Introduction to Forces: Common Misconceptions

Introduction to
Common Misconceptions
Common Misconceptions about Forces
True or False
____ When a ball has been thrown, the
force of the hand that threw it remains on
____ A force is needed to keep an object
____ Inertia is a force
____ Air does not exert a force
____ The quantity ma is a force
Common Misconceptions about Forces
Short Answer
If there is no net force the object will continue with
________ velocity (unchanged, increased, or
Inertia is the tendency of an object to resist
____________________. (motion, change in
motion, increase in motion, or decrease in motion)
The force exerted by air is a ____________ force.
(small, balanced, or negative)
The equation F= _____ is a mathematical model of
Newton’s 2nd Law.
Newton’s First Law of
( The Law of Inertia)
Read 4.1-4.3 and be prepared for a brief quiz
Aristotle and the Early Greeks
(Around 300 B.C.)
► Believed
there were
two types of motion
and they were:
Natural motion – objects were moving
because they were moving toward their
natural resting place
Violent motion – imposed motion. This was
the result of an object being pushed or
A More Modern View:
Galileo Galilei (1564-1642)
Used his telescope to make such
astronomical discoveries such as the
rough surface of the moon, the four
moons revolving around Jupiter,
“sunspots”, the fact that the Milky Way
is made up of a myriad of single, faraway stars and the phases of Venus.
► Used the Leaning Tower of Pisa as a
laboratory to test his law of falling
► Was the first “scientist” because he used
experiments to prove/disprove his ideas.
Galileo and His Ramp Experiment
Galileo did an experiment with two ramps.
The ball rolls down the incline and up the
opposite incline and reaches almost its initial
He replaced the second ramp with a longer,
less-steep ramp and released the ball on the
first ramp. The ball rolled further and up the
second incline to almost the same height from
where it was released.
Hypothetically Galileo reasoned that if the
second ramp was removed altogether, the ball
would roll down the first incline plane and roll
forever and ever trying to attain the same
height from where it was released.
Comparison between Aristotle and Galileo
► Things at rest were at
their natural resting
► Objects that are
moving away from
their natural resting
place need to be
continuously forced to
keep them moving
► Objects at rest stay at
rest until something
forces them to do
► Objects in motion want
to continue in motion
unless something stops
► Idea
that Galileo came
up with
► Derived from Latin
meaning “idleness” or
► Definition: an objects’
resistance to change
its state of motion
Sir Isaac Newton
► Born
on Christmas Day
in 1642
► Please see thumbnail
sketch of his life
► “If I have been able to
see further, it was only
because I stood on the
shoulders of giants.”
Newtons 1st Law of Motion
(The Law of Inertia)
► Every
object continues in a state of rest, or
of motion in a straight line at constant
speed, unless it is compelled to change that
state by forces exerted upon it.
► In other words:
 Objects at rest, stay at rest
 Objects in motion, stay in motion.
Vocabulary terms
► Inertia
– Tendency of an object to resist
change in motion
► Equilibrium
– an object is in equilibrium if
it is at rest or moving with a constant
velocity (note: rest is a constant velocity of
zero). (For this to be achieved the NET force
has to be zero, more about this later)
Vocabulary Terms continued
► contact
forces – push or pull on an object
through touching it (contact)
► Long range – push or pull on an object
without direct contact
► Agents – cause of force (i.e. “applied”,
gravity, “normal”, friction, … among others)
What gives an object a lot of Inertia?
► Is
it volume?
Volume – the amount of
space an object takes
What gives an object a lot of Inertia?
► Is
it weight?
Weight – the effect that
gravity has on an
What gives an object a lot of Inertia?
► Is
it mass?
Mass – the amount of
matter an object
-the amount of “stuff”
something has
Can you measure mass in space?
Yes. Since astronauts are
weightless during their
mission, keeping track
of their weight can not
be accomplished by
asking them to step on
the traditional
bathroom scale. They
are strapped into a
device called an inertial
balance and vibrated
back and forth. What
would happen to the
period of vibration if
the astronaut losses
mass? How about if
he/she gains mass?
A concussion occurs when
the brain collides with the
skull and the result is
bruising of the brain.
Sometimes the person
loses consciousness. New
technology in the design of
helmets helps to reduce
the chance of a concussion
► Whiplash
is the term
that is used when
someone is struck from
behind, either in a car,
or not and the head
stays at rest while the
body moves forward
causing the head to
make a whipping
Homework Assignment
► Due
tomorrow, you are to find a specific
example of Newton’s 1st Law. Your example
 be written in your notebook
 include diagrams
 be unique.
Any duplications will receive a zero (0). Any
examples discussed in class may not be used.
How do you change the state of
motion of an object?
Apply a Force!
Force- a push or a pull
-measured in Newtons (N)