Science A–Z Physical Series
Word Count: 1,746
Written by Ron Fridell
2. Force and Key elements Used in This Book
The Big Idea: Force and motion are fundamental to all matter in the
universe. A force is anything that can push or pull on an object. Forces
influence objects that are at rest or that are already in motion. Isaac
Newton’s three laws of motion involve inertia, mass, velocity, and
momentum. Key forces include gravity, friction, and magnetism. A force
is required to do work, and generating a force requires energy. Energy
can be stored as potential energy, or it can have kinetic energy—the
energy of motion. Energy can also be converted and exchanged
through energy transfer. Objects move in predictable ways. By learning
about force and motion, we come to understand how using forces can
produce motions that allow us to be safe and to enjoy ourselves.
Key words: attract, direction, distance, electricity, electromagnetism, energy,
energy transfer, engine, force, friction, generator, gravity, heat energy, inertia,
kinetic energy, law, light energy, lines of force, magnetic field, magnetism, mass,
momentum, motion, potential energy, reaction, repel, rest, sound energy, speed,
velocity, weight, work
Key comprehension skills: Cause and Effect
Other suitable comprehension skills: Classify information; compare and contrast;
elements of a genre; identify facts; interpret charts, graphs, and diagrams; main
idea and details
Key reading strategy: Visualize
Other suitable reading strategies: Ask and answer questions; connect to prior
knowledge; retell; summarize
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Illustration Credit:
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Written by Ron Fridell Force and Motion
© Learning A–Z
Written by Ron Fridell
All rights reserved.
3. Table of Contents
ro nn
n g
i n n
p n
Introduction............................................................... 4 p s iiin
sl ssp
Motion Needs a Force............................................... 5
jumping i s e in
th g
The Laws of Motion.................................................. 7
Newton’s First Law of Motion................................. 8
c r e e p i n g
Newton’s Second Law of Motion........................... 10 Introduction
Newton’s Third Law of Motion.............................. 12
Each word above tells how objects can move.
Types of Forces........................................................ 13 Some travel in a straight, flat line, such as a ball
Gravity................................................................... 13 rolled along the ground. When you kick the ball
Friction................................................................... 16 into the air, it flies in a curved path until it bounces
Magnetism............................................................. 18 on the ground. A merry-go-round turns in circles,
Force, Motion, and Work........................................ 20 while lightning strikes in a jagged path.
Energy....................................................................... 21 Motion is all around you. It’s even inside you
Potential and Kinetic Energy................................. 21 as your blood moves through your body.
Energy Transfer...................................................... 23
For every motion, there is a force. In this book,
Conclusion............................................................... 24
you will learn about the forces that make things
Glossary.................................................................... 25 move, stop, and change direction or speed.
Index......................................................................... 26

3 4
4. Motion Needs a Force Machines can provide
force to create motion. A
To make something move, a force is needed. huge engine can produce
Lifting, pushing, and pulling are all forces. At enough force to propel a
times, a person provides the force. massive rocket into space.
When you lift a stack of books, Motors in cake mixers,
push a broken-down car, or pull fans, and blenders use
a rope in a tug-of-war game, you electrical force to make
are the force. things move.
push Nature can also be a
strong force. Breezes make
leaves dance in the treetops.
Ocean waves make boats
bob up and down. Forces deep underground
cause earthquakes that make the ground tremble.
Earth’s crust is made up of
moving plates. These plates
can shift in several ways.
The force of these shifting
plates causes earthquakes.
5 6
5. The Laws of Motion Newton’s First Law of Motion
Isaac Newton is known for his three laws of
Much of what we know about
motion, which explain how things move. The
motion comes from scientists
first part of the first law says that an object at
who lived hundreds of years ago.
rest will remain at rest unless a force acts upon it.
They conducted tests on moving
For example, your bike will stay parked where it
objects. British scientist Isaac
is unless something moves it.
Newton discovered gravity, the
invisible force that causes things to fall toward
the ground. You may have heard that he was
sitting under a tree one day when a falling apple In government, a law is a rule that
hit him on the head. The story goes that this people must obey. In science, a law is
accident led him to discover gravity. a statement that explains how things
always work in the physical world.
Isaac wasn’t really hit on the head by a falling
apple. That’s just a myth. But over time, he
The second part of this law states that an object
observed that objects always fall down toward
in motion will continue moving in the same way
the ground. And that led him to discover gravity.
unless a new force changes the motion. It will keep
moving at the same speed and in the same direction.
SIr Isaac Newton (1642–1727)
So your moving bike will continue moving at the
Isaac Newton lived on a farm in England. Instead of
farming, he chose to study math and science in London. same speed and in the same direction until you
At age 27, Isaac did experiments with light. He was the pedal faster,
first to suggest that
light was actually coast, brake,
made up of all the or turn.
colors of the rainbow.
He is now one of
the most famous
scientists of all time!
Why is it important
to wear a seat belt?
Think about Newton’s
first law of motion.
7 8
6. Every object, at rest or in motion, has inertia. Newton’s Second Law of Motion
Inertia is what makes an object keep doing what Mass is the amount of matter in an object.
it is already doing. An object at rest will remain Newton’s second law of motion deals with mass
at rest unless a force moves it. An object in motion and motion. It explains that an object’s motion
will remain in motion unless a force stops it. depends upon its mass and the amount of force
Newton’s first law of motion is sometimes called needed to move that mass.
the Law of Inertia.
Why is it easier to move a small rock than
a boulder? Since a boulder has much more mass
than a small rock, much more force is needed
to overcome the boulder’s inertia. Much less
force is needed to make small rocks move, stop,
or change direction.
An object has inertia whether it is at rest or in motion. In either
case, a force must overcome that inertia. The force may create
motion, stop it, or change its direction.
9 10
7. Newton’s Third Law of Motion
Newton’s third law of motion says
This train has a large
mass and a high
that for every force or action, there
speed, so it has a is an equal and opposite reaction.
lot of momentum.
It would take a very
So, if you lift a 9 kg (20 lb.) box,
strong force to slow the box pulls down with
or stop this train!
an equal force of 9 kg
(20 lbs.) in the opposite
This law of motion also deals with speed. direction.
Speed measures how far something travels in
This law explains
a certain amount of time. The faster an object
why a balloon full of air
moves, the more force is needed to stop it.
goes flying when you let
Think about a locomotive speeding down a it go without tying it. Air
track. It has a lot of mass and speed. So a great rushes out the open end.
deal of force will be needed to overcome the An opposite force pushes
inertia of the train’s motion and make it stop. on the far end of the balloon,
making it fly.
Momentum is the amount of force in a
moving object. The more speed and mass a
moving object has, the more momentum it has.
A small, slow scooter can stop much faster than
a massive, fast train can. As air rushes out of the
B open balloon (arrow A),
an equal force pushes
in the opposite direction
A (arrow B). In which
Velocity describes an object’s speed in a certain direction will it fly?
direction. Scientists calculate momentum by using
this formula:
mass x velocity = momentum
11 12
8. Types of Forces Imagine standing next to a tall building. Both
you and the building have mass, so you both
You’ve learned that people, machines, and exert a gravitational pull. The building has much
nature can all provide a force. Here are three more mass, so it pulls on you much more than
more forces you should be familiar with. you pull on it.
Isaac Newton observed that all objects pull on
each other due to gravity. His findings became
known as the Universal Law of Gravity. The pull
of gravity depends on the mass of the objects and
the distance between them. Greater masses have
a stronger pull. If they move farther apart, the
pull between them gets weaker.
Mass, Distance, and Gravity
In which situation is the gravitational pull strongest?
In which situation is the gravitational pull weakest?
However, both you and the building are
standing on Earth. Planet Earth is far more
massive than either you or the building. So
• two small masses • two small masses Earth exerts a much stronger gravitational
• short distance • greater distance
pull on you and the building than you and
C D the building exert on each other.
On Earth, an object’s mass is measured as
weight. The greater the mass of an object, the
• t wo large masses • two large masses more the force of Earth’s gravity pulls on it,
•s  hort distance • greater distance
Strongest: C; Weakest: B and the more it weighs.
13 14
9. Friction
Friction is an invisible force that both slows
down moving things and heats them up. One
kind of friction you know well is sliding friction.
When you rub your hands together,
they create sliding friction. This
friction produces heat energy,
causing your skin to warm up.
A rolling bowling ball has
another kind of friction—rolling
friction. Rolling friction will slow down
a moving object, but not as much as sliding
friction does.
Placing rollers
If you were in space, twice as far from the center of Earth as you under a box
are now, the pull of gravity would be only one-quarter as strong.
You would still have the same mass, but you would weigh only makes it easier
one-quarter of what you weigh on Earth. to move. Rolling
reduces friction.
Now let’s move from Earth to the Sun. The
Sun has much more mass than anything else in
our solar system. It exerts enough gravitational
pull to keep all the planets from flying off into
space. Instead, they orbit the Sun.
Rough surfaces exert more
Remember, distance is also important. While friction against each other
the Sun exerts more gravitational pull than Earth, than smooth surfaces do.
you are much closer to Earth than to the Sun. Putting a lubricant such as
oil or grease between two
So Earth’s gravity keeps you from getting pulled
surfaces will reduce friction.
up to the Sun!
15 16
10. Fluid friction happens when an object moves Magnetism
through a fluid, such as water or air. You can Magnetism can push or pull objects made of
move your finger through water faster than honey certain metals, such as iron. Magnets also push
because honey creates more fluid friction than and pull each other.
water. Engineers design airplanes to reduce the
fluid friction caused The force of lines of force
by the air. Fluid magnetism can
move objects N+
friction can help + ++
without touching +
objects move more + +–
them. How? + –
easily, such as – –
a puck floating Invisible lines – –

on air in an air of force enter and – S
hockey game. leave at opposite
ends, or poles, of
a magnet. The field
magnet attracts certain
objects that enter its magnetic field.
Each magnet has a north and south pole.
Two opposite poles attract each other. But two
of the same poles repel, or push each other away.
Imagine that S N
you wanted
to race your friends N S
down a snowy hill.
How could thinking N N
Opposite poles attract.
about friction help
you win the race?
Like poles repel.
17 18
11. Force, Motion, and Work
In science, work happens when you move
something. The amount of work you do depends
on the distance the object moves and the force it
takes to move it.
Scientists measure work by multiplying the distance
an object moves by the force used to move it.
force x distance = work
Generators like these use magnets to make electricity flow
through wires.
Magnets can make electricity flow through most work
wires. Power plants use magnets to produce
On the other hand, electricity can turn some
4 blocks, no baby 4 blocks, with baby
metals into magnets. Electricity and magnetism
are part of a single force called electromagnetism.
least work
2 blocks, no baby 2 blocks, with baby
What would happen It takes less work to push an empty stroller than a stroller with
to the paper clips a baby in it because an empty stroller has less mass. Also,
if the wire weren’t pushing the stroller four city blocks requires twice as much work
touching the battery? as pushing it only two blocks.
19 20
12. Energy Can you guess which type of energy a tank full
of gasoline contains? Fuels such as gasoline have
Energy is the ability to potential energy. Burning gas to make a car move
do work. The more energy changes that potential energy into kinetic energy.
you have, the more work
you can do.
Potential and Kinetic Energy
Energy comes in two Why can’t a
motorcycle that is
basic types: potential and kinetic. Take a rubber
out of gas be ridden? Think about
band, for example. When you pull it back, it has potential and kinetic energy.
potential energy. This energy is stored and ready
to use. The farther back you pull the rubber band,
the more potential energy it has. When you let it go, The food you eat also has potential energy.
the rubber band flies away. That potential energy Plants gather energy from the Sun. Animals get
changed into kinetic energy. Kinetic energy is energy from eating plants and other animals.
energy in motion. When you eat food, you store up energy. Then
that energy makes your heart beat and makes
your lungs expand and contract.
21 22
13. Energy Transfer Conclusion
Energy can be changed, or converted, from
You’ve learned that it takes force to make things
one kind to another. Energy can also be moved,
move or stop and that forces affect the speed and
or transferred, from one object or place to another.
direction of a moving object. You read about Isaac
Changing and moving energy is called energy
Newton’s laws of motion. You learned that the
mass of an object is related to the force that makes
Solar energy becomes it move. Simple pushes and pulls as well as lifting
stored energy in plants. are examples of force. Other types of force include
When you eat, the stored gravity, friction, and magnetism. People, machines,
energy in plants can be and nature can all exert a force.
transferred to your muscles
Work is done when a force moves an object
and converted into motion in
over a distance. It takes energy to do work.
your arms. Then this motion
This energy is either potential (stored) or kinetic
can be transferred to moving
(motion). Energy can transfer by moving or by
drumsticks. The sticks can
changing from one type of energy to another.
make a drum vibrate,
Forces and motion are everywhere, all around you.
creating sound energy.
The energy stored in batteries
can make a toy move and
make sounds. It can
also be converted
into light energy
in a flashlight.
What other examples
of energy transfer can
you think of?
23 24
14. Glossary mass the amount of matter,
measured on Earth by its
direction the way or course toward which
weight (p. 10)
something moves or faces (p. 4)
momentum the strength or force that keeps
distance the amount of space between
something moving (p. 11)
things (p. 13)
motion the act of going from one place
electromagnetism the combined force of electricity
to another; movement (p. 4)
and magnetism (p. 19)
potential energy the energy a body has because
energy transfer the movement of energy from
of its position, electrical
one object to another or the
charge, or structure; stored
change of energy from one
energy (p. 21)
form to another (p. 23)
speed the rate of movement (p. 4)
force the strength or energy that
moves an object (p. 4) weight how heavy something is,
determined by the pull of
friction a force that slows down
gravity on the object’s mass
moving things (p. 16)
(p. 14)
gravity the force that pulls things
work the act of using force to move
toward the center of Earth
something over a certain
or any other object that has
distance (p. 20)
mass (p. 7)
inertia the tendency of an object to Index
resist change in the direction lines of force, 18 Universal Law of
or speed of its motion (p. 9) Newton, Isaac, 7–10, Gravity, 13
kinetic energy the energy that a moving body 12, 13, 24 speed vs. velocity, 11
has because of its motion (p. 21) first law of motion, 8, 9 types of friction
second law of fluid friction, 17
magnetism a force that pushes and pulls motion, 10, 11 rolling friction, 16
certain metals (p. 18) third law of sliding friction, 16
motion, 12
25 26