Download 1 – Types of Forces Reading ver A

Name: ________________________ Per. ______ Date: ______________
Types of Forces Reading
Each of these basketball players is trying to push the ball. One player is trying to
push it into the basket, and the other player is trying to push it away from the
basket. If both players push the ball at the same time, where will it go? It
depends on which player pushes the ball with greater force. Forces like this
come into play in every sport, whether it’s kicking a soccer ball, throwing a
baseball, or spiking a volleyball. Forces are involved not only in sports such as
these but in every motion in our daily lives. In this reading, you’ll see how forces
affect the motion of everything from basketballs to planets.
Force is defined as a push or a pull acting on
an object. Any time the motion of an object
changes, a force has been applied. Force can
cause a stationary object to start moving or a
moving object to accelerate. The moving
object may change its speed, its direction, or
both. How much an object’s motion changes
when a force is applied depends on the
strength of the force and the object’s mass.
Force is a vector because it has both size and
direction. The Figure below shows some
examples. The length of each arrow
represents the strength of the force, and the
way the arrow points represents the direction
of the force.
What is force? What does force have to do
with the motion of an object?
Which example shows forces that are
unequal? How do you know?
Which example shows forces that are in
opposite directions? How do you know?
The SI unit of force is the newton (N). One
newton is the amount of force that causes a
mass of 1 kilogram to accelerate at 1 m/s2.
Thus, the newton can also be expressed as
kg·m/s2.
Applied Force (Fapp)
An applied force is a force that is applied to an
object by a person or another object. If Jack
Sparrow is pushing a boat across the sand,
then there is an applied force acting upon the
object. The applied force is the force exerted
on the boat by the person.
Draw a picture of something putting applied
force on Jack Sparrow
(pictured here).
Normal Force (Fnorm)
The normal force is
the support force
exerted upon an
object that is in
contact with another
stable object. For
example, if a bird is
resting on a rock,
then the rock is
exerting an upward force upon the bird in order
to support the weight of the bird. On
occasions, a normal force is exerted
horizontally between two objects that are in
contact with each other. For instance, if a
person leans against a wall, the wall pushes
horizontally on the person.
What is exerting normal force on the birds in
the picture to the left?
What is exerting normal force on you right
now?
Friction Force (Ffrict)
Friction is a force that opposes motion
between two surfaces that are touching. For
example, if a book slides across the surface of
a desk, then the desk exerts a friction force in
the opposite direction of its motion. Friction
results from the two surfaces being pressed
together closely. As such, friction depends
upon the nature of the two surfaces and upon
the degree to which they are pressed together.
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. Look at the metal
surfaces in the Figure below. The metal foil is
so smooth that it is shiny. However, when
highly magnified, the surface of metal appears
to be very bumpy. All those mountains and
valleys catch and grab the mountains and
valleys of any other surface that contacts the
metal. This creates friction.
In the picture of the polar bear, why is the bear
sliding on ice? Use friction in your response.
The bear will not slide forever. Which forces
could cause the bear to stop, and why?
Air Resistance Force (Fair)
Air resistance is a special type of frictional
force that acts upon objects as they travel
through the air. The force of air resistance is
often observed to oppose the motion of an
object. It is most noticeable for objects that
travel at high speeds (e.g., a skydiver or a
downhill
skier) or for
objects with
large
surface
areas.
Why is running with a parachute a better
workout than running without one? Use the
information to the left about air resistance and
force in your explanation.
Why is the picture of the light bulbs an
Tension Force (Ftens)
example of tension force?
The tension force is the force that is
transmitted through a string, rope, cable or
wire when it is pulled tight by forces acting
from opposite
ends. The tension
force is directed
along the length of
the wire and pulls
equally on the
objects on the
opposite ends of
the wire.
Gravity Force (Fgrav)
When the universe was still young, an
incredible force caused dust and gas particles
to pull together to form the objects in our solar
system. From the smallest moon to our
enormous sun, this force created not only our
solar system, but all the solar systems in all
the galaxies of the universe. The force is
gravity.
Gravity helped to form our solar system and all the other solar
systems in the universe.
What does gravity force do? How did this help
form the solar system?
Gravity has traditionally been defined as a
force of attraction between two masses.
According to this conception of gravity,
anything that has mass, no matter how small,
exerts gravity on other matter. The effect of
gravity is that objects exert a pull on other
objects. Unlike friction, which acts only
between objects that are touching, gravity also
acts between objects that are not touching. In
fact, gravity can act over very long distances.
Is gravity force acting on the light bulbs on the
previous page? How do you know?
Electric Force
(Felec)
An electric force is
an attractive or
repulsive force
between two charged objects. Electric forces
are attractive when two objects have
opposite charges and repulsive when two
objects have like charges. Electrons and
protons in an atom have opposite charges,
so atoms are in part held together by the
electric force between protons and electrons.
Magnetic Force (Fmag)
Magnetic force is the force that is created by
two magnets. A single magnet cannot create a
magnetic force. Magnetic forces are created
when a magnet, a magnetic material, or a
current carrying wire is placed at an external
magnetic field.
Electric forces can be produced by either
stationary or moving electric charges, whereas
magnetic forces can be produces only by
moving charges.
Magnetic force on a moving charge is always
normal to the direction of the movement and
the magnetic field whereas the force by an
electric field on a moving charge is always
parallel to the electric field and does not
depend on the direction of movement.
What are two differences between electric
force and magnetic force?