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 LawObjects at rest tend to stay at rest.Objects in motion tend to stay in motion.

Newton’s First Law

If there is no net force on an object, the object won’t accelerate.

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 an “object”?

Anything made of matter is an “object”.

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.

“object won’t accelerate” means:

the object:won’t speed up.won’t slow down.won’t change direction.

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).

Newton’s Second Law In symbols:

a = Fnet

m

Fnet = ma

Fnet

m a

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 In symbols:

Fg = mg Fg

m g

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.

Newton’s Third Law

For every action, there is an equal and opposite reaction.

What are “action” and “reaction”?

“Action” and “Reaction” are names of forces.

“For every action…reaction”

“For every action force, there is … a reaction force” means:Forces ALWAYS occur in pairs.Single forces NEVER happen.