Concept Summary Batesville High School Physics. Newton’s Second Law If there is a net force on an...
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Transcript of Concept Summary Batesville High School Physics. Newton’s Second Law If there is a net force on an...
Concept Summary
Batesville High School Physics
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:
W = mg Wm g
Weight of a 1 kg object
Since W = mg, the weight of a 1 kg object is: W = (9.8 m/s2)(1 kg) = 9.8 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.
Force is not Pressure
Force determines how much an object will accelerate.
Pressure determines how that acceleration will feel.
What does pressure depend on?
The pressure exerted on an object depends on:
The force exerted on the object. In fact, pressure is directly proportional to
force.
The area over which the force is applied. In fact, pressure is inversely proportional
to area.
Pressure Units
A force of 1 N applied over an area of 1 m2 exerts a pressure of 1 Pascal.
Another common unit of pressure is pounds per square inch (lb/in2).
Pressure
In symbols:
P = FA
FAP
The End.