Purpose: To further explore Newton's Second Law
Introduction: You will now have the opportunity to further explore Newton's
Second Law using a tutorial and a concept development practice page developed
by Paul Hewitt. Newton's Second Law states that the acceleration of
an object is directly proportional to the net force acting on the
object, is in the direction of the net force, and is inversely proportional
to the mass of the object. The mathematical statement of the Second Law
is a = F/m.
Again I want to remind
you that students often hold misconceptions that can really "block"
the learning of physics concepts. Please review each section as often
as needed.
Materials: none
Procedure:
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Study the
Newton's Second Law tutorial. Review the tutorial as needed to develop
your understanding of the concepts.
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Print the Concept Development
5-2 page for your notebook. Complete the questions to the best of
your ability. You may want to refer to the information in the previous
lesson.
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After you have completed
all questions, you can check your answers by using the link at the
end of the lesson.
Newton's
Second Law Tutorial:
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Definitions:
Newton's
Second Law states that the acceleration of an object is directly proportional
to the net force acting on the object, is in the direction of the
net force, and is inversely proportional to the mass of the object. To understand
this law, there are several terms that we must discuss. Read each definition
and accompanying comments. You may want to print this for your notebook.
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Acceleration
Acceleration is the rate
at which an object's velocity changes with time; the change in velocity
may be in magnitude (speed) or direction or both. In Kepler Kingdom
we explored this term and looked at mathematical relationships related
to this topic. We learned that acceleration is a change in motion,
but we did not study the cause of the change in motion. We will examine
the "cause" later in this activity.
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Directly
Proportional
To state that two quantities
are directly proportional to each other means that as one quantity
increases, the other quantity increases, too. It also means that as
one quantity decreases, the other quantity decreases, too. We have
previously explored this topic in Galileo Gardens with the various
graphing activities.
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Force
Most textbooks define a
force as a push or a pull. I like the definition Paul Hewitt gives
in his textbook Conceptual Physics. He defines force as
"any influence that can cause an object to be accelerated, measured
in newtons." Notice that the metric unit for force honors Sir
Isaac Newton. We have previously discussed force in our study of vector
quantities.
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Net
Force
The net force is a resultant
of all the forces that act on an object. From this definition we learn
that more than one force can act on an object at a given time. All
of these forces taken together can cause a change in motion of an
object. Remember that force is a vector quantity. The vector sum of
all of the forces acting on an object is the net force. The key word
here is "net."
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Inversely
Proportional
To state that two quantities
are inversely proportional to each other means that as one quantity
increases, the other decreases proportionally. We have previously
explored this topic in Galileo Gardens with some of the graphing activities.
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Mass
Mass is usually defined
as the quantity of matter in an object. This can be confusing because
matter is defined as anything that has mass and volume. In other words,
we are using the terms to define each other, and this is not a good
technique. I really like Paul Hewitt's definition: "Mass is the measurement
of the inertia or sluggishness that an object exhibits in response
to any effort made to start it, stop it, or change in any ways its
state of motion." He goes on to define mass as a form of energy. I
also want to mention that I once was in a discussion with several
physics teachers regarding the definition of mass. We decided that
a good definition of mass was the amount of "stuff" in an
object.
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Explaining
the Second Law
Here is the statement
of Newton's Second Law: The acceleration of an object is directly proportional
to the net force acting on the object, is in the direction of the
net force, and is inversely proportional to the mass of the object.
Let's explore the Second Law.
Image: ©2003flvs
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the
net force: the vector sum of all the forces acting on the object
Summary: The change in
motion of an object increases or decreases proportionally with the
sum of the forces acting on the object. Acceleration and net force
are directly proportional.
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is
in the direction of the force: the direction of the net force and the acceleration of the object
is the same
Summary: Net force and
acceleration are vector quantities. This means that both a magnitude
and direction are necessary for a complete description. If the net
force is "to the north," then the acceleration is "to
the north."
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and
is inversely proportional: one quantity decreases and the other increases
Image:
©2003flvs
- to
the mass of the object: the
acceleration increases if the mass decreases (assuming other conditions
are controlled)
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A
Mathematical Look at the Second Law
Newton's Second Law states
that the acceleration of an object is directly proportional to the
net force acting on the object, is in the direction of the net force,
and is inversely proportional to the mass of the object. Let's express
this in mathematical terminology.
The relationship may be
expressed with this equation: a =
F/m
Acceleration (a) is expressed
in m/s2, net force (F) is expressed in newtons, and mass
is expressed in kilograms. |
Suppose A is still a 1-kg block, but B is a low-mass feather (or a coin.). 
Suppose A is a feather or coin, and B has a mass of 1 kg. 
