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Newton's Laws. Aristotle’s View. Two 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.
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Aristotle’s View • Two 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 View • Realized 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 Law • Whatever 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 • Objects 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.
Inertia • Newton’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 Law • If there is a net force on an object, the object accelerates. • Its acceleration is directly proportional to the net force • Its acceleration is inversely proportional to the object’s mass • Its 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: Fnet a = Fnet m a m Fnet = ma
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 Weight • Mass is a property of an object that measures how much it resists accelerating. • An object is difficult to accelerate because it has mass.
Weight • Weight 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 g m
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. Weight • We 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.
