DYNAMICS The study of why objects move or don’t move.

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
• His experiments represent the birth of
modern day science.
• He also suffered persecution for his
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.
He changed the angle of the second
ramp and observed the ball rolled further
before reaching hf.
He than imagined what would happen if
the second ramp was flat. Hmmm?
What would happen? (remember to
neglect friction)
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
Sir Isaac Newton was the next
major player on the scene
• He was born on Christmas day, the year Galileo
• 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.
• 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).