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Aristotle’s ViewTwo types of motion:Natural motion - what an object
“naturally wants to do”Violent motion - what an object has to
be forced to do
Aristotle’s View In order for a “normal” object to move at
constant velocity, something must be pushing on it. When the pushing stops, the object (perhaps gradually) comes to a stop.
Galileo’s ViewRealized that the classical view of
motion did not recognize the role of friction
If left to themselves, moving objects don’t slow down.
Newton’s First LawWhatever an object is doing, that’s what
it wants to keep doing. If left to themselves, objects will keep
doing whatever they’re doing.
Newton’s First Law
The converse is also true:If an object is not accelerating, then
there is no net force on it.
What is a “force”?
A force is an interaction between 2 objects involving a push or a pull.
Forces are vectors - they have a direction in space.
Common units of force are: pounds or Newtons
What is a “net force”?The net force on an object is the vector
sum of all of the forces that push or pull on the object.
“no net force on an object” means:
either there are no forces on the object, or:
the forces that push or pull on the object all cancel exactly.
An object in equilibrium has a net force of zero.
Newton’s First Law can be stated:
If no forces push or pull on an object, or if the forces that do push or pull on it all cancel exactly, then the object will not speed up, slow down, or change direction.
InertiaNewton’s First Law says that objects do
not accelerate spontaneously.This property of matter, which causes
objects to resist acceleration, has been named “inertia”.
Newton’s First Law is often called the Law of Inertia.
Equilibrium
An object that is not accelerating is said to be “in equilibrium.”
If an object is at rest and not accelerating, it is said to be “in static equilibrium.”
Newton’s First Law Again
If an object is in equilibrium, the net force on it is zero, and
If the net force on an object is zero, the object is in equilibrium.
Newton’s Second LawIf there is a net force on an object,
the object accelerates.Its acceleration is directly
proportional to the net forceIts acceleration is inversely
proportional to the object’s massIts acceleration is in the same
direction as the net force.
“directly proportional” means: If the net force doubles, the acceleration
doubles. If the net force triples, the acceleration
triples. If the net force is half as much, the
acceleration is half as much.Etc.
“inversely proportional” means:
If the object’s mass doubles, its acceleration will be half as much.
If the object’s mass triples, its acceleration will be one-third as much.
If the object’s mass is half as much, its acceleration doubles.
Etc.
What is “mass”?Mass measures the inertia of an object.All objects made of matter have inertia -
that is, they resist accelerations (Newton’s First Law), but some objects resist more than others.
Mass is a scalar quantity.SI unit of mass is the kilogram (kg).
Preconceptions
There are 2 major preconceptions to address:Mass is not the same as weight.Force is not the same as
pressure.
Mass is not WeightMass is a property of an object that
measures how much it resists accelerating.
An object is difficult to accelerate
because it has mass.
WeightWeight is a force - an interaction
between 2 objects involving a push or a pull. One of these objects is typically VERY big - the Earth or the Moon, for instance.
Weight is NOT a property of an object.
What does weight depend on?The weight of an object depends on the
object’s mass. In fact, an object’s weight is directly
proportional to the object’s mass.The weight of an object also depends
on the object’s location. In fact, an object’s weight is directly
proportional to its free fall acceleration, g at its current location.
Weight of a 1 kg object
Since W = mg, the weight of a 1 kg object is: W = (-10 m/s2)(1 kg) = -10 N on Earth W = (-1.6 m/s2)(1 kg) = -1.6 N on the
Moon
Mass vs. WeightWe typically think that an object is
difficult to accelerate because it is
heavy (has weight) - but it is heavy because it has mass.
So, objects are difficult to accelerate
because they have mass.