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Forces and Motion
Forces
When you pull open a door, you apply a force on
the door. A force is a push or pull that acts on an
object. Every force has strength. Another word for
strength is magnitude. Force is measured in units
called newtons (N).
Forces can change the way objects move. When
an object begins to move, it happens because a
force has made it move. When an object is already
moving, forces can make it change speed. They
can make it speed up, slow down, stop, or change
direction.
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Contact Forces
A force that requires two pieces of matter to touch
is called a contact force. There must be contact
with the object for the force to act. A contact force is
acting when you push or pull a wagon.
One kind of contact force is friction. Friction is
the force that results when two materials rub against
each other. The amount of friction between two
objects depends on their shapes, speeds, and weights.
A dry surface causes more friction than the same
surface would if it were wet.
Air and water can also cause friction. This
happens when particles of air or water contact a
surface. Submarines and ships are shaped in ways
that reduce friction. Their shapes allow them to
move through water easily.
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Non-Contact Forces
For friction to work, two things must touch.
But there are forces that work at a distance. They
work even if the object that is pushing or pulling
is not touching another object. A force that acts at
a distance is called a noncontact force. Three
examples of non-contact forces are gravity, electric
forces, and magnetic forces.
Gravity
The force of attraction between any two objects is
called gravity. Gravity pulls objects toward Earth’s
center. Gravity is a non-contact force.
The weight of an object is a measure of the force
of gravity on that object. The farther an object is
from Earth, the less it will weigh. The pull of Earth’s
gravity becomes weaker the farther away an
object is.
Electric and Magnetic Forces
Electric forces act between objects that are
electrically charged. Objects with opposite charges
are pulled toward each other. Objects with the same
charge are pushed away from each other.
Magnets will pull strongly on objects made of
some metals, like iron and nickel. Every magnet
has a north pole and a south pole. Magnetic force
is strongest at a magnet’s poles. The north pole of
one magnet will pull on the south pole of another
magnet. The north poles of two magnets will push
away from each other. The south poles of two
magnets will push away too.
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Changes in Motion
Have you ever watched a car moving down the
road? The driver steps on the brake pedal as the
car comes to a red light. The speed of the car slows
down to zero. It stops. When the light turns green,
the driver steps on the gas pedal. The car speeds up.
If the car has to turn a corner, the driver turns the
steering wheel. The car changes direction.
The motion of an object changes when it speeds
up, slows down, or changes direction. The rate at
which the speed or the direction of motion of an
object changes over time is its acceleration.
In science the word acceleration means any change
in motion. For example, the cars on a Ferris wheel
are accelerating whenever they are moving. The
riders in the cars change direction all the time. They
go up, then forward, then down, and then backward.
A moving object has no acceleration if it does
not change its speed or direction. Motion without
acceleration is called uniform motion. The word
uniform tells you that the motion stays the same. It
does not change. A train traveling at the same speed
on a straight track has uniform motion. Even a book
sitting on a table has uniform motion. Its speed
is zero.
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Newton’s First Law
Newton’s first law of motion says that an object at
rest will stay at rest. An object in motion will keep
the same speed and direction. A marble will stay
still on the floor unless you push it. If the marble is
already moving, it will keep moving. It will move at
a constant speed in a straight line until a force acts
on it. Friction will slow down a rolling marble until
it stops.
Objects you push or throw will stop moving in
time. They stop because there are other forces acting
on them. For example, a bowling ball will slow down
because of friction between the ball and the
bowling lane.
The tendency of an object to resist any change
motion is known as inertia. Large, heavy objects
have more inertia than lighter, smaller objects
moving at the same speed.
Inertia is what pushes you against the side of a
car when the car makes a turn. Your body continues
moving in a straight line when the car changes
direction.
Inertia causes you to rise up from your seat as the
car goes over a hill. When the car begins to move
down the hill, your body continues moving upward.
Gravity or your seatbelt will stop the upward motion.
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Newton’s Second Law
Newton’s second law of motion describes how
acceleration, mass, and force are related. Force is the
product of mass and acceleration. The force acting
on an object can cause it to speed up, slow down, or
change direction.
Same Force, Different Masses
A given force will cause a smaller change in the
motion of a large mass than of a smaller mass. In
other words, the same force will cause a large mass
to accelerate less than a small mass.
Large masses are harder to speed up and harder
to slow down or stop. The engine and brakes of
a truck can produce the same forces whether the
truck is empty or full. A full truck has more mass. It
will accelerate more slowly than an empty truck. It
will also take longer to stop. Suppose the same size
engine were put on each boat. The larger boat would
have less acceleration because it has more mass.
Same Mass, Different Forces
Newton’s second law of motion also says that a
stronger force will cause a greater change in motion.
A large force will cause more acceleration on an
object than a smaller force.
In the sport of archery, arrows are shot at targets.
The arrow depends on acceleration to reach the
target. The archer increases the force acting on the
arrow by pulling on the bow string. The farther the
archer pulls the bow string, the greater the force. The
same arrow can be given different accelerations by
changing the amount of force that is put into pulling
the bow.
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Newton’s Third Law
Newton’s third law of motion states that when one
object exerts a force on a second object, the second
object exerts a force on the first. The forces are equal
in strength. They are opposite in direction.
It is impossible to have one force without an equal
and opposite force. For example, when bumper
cars crash, they exert forces on each other. When a
moving car bumps into a stopped car, both drivers
feel the force of the bump. The driver of the stopped
car feels one force, and the car starts to move. The
driver of the moving car feels an opposite force. It
slows the moving car.
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Isaac Newton
Isaac Newton is one of history’s greatest scientists.
He developed the laws of motion. His laws and his
definition of force made people think about science
in a new way.
Newton wanted to know more about nature. He
looked for ways to explain what he saw. He put
together ideas in creative ways.
Newton graduated from Cambridge University
in England in 1665. For 30 years, he was a teacher
there. Later he became the head of England’s mint. A
mint makes and prints money. His theories still form
the basis of our understanding of the universe.
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Adding Forces
Have you ever asked a friend to help you push
something heavy? When you push together, you
can combine forces into a stronger single force. This
combined force can have a bigger result.
When two forces act on the same object in the
same direction, the strength of each force can be
added together. The combined force will act in the
same direction as the two single forces. For example,
suppose two people lift a box together. Their forces
can be added because both forces point in the same
direction. If each person applies a force of 100
newtons, their total force will be a stronger upward
force. A force of 100 newtons plus 100 newtons
equals 200 newtons.
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Balanced Forces
When you hold a book in your hand, your hand
is pushing the book up. At the same time, Earth’s
gravity is pulling the book down. The two forces
push against each other and keep the book
from moving.
Two forces of equal strength that act on the
same object in opposite directions are balanced.
Balanced forces cancel each other out. They cannot
change the motion of the object.
Several forces can be balanced at the same time.
More than one rope can pull on a camping tent
without causing a change in motion. The total of all
the forces acting on an object is called the net force.
When the forces are balanced, the net force is zero.
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Glossary
acceleration
t he rate at which the speed or
the direction of motion of an
object changes over time
balanced
escribes forces of equal
d
strength that combine to act
on the same object but in
opposite directions
contact force
force that requires two
a
pieces of matter to touch
force
push or pull that acts on
a
an object
friction
t he force that results when
two materials rub against
each other
gravity
t he force of attraction
between any two objects
inertia
t he tendency of an object to
resist any change in motion
non-contact force
a force that acts at a distance
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