Transcript Dynamics-cause of motion

Warm Up 5
10.25.11
 What
causes motion? In other words, what
causes things to start, stop and change
directions?
 Sorry,
computer issues have made me use
a powerpoint from the web
Dynamics
- the study of what causes motion
Aristotle (384-322 BCE)


Greek philosopher who
arrived at several principles
of nature.
Greeks did not use
experimentation because
they believe that the senses
can be deceived- the correct
way to do science is to think
about it.
He proposed the 2 Types of
Motion
a. Natural Motion
b. Violent Motion
Natural Motion

Occurs when objects
fall to rest on the
surface of the earth
or when they are
sailed up into the sky,
like a puff or smoke.
Objects seeking their
natural place in the
world.
Violent Motion

Occurs when a moving
object is moved away
from its natural place, such
as picking up a bottle from
the ground or throwing the
ball into the air.
 The object in violent
motion must be kept in
motion by a violent force
or it will come to rest
Aristotle vs Galileo
 Aristotle’s
views
were widely held
for centuries.
 It wasn’t until
Galileo came along
that his theories
were completely
disproved.
6 Notions of Force and Motion
1.) If there is motion there is force acting.
2.) There cannot be a force without motionif there is no motion then there is no force
acting.
3. When an object moving there is a force in
the direction of motion.
6 Notions of Force and Motion
4.) A moving object has a force within it
which keeps it moving.
5.) A moving object stops when its force is
used up.
6.) Motion is proportional to force acting.
Therefore a constant speed result from a
constant force.
Answers?
NOT one of these notions are
correct.
These are 6 commonly held
misconceptions of force and motion,
and they all originate form Aristotle’s
views.
Galileo (1564- 1642)

Conceived of motion as
continuous.
 Conceived of motion on a
frictionless surface
 “ An object once placed in
motion , will move further
until another force brings it
to stop”.
 Implied that the natural
state of objects is
continuous motion.
Galileo’s Breakthrough

He realized that things
slowed down due to
friction .
He called it RESISTANCE
 In order to understand
force and motion, you
need to envision motion
without friction.
“INERTIA”
 Objects
on frictionless surface has it.
 Objects will never stop, will go
forever.
 Objects needs no force to keep it
moving
 Newton incorporated this into his
work and called the property “inertia”.
Newston’s First Law
The Law of Inertia
 When no external , unbalanced force acts on an
object its velocity remains constant. or
 If no net force acts on an object, it maintains its
state of rest or its constant speed in a straight
line or
 An object at rest will remain at rest, an object in
motion will remain in motion in a straight line
unless either are acted upon by an external
force.
Implications
 External
force is required to change
the velocity of an object.
 External force must be unbalanced.
 Helps explain the behavior of objects
in motion- they resist change in
motion due to their inertia.
Mass – a measure of inertia
Inertia is directly proportional to the mass of
the object.
 A large object at rest is hard to move.
 A large object having constant velocity is
hard to stop.
Mass is defined as the amount of matter in
an object. In the book it is defined as the
“laziness “of an object.
Example
 An
elephant at
rest has a lot of
“laziness” so it
has a greater
tendency to
remain at rest.
 (hard to move)
Example
 A runaway
train at constant velocity has
a lot of “laziness” so it has a greater
tendency to stay in motion.
 (hard to stop)
Differentiating Mass and Weight
 A given
quantity of mass will remain
the same wherever it is in this
physical world.
 The
weight depends on the mass of
an object and it is equal to the force
of gravity on an object.
Examples
 If
you have a mass of 75 kilograms,
your mass on earth, moon and Jupiter
will be 75 kilograms.
 Your
weight depends on the force of
gravity exerted on you by the other
mass (earth, moon and Jupiter).
MASS and WEIGHT are DIRECTLY
PROPORTIONAL TO EACH OTHER.
More mass means more weight.
Conversion of your weight in pounds to your
mass in kilograms.
2.2 pounds = 1 kilogram
MASS and WEIGHT are DIRECTLY
PROPORTIONAL TO EACH OTHER.

If your weight is 50 lbs, what is your mass in kg?
50 lbs ( 1 kg/ 2.2 lbs) = 22.73 kg

The earth is pulling you with a force of 50 lbs
towards its center. Also equal to a force of 490
newtons.

Do you want your weight
to be in newtons?
50 lbs = 490 newtons
NET Force – resultant force.
 Which
object will remain at rest, according
to Newton’s First Law?
5N
5N
5N
8N
The cat will move
 Which
object will continue moving with
constant velocity, according to Newton’s
First Law?
5N
5N
5N
This car will accelerate.
8N
Light Bulb Moment
 No
matter how many applied forces
are acting on an object AND the net
force is equal to zero, the object will
maintain its inertia.
 Balanced
forces on an object,
maintains the state of inertia of an
object.
If the forces acting on an object are cancelled, the
object is said to be in a state of equilibrium.
Meaning the net force is equal to zero and the
object maintains its inertia.
Weight
Weight
Weight
Air Friction
Normal Force
by the ground
The object is at rest
Air Friction
Falling objects will experience
terminal velocity
Example
If you are standing, at
rest, the balanced
forces acting on you
are:
a.) your weight and
b.) the upward support
force of the ground
called the
normal force.
Questions
 1.
Why do professional photographers use
heavy cameras for their shots?
 2. While making a turn to the right, why is
your body “moving towards the left side of
the car”?
 3. Why do we use safety seat belts?
 4. In which location will it be difficult to
shake a 50 kg rock, on earth or in space?
Questions
 5.
Which object has more inertia, a 15 kilogram
bowling ball or a 15 kilogram bag of pure
cotton?
 6. While standing still what is your inertia with
respect to the sun? with respect to the ground?
 7. What is the value of the normal force if you
are standing on the table?
 8. If you throw a ball horizontally and we
neglect air friction and the force of gravity, how
will you describe the motion of the ball?