Performance Benchmark P.12.B.1
Students know laws of motion can be used to determine the effects of forces on the motion
of objects. E/S
Sir Isaac Newton developed his Three Laws of Motion from centuries of thought and
observation. In a letter to Robert Hook, Newton wrote “If I have seen further, it is by
standing on the shoulders of giants.” In particular two “giants” that helped Newton
develop his work were the famous scientists Aristotle and Galileo. To understand these
two scientists, is to understand Newton and his laws of motion.
For more information about the physics of Aristotle versus Galileo, go to
http://csep10.phys.utk.edu/astr161/lect/history/aristotle_dynamics.html
Newton’s First Law of Motion (Law of Inertia)
Newton’s First Law states that an object at rest remains at rest, and an object in motion
continues in motion at a constant velocity in a straight line, unless acted upon by an
external force or unbalanced force. An external force or unbalanced force is crucial for
students to comprehend. Below are two illustrations of forces acting on a book in a
balanced and unbalanced state.
Figure 1: Forces (from http://www.physicsclassroom.com/Class/newtlaws/U2L1d.html)
)
For example, Marks’s car is stuck in a snowdrift, so he asks Bob sitting in the passenger
seat to push him out of the snow. He agrees and starts pushing as hard as he can on the
dashboard; yet the car doesn’t move. Bob, in this example, is considered the internal
force. In order for the car to move, he should have stepped out of the car and pushed from
there; thus becoming the external force needed to cause the car to move.
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Figure 2: Newton’s 1st Law (from http://www.physicsclassroom.com/Class/newtlaws/U2L1a.html)
An object resisting a change in its “natural state of motion” (stopped or moving in a
straight line) is what Newton referred to as inertia. This is why Newton’s First Law of
Motion may as well be coined the Law of Inertia; the resistance an object has to a change
in its state of motion.
To learn more about Newton’s First Law, go to
http://www.astronomynotes.com/gravappl/s2.htm#A1.1
Newton’s Second Law of Motion
Sir Isaac Newton wrote his three laws of motion in his book in a specific order being that
each one builds upon the each other. Newton’s First Law stated that an object at rest will
remain at rest, and an object in motion will continue in motion at a constant velocity in a
straight line, unless acted upon by an external force or unbalanced force. Thus, the First
Law describes what will occur if there is no force. However, Newton’s Second Law
describes what will happen if there is an external and unbalanced force.
Newton’s Second Law states when an external, unbalanced force acts on an object,
the object will accelerate in the same direction as the force. The acceleration varies
directly as the force, and inversely as the mass. This in itself may be a bit confusing for
the students. So, present it to them using an equation.
When an external, unbalanced for acts on an object, the object will accelerate in the same
direction as the force. For example, the object might be moving to the right, while a force
is pushing it to the left causing the object to slow down. Its acceleration is in the direction
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of the force, which is to the left, but it is still moving to the right. The acceleration varies
directly as the force, which means that if the force increases, the acceleration will also
increase and vice versa if the force decreases, the acceleration will also decrease. For
example, push something harder and it will accelerate more. They are directly dependent
on each other. Though acceleration and force may vary directly; acceleration inversely
varies with mass. This means that if the mass is larger, the acceleration is less and vice
versa if the mass if less, the acceleration is more. In other words, if something has less
mass, it is easier to make it move faster. They depend inversely on each other. This may
be written mathematically as shown below:
aF
m
Figure 3: Newton’s 2nd Law (from http://www.astronomynotes.com/gravappl/s2.htm#A1.2)
Figure 4: Newton’s 2nd Law (from http://hyperphysics.phy-astr.gsu.edu/hbase/newt.html#nt3)
To learn more about Newton’s Second Law, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l3a.html
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Newton’s Third Law of Motion (Action-Reaction)
To review, Newton’s First Law describes what happens when there is no force. His
Second Law describes what happens when there is a force. And lastly, his Third Law
describes what happens when objects interacting.
Newton’s Third Law states that for every action force, there is an equal and opposite
reaction force. This law is also known as the Law of Action-Reaction Pair. A force is a
push or pull upon an object, which results from its interaction with another object.
According to Newton, whenever object A and object B interact with each other; they
exert forces upon each other both equal in magnitude and opposite in direction. For
example, when sitting in a chair, your body exerts a downward force on the chair and the
chair exerts an upward force on your body. These two forces are called action-reaction
pair because they always come in pairs.
Figure 5: Action-Reaction Pair Forces (from http://www.physicsclassroom.com/Class/newtlaws/U2L4a.html)
An important concept to illustrate when looking at action-reaction pairs is that the two
forces are acting on different objects, not on the same object. For example, have the
students stand on the ground and identify the action-reaction pair forces. The students are
pushing on the ground with a force due to gravity (Fg down) and the ground is pushing
upon them (FN up). The FN is the normal force that balances out the force due to gravity
down. It is always perpendicular to the surface the object is on.
Lastly, action-reaction pair forces may either be in direct contact or action-at-a-distance
force. Here are some examples of action-reaction forces that depend on the objects being
in direct contact, meaning that the two objects involved are touching each other to exert
forces in equal magnitudes and opposite directions.
1. The baseball forces the bat to the right (an action); the bat forces the ball to
the left (the reaction).
2. Athlete pushes bar upward (an action); the bar pushes athlete downwards (the
reaction).
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Here Figure
are some
examples
of(from
action-reaction
pairs occurring without friction, or even
6: Contact
Forces
http://www.physicsclassroom.com/Class/newtlaws/U2L4b.html)
without direct contact, known as action-at-a-distance force.
1. A rocket pushes out exhaust (an action); the exhaust pushes the rocket forward
(the reaction).
2. The earth pulls down on a ball (an action); the ball pulls up on the earth (the
reaction).
3. If I push on a lawn mower, it pushes back on me with an equal, but opposite
force. Explain why we don’t both just stay still.



The forces are acting on different bodies (and there are other forces to
consider).
It doesn’t matter to the lawn mower that there is a force on me… all that
matters to the lawn mower is that there is a force on it, so it starts to move!
Another action-reaction pair you need to consider is that I am pushing
backwards on the ground, and it pushes forwards on me.
Figure 7: Action-Reaction Pair
(from http://www.studyphysics.ca/newnotes/20/unit01_kinematicsdynamics/chp05_forces/lesson17.htm)
To learn more about Newton’s Third Law, go to
http://theory.uwinnipeg.ca/mod_tech/node24.html
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Performance Benchmark P.12.B.1
Students know laws of motion can be used to determine the effects of forces on the motion
of objects. E/S
Common misconceptions associated with this benchmark:
1. Students have the incorrect idea that sustaining motion requires a continued
force.
Sir Isaac Newton built on Galileo’s thoughts about objects in motion. Newton’s First Law
clearly states that a force is not needed to keep an object in motion. Slide a physics book
across a tabletop and watch it slide to a rest position. The book in motion on the tabletop
does not come to a rest position because of the absence of a force, rather the presence of a
force, a force being the force of friction. The force of friction is what brings the book to a
rest position. In the absence of friction, the book would continue in motion with the same
speed and direction forever or at least until the end of the tabletop. Thus, a force is not
required to keep any object horizontally moving in motion.
To learn more about this misconception, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1b.html#Misconception
2. Students incorrectly think that if an object has a speed of zero (even
instantaneously), it has no acceleration, and they also incorrectly believe that
the “natural motion” for objects is to be at rest.
Aristotle said that if you stop pushing an object, it would stop moving or come to rest. He
as well believed that “at rest” was the natural state for any object. Unfortunately both
Galileo and Newton proved Aristotle to be incorrect. According to Newton’s First Law of
Motion, also referred to as the Law of Inertia, is defined as the tendency of an object to
resist changes in its state of motion. An object at rest has zero velocity and in the absence
of an unbalanced force, it will remain with a zero velocity. It will not change its state of
motion (velocity). Thus, inertia could be redefined as the tendency of an object to resist
accelerations. For example, an object in motion with a velocity of
3 m/s, East will (in
the absence of an unbalanced force) remain in motion with a velocity of 3 m/s, East. It
will not change its state of motion (velocity). Thus, the “natural motion” for objects is not
to be at rest but to resist changes in their velocity.
To learn more about the state of motion, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1b.html
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3. Students incorrectly think that acceleration always occurs in the same direction
as the motion.
Newton’s Second Law describes objects experiencing a force. According to Newton, an
object will only accelerate if there is a net force or unbalanced force acting upon it. The
presence of an unbalanced force will accelerate an object by changing its speed,
direction, or both its speed and direction. [Remember, acceleration occurs anytime an
object's speed increases, speed decreases, or direction of motion changes.] Thus, the
acceleration of an object as caused by a net force will be directly proportional to the
magnitude of the net force, in the same direction as the net force, and inversely
proportional to the mass of the object. In essence, the direction of acceleration is in the
same direction as the net force.
To learn more about acceleration and its state of motion, go to:
http://www.hypertextbook.com/physics/mechanics/acceleration/
4. Students incorrectly think that large objects exert a greater force than smaller
objects.
Force is directly proportional to mass and acceleration, according to Newton’s Second
Law of Motion. For example, imagine a ball of certain mass moving at a certain
acceleration. This ball has a certain force. Now imagine the ball becomes twice as big
(double the mass) but keep the acceleration the constant. Newton’s Second Law equation,
F=ma, says that this new ball will have twice the force of the original ball. Now imagine
the original ball moving at twice the original acceleration. Newton’s Second Law
equation, F=ma, says that this new ball will have twice the force of the original ball at its
original acceleration. In other words, if you double the mass, you double the force. If you
double the acceleration, you double the force as well. The force of an object is derived
from both its mass and acceleration. For example, something very massive (high mass)
that is changing speed very slowly (low acceleration), like a glacier, can still have a great
force. On the other hand, something very small (low mass) that is changing speed very
quickly (high acceleration), like a bullet, can still have a great force. In addition,
something very small, changing speed very slowly will have a weak force.
To learn more about this misconception and others related to force, go to
http://modeling.asu.edu/R&E/FCI.PDF
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Performance Benchmark P.12.B.1
Students know laws of motion can be used to determine the effects of forces on the motion
of objects. E/S
Sample Test Questions
1. A student hits a hockey puck which slides across a frozen lake. The force
required to keep the puck sliding at constant velocity across the ice is:
a.
b.
c.
d.
zero Newton’s.
equal to the weight of the puck.
the weight of the puck divided by the mass of the puck.
the mass of the puck multiplied by the weight of the puck.
2. If a hockey puck with twice the mass were substituted in #1. above, and hit
with the same impulse, the puck's speed would be:
a.
b.
c.
d.
twice as great.
half as great.
the same.
at rest.
3. Identify which of the following accurately describe Newton’s Second Law of
Motion.
I. Big masses are hard to accelerate. Big masses require big forces to change
speed.
II. Small masses are hard to accelerate. Small masses require large forces to
change speed.
III. Big masses are easy to accelerate. Big masses require small forces to
change speed.
IV. Small masses are easy to accelerate. Small masses require small forces to
change speed.
a.
b.
c.
d.
II and III
I only
I and IV
IV only
4. A hammer strikes a nail and drives the nail into a block of wood. If the
action force is the hammer striking the nail, the reaction force pair is
a.
b.
c.
d.
The nail striking the wood with an equal and opposite force.
The nail striking the hammer with an equal and opposite force.
The wood striking the hammer with an equal and opposite force.
The wood striking the nail with an equal and opposite force.
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5. A 10 kilograms truck traveling to the right experiences a constant force of 20
Newtons. A constant frictional force of 7 Newtons acts to the left. What is
the acceleration of the truck?
F=7N
a.
b.
c.
d.
F=20N
1.0 m/s2
1.3 m/s2
0.77 m/s2
3.0 m/s2
6. In the top picture, a physics student is pulling upon a rope which is attached to
a wall. In the bottom picture, the physics student is pulling upon a rope which
is held by the Strongman. In each case, the force scale reads 500 Newton’s.
The physics student is pulling:
a.
b.
c.
d.
with more force when the rope is attached to the wall.
with more force when the rope is attached to the Strongman.
with less force when the rope is attached to the wall.
the same force in each case.
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Students know laws of motion can be used to determine the effects of forces on the motion
of objects. E/S
Answers to Sample Test Questions
1.
2.
3.
4.
5.
6.
(a)
(b)
(c)
(b)
(b)
(d)
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Performance Benchmark P.12.B.1
Students know laws of motion can be used to determine the effects of forces on the motion
of objects. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.
1. Part II: Forces and Newton’s Second Law PhysicsQuest
The PhysicsQuest for Part II: Forces and Newton’s Second Law PhysicsQuest
is an interactive website that is maintained by Dolores Gende in which
provides the students opportunities to learn about weight, mass, and net force
(vector sum of all forces) by finding the value of individual forces of
acceleration using Newton’s Second Law equation.
You can access this interactive site at
http://physicsquest.homestead.com/quest4B.html
2. Newton’s Challenge
Newton’s Challenge consists of three simple laboratory experiments, one for
each law, which allows the students to obtain and comprehend a better
understanding of the three laws of motion. Trimpe creates the experiment
“pull the table cloth” trick for Newton’s First Law, hot wheelers carrying
various masses down a ramp to represent Newton’s Second Law, and the use
of straws and balloons to investigate Newton's Third Law by experimenting
with several variations (angles), allowing the students to construct their own
understanding of this law.
The challenges can be accessed at
http://sciencespot.net/Pages/classphys.html#Anchor9
And the worksheets for these activities are at
http://sciencespot.net/Media/newtonlab.pdf
3. Mulitmedia Physics Studio – Newton’s Laws of Motion
This site was created by Physicsclassroom.com which provides several
illustrations via multimedia animations in order to help the students to
visualize and understand Newton’s three laws of motion.




The Car and the Wall –
http://www.physicsclassroom.com/mmedia/newtlaws/cci.html
The Motorcyclist http://www.physicsclassroom.com/mmedia/newtlaws/mb.html
The Truck and the Ladder http://www.physicsclassroom.com/mmedia/newtlaws/il.html
The Elephant and the Feather – Free Fall
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

http://www.physicsclassroom.com/mmedia/newtlaws/efff.html
The Elephant and the Feather – Air Resistance
http://www.physicsclassroom.com/mmedia/newtlaws/efar.html
Skydiving
http://www.physicsclassroom.com/mmedia/newtlaws/sd.html
4. Mass, Force, and Acceleration
If you have access to an Internet Lab, Harcourt School Publishers created an
interactive game for students to utilize towards their comprehension between
mass, force, and acceleration. Students will fill out the chart to observe how
mass, force, and acceleration are related. When they are done, allow the
students the opportunity to write a rule.
To access the interactive activity, go to
http://www.harcourtschool.com/activity/newton/index.html
5. The Ramp Simulation
Physics Education Technology has developed a java applet for students to
gain a better understanding regarding Newton’s Laws of Motion. Students
will be able to explore forces, energy and work as they push household objects
up and down a ramp. They will lower and raise the ramp to see how the angle
of inclination affects the parallel forces acting on the file cabinet. Graphs will
show forces, energy and work.
To get to this applet, go to http://phet.colorado.edu/web-pages/simulationsbase.html. Once on the site, click on “Motion” in the left-hand toolbar, and
then click “The Ramp.”
6. Forces in 1 Dimension
Physics Education Technology has developed a java applet for students to
gain a better understanding regarding Newton’s laws of motion. Students will
be able to explore forces at work as they push a filing cabinet. They will
create an applied force and observe the resulting friction force and net force
acting on the cabinet. Charts will show forces, position, velocity, and
acceleration versus time. They will be able to apply their knowledge of
Newton’s three laws of motion via free body diagrams.
To get to this applet, go to http://phet.colorado.edu/web-pages/simulationsbase.html. Once on the site, click on “Motion” in the left-hand toolbar, and
then click on “Forces in 1 Dimension”
7. Newton’s Law Booklet
This site developed by NASA’s Swift Mission Education and Public Outreach
Web site, which examines Newton’s First Law by having the students
complete the following links and create an activity booklet called Newton’s
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Law book. The students will be able to take notes and track their findings
from the scientific experiments offered. Specifically, the following activities
deal with the Law of Inertia:



Activity #1: Inertia – A Body at Rest
http://swift.sonoma.edu/education/newton/newton_1/html/newton1.ht
ml
Activity #2: Inertia – A Body in Motion
http://swift.sonoma.edu/education/newton/newton_1/html/newton1.ht
ml
Activity #3: And They’re Off
http://swift.sonoma.edu/education/newton/newton_1/html/newton1.ht
ml
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