DYNAMICS
The study of why objects move or
don’t move.
What is the natural state of
motion of an object?
Aristotle Observed:
• All objects eventually come to a stop.
• Therefore he concluded the natural state
of motion for an object, is for the object to
be at rest.
• He Incorrectly concluded that the only way
an object stays in motion is if it has a force
acting on it.
• But, what is a force?
Force – A push or a pull caused by
the interaction of two objects.
•
•
•
•
F = Force
Units: Newtons (N) or pounds (lb)
1lb = 4.45N
A force is a vector quantity – It has
magnitude (value) and direction.
Weight is a Force
• It is caused by the interaction between the
earth and an object.
• It has a value and a direction (down).
• Objects have the same mass everywhere,
they only have weight when they interact
with a planet, like earth.
• A 150lb person has a weight of 668N.
Copernicus Observed:
• The motion of the stars and planets.
• He concluded that the earth was in
constant motion.
• This idea was highly controversial.
• People preferred to think the earth was the
center of the universe.
• As a result he was persecuted and had to
work in secret.
Galileo devised experiments to
discover the natural state of
motion.
• His experiments represent the birth of
modern day science.
• He also suffered persecution for his
efforts.
Galileo’s Experiments:
A ball rolling down a ramp, rolls to the
same height on a ramp across from it.
He worked very hard to eliminate friction,
but of course he never could completely.
ho
hf
ho=hf
He changed the angle of the second
ramp and observed the ball rolled further
before reaching hf.
ho
hf
ho=hf
He than imagined what would happen if
the second ramp was flat. Hmmm?
What would happen? (remember to
neglect friction)
ho
hf
The ball is trying to roll back to a place where
ho = h f
The ball would roll forever with a
constant velocity!!!!!!
Galileo’s experiments
demolished the idea that a
force is necessary to keep an
object moving.
The natural state of motion for
an object is to be moving with a
constant velocity, which could
be zero.
So an object at rest is just a
specific example of an
object with a constant
velocity.
Sir Isaac Newton was the next
major player on the scene
• He was born on Christmas day, the year Galileo
died.
• His work influenced today’s world at a
monumental level.
• He developed three laws that describe the
motion of everything.
• He very well may have been one of the most
influential human beings to ever live.
• And you thought he was just some dumb guy
who got hit on the head with an apple.
1642-1727
Inertia
• The natural tendency of an object to resist
a change in motion.
• Mass is a measure of inertia.
Newtons 1st Law:
An object with constant velocity (which
could be zero) continues with constant
velocity (same speed and direction) unless
an unbalanced force acts on it.
Newtons 2nd Law:
The sum of the forces on an object is
equal to the mass times the acceleration
of the object.
 F  ma
The sum of
Newtons 3rd Law:
For every force (action), there is an equal and
opposite force (reaction).