Content Benchmark P.8.B.1
Students know the effects of balanced and unbalanced forces on an object's motion. E/S
When asked if an object is balanced, a person may check to see whether the object is still or
wobbling back and forth. Students often realize that the occurrences discussed in physical
science are more detailed explanations of daily experiences. To help students fully grasp the
effects of balanced and unbalanced forces on an object’s motion, the content background for this
benchmark will include, motion, graphing motion, speed, velocity, acceleration, Newton’s three
laws of motion, and friction.
Motion is defined as an act or instance of moving. It can be described as any change in position.
If you raise your arms in the air, you can say that motion has occurred. Everything in the world
is moving. Even things that seem still are in motion because the atoms are vibrating. All things
on Earth including our atmosphere are rotating with the Earth.
You don’t have to actually see motion take place to prove that an object has moved. To know
whether the position of something has changed, you need a reference point. A reference point
is a specific location that can be used to compare if something is moving (i.e., if an object
changes its position relative to the reference point, then we consider that the object has moved).
Speed is the measure of motion. You can find it by dividing the distance covered by the time it
takes to travel that distance. Velocity is the speed of an object moving in a particular direction.
For more information on speed and velocity, go to
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/1DKin/U1L1d.html
Graphing Motion
A distance-time graph makes it possible to “see” the motion of an object over a period of time.
Referring to Figure 1, the green line covers the greatest distance in the least amount of time and
therefore has the greatest speed. The green line also shows a constant speed because the speed
does not vary with time (i.e., the line is straight with a constant slope).
Figure 1. Distance-Time graph for three different objects.
(From http://www.bbc.co.uk/schools/gcsebitesize/physics/forces/speedvelocityaccelerationfhrev2.shtml)
The red line shows how an object has speed changing with time. First the object has a constant
speed (shown by the straight line with the positive slope). Then the object stops moving because
the time continues to increase, but the distance stays the same (i.e., the slope of the line is zero).
The object then returns to its start position with a constant speed (straight line and negative
slope) because the distance goes back down to where it began. With students, you could say that
someone went somewhere stayed for a while and went back home.
The blue line shows an object where the velocity is continually changing or accelerating. The
object is starting off slow because it is covering a small distance in a greater amount of time and
then the distance increases substantially within a shorter time period. In this case, the object is
speeding up. In physics, acceleration is any change in velocity and occurs when an object speeds
up (positive acceleration), slows down (negative acceleration, or more commonly, deceleration),
or changes direction.
For background notes supporting interpretation of motion graphs visit
http://cmp.ameslab.gov/physics106/lecture5.pdf
The following graph shows a car traveling at a constant speed because every second its position
increases 10 meters. You can say the car is traveling 10 m/s.
Figure 2. Data and graph illustrating constant speed.
(From http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L3a.html)
The following graph shows a car accelerating because each time interval the car covers a
different amount of distance. Again, acceleration is any change in velocity, whether it goes
faster, slower, or changes direction.
Figure 3. Data and graph illustrating acceleration.
(From http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L3a.html)
For more information about motion graphs, go to
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/1DKin/U1L3a.html
Balanced and Unbalanced Forces
A force is defined as a push or a pull and more than one force can and usually does act on an
object at the same time. Every force has a certain strength or magnitude. Forces also have a
direction. The net force on an object is the sum (in both magnitude and direction) of all the
forces acting on it. If the net force is zero, the forces are balanced. Balanced forces produce no
changes in motion of an object. In the picture below, there are two forces acting upon the
person. The gravitational force exerts a downward force. The floor exerts an upward force.
Because these two forces are of equal magnitude and in opposite directions, they balance each
other. The person is at equilibrium because there is no unbalanced force acting upon the person,
and thus, the person maintains his/her state of motion (in the case the motion is zero or at rest).
Figure 4. Object at rest with no net force.
(From http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1d.html)
Objects moving at a constant velocity are also balanced and have a net force of zero. A plane at
cruise altitude has four primary forces (weight, lift, thrust and drag) that when balanced, the plan
travels at a high yet constant speed.
Figure 5. Object in motion with constant speed and no net force.
(From http://www.grc.nasa.gov/WWW/K-12/airplane/forces.html)
When the net force is greater than zero, the forces acting on the object are not balanced.
Unbalanced forces cause an object at rest to move and an object in motion to change speed
and/or direction. The animation below demonstrates an unbalanced force.
Figure 6. Objects in motion with net force.
(From http://www.schools.utah.gov/curr/science/sciber00/8th/forces/sciber/forces.htm)
For more information and sample problems on balanced and unbalanced forces, go to
http://www.schools.utah.gov/curr/science/sciber00/8th/forces/sciber/forces.htm
For a table with types of forces and descriptions for each one, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l2b.html
Friction
Friction is a force that opposes motion between two surfaces that are in contact with each other.
The amount of friction depends on two things: the type of surface and how hard they press
against each other. When we consider “how hard” two surfaces are pressing against each other,
we are really talking about the force of contact between the surfaces.
It can be beneficial to increase or decrease friction. Students lubricate the keys on their brass
instruments to decrease friction and avoid sticking. On the other hand, people can increase
friction to avoid slipping.
Figure 7. Friction in footwear.
(From http://revelsports.com/yaktrax.asp)
Three types of friction are sliding friction, rolling friction and fluid friction. Sliding friction is
the friction between two solid surfaces as shown below. If it weren’t for friction, students would
slip right off their chairs and pencils would slide right out of their hands.
Figure 8. Sliding Friction.
(From http://www.school-for-champions.com/science/friction.htm)
Rolling friction is much less than sliding friction. It is the resistance that occurs when an object
rolls. People use wheel barrels and dollies to help move large or heavy objects.
Figure 9. Rolling Friction.
(From http://www.school-for-champions.com/science/friction.htm)
Fluid friction occurs when a solid object is in contact with a fluid, such as a liquid or gas. When
a force is applied to either the object or the fluid, there is a friction force that resists the motion.
Examples where fluid friction occurs are water flowing through a hose, an airplane flying
through the atmosphere, and oil lubricating moving parts. Air resistance is a type of fluid
friction. As the force of gravity pulls an object down, fluid friction from the air pushes back up
on the object. The diagram below illustrates the amount of force acting on a skydiver throughout
their fall. If in free fall long enough, the person will reach terminal velocity when the force of air
resistance is balanced with the force of gravity. To increase fluid friction, a person uses a
parachute to increase surface area. This allows the person to “catch air”.
Figure 10. Fluid Friction.
(From http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/newtlaws/u2l3e.html)
For more information on friction and the types of friction, go to
http://www.school-for-champions.com/science/friction.htm
Newton’s Three Laws of Motion
Newton’s Three Laws of Motion are valuable for teaching and tying together force and motion
vocabulary and concepts. Vocabulary will be highlighted within this section to show where
these relationships occur.
Newton’s First Law of Motion
Newton’s First Law of Motion 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. The First Law of Motion is also known as the Law of
Inertia. Inertia is the tendency in all objects to resist any change in motion. A person can pull a
tablecloth out from under dishes because the dishes have inertia. The more mass an object has
the more inertia it has. An object in motion has inertia because it won’t stop unless an
unbalanced force acts on it. For example, if a ball were rolled down a street it would keep going
in a straight line forever until a force changes its speed or direction (in this case, friction forces
would eventually slow the ball to a stop). Acceleration is any change in speed or direction, so
you can say an object will remain at a constant speed and straight line unless a force causes it to
accelerate. And if an object has more mass, it is much harder to accelerate when an unbalanced
force is applied. Going at the same speed, it is harder to stop a large truck than a motorcycle.
The picture below is a demonstration of inertia. If a person is not wearing a seatbelt when the
car is abruptly stopped, the person will continue forward at the same speed the car was originally
going. People wear seatbelts to attach them to the car. Students are surprised to hear that when a
car is going 75 mph, everything in the car is also going 75 mph and if you’re not attached to the
car and it stops, you will continue to travel at 75 mph. All students have experienced inertia in a
car. When brakes are slammed, everyone in the car leans forward. When cars turn, our bodies
lean because inertia is the tendency to keep going straight.
Figure 11. Demonstration of inertia.
(From http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1a.html)
For examples of inertia and a home experiment, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1a.html
Newton’s Second Law of Motion
Newton’s Second Law of Motion says that net, unbalanced forces acting on an object cause the
object to accelerate in the direction of the net force. If a boy is riding a bike and the wind blows
against him, then the boy will slow down and therefore negatively accelerate or decelerate.
The acceleration is determined by the size of the force and the mass of the object. If the wind is
not a strong enough force to slow the boy down then there will be no negative acceleration. If
the mass of the boy is small, and the force of the wind is great, then there may be a large
negative acceleration. Again, it takes more force to cause a massive object to accelerate.
The relationship between acceleration, mass, and force can be written mathematically, as
follows: Force = mass x acceleration, or F = ma. This picture illustrates the relationship between
acceleration, mass, and force.
Figure 12. Newton’s 2nd Law.
(From http://www.astronomynotes.com/gravappl/s2.htm#A1.2)
For more information on Newton’s 2nd law of motion and sample problems, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l3a.html
Newton’s Third Law of Motion
Newton’s Third Law of Motion states that when one object exerts a force on a second object, the
second one exerts a force on the first that is equal in size and opposite in direction. This can also
be stated as every action has an equal and opposite reaction. When a person jumps on a
trampoline, they exert a downward force on the trampoline and the trampoline exerts an equal
force upward, sending the person into the air. A person swimming exerts a force on the water
and the water pushes back on the swimmer. Rockets illustrate Newton’s Third Law because the
burning fuel inside produces hot gases that push against the inside of the rocket and escape out
the bottom. The downward push of the gases results in a “paired” force, which is opposite in
direction, and therefore, pushes the rocket upward.
Figure 13. Action-Reaction pairs.
(From http://www.schools.utah.gov/curr/Science/sciber00/8th/forces/sciber/newton3.htm)
Content Benchmark P.8.B.1
Students know the effects of balanced and unbalanced forces on an object's motion. E/S
Common misconceptions associated with this benchmark
1. Students inaccurately assume that the lines on a distance-time graph represent a hill on
which an object is traveling.
Instead of referring to the variables to read a graph, students will look at the picture only and
make assumptions. When students look at the blue line, many incorrectly think that the
object has slowed down because the line is getting steeper and steeper. However, the blue
line actually shows an object that is accelerating, with its velocity increasing steadily (e.g., an
object in freefall). Students may incorrectly think that the green line represents an object that
couldn’t go very fast because the hill was too steep, and the red line represents an object that
didn’t have a hard time at all because for a while, the hill was flat.
When graphing the position of an object or distance traveled as a function of time, the slope
at any point on the curve gives the velocity.
Figure 1. Distance-Time graph for three different objects.
(From http://www.bbc.co.uk/schools/gcsebitesize/physics/forces/speedvelocityaccelerationfhrev2.shtml)
For more information about graphing misconceptions, go to
http://physics.montana.edu/physed/misconceptions/graphs/graphs.html
2. Students incorrectly believe that acceleration is only increasing speed.
Acceleration is often used to describe an object that is going faster, but acceleration is not
just increasing speed or going fast. Any change in velocity is acceleration. Velocity is
defined as the speed and direction of a moving body. Therefore, acceleration can be an
increase in speed, decrease in speed, or change in direction. A Ferris wheel going a constant
speed is still accelerating because it is continuously changing direction.
For more information on this misconception, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L1e.html
3. Students incorrectly believe that sustaining motion requires a continued force.
If the forces acting on an object are balanced and the object is in motion, then it will continue
in motion with the same velocity. Remember: forces do not cause motion. Forces cause
changes in motion (i.e., acceleration). It is forces that bring objects to rest, as well as
increase an object’s speed or change its direction. Students incorrectly think that force is an
inherent property within an object, commonly called force impetus.
For more information on this common misconception and other misconceptions related to
force and motion, go to http://www.physicsfirstmo.org/files/Misconceptions.pdf.
(From http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l1b.html#Misconception)
Content Benchmark P.8.B.1
Students know the effects of balanced and unbalanced forces on an object's motion. E/S
Sample Test Questions
Use the graph to answer questions 1-3
1. What two variables are used to calculate speed?
a. mass and time
b. mass and velocity
c. distance and time
d. distance and velocity
2. What was the speed of object A at five seconds?
a. 3 m/s
b. 5 m/s
c. 15 m/s
d. 45 m/s
3. Which statement is true about the graph?
a. Object B traveled at a constant speed
b. Object A traveled 30 meters in 10 seconds
c. Object A was accelerating
d. Object B traveled 15 meters in 10 seconds
4. Which of the following statements about acceleration is NOT true?
a. acceleration occurs when objects change direction
b. acceleration occurs when objects are only going fast
c. acceleration occurs when objects are increasing speed
d. acceleration occurs when objects are decreasing speed.
5. Mass is the amount of matter in an object, whereas weight depends on
a. friction
b. speed
c. velocity
d. gravity
6. A car parked on a hill may start to roll back because of
a. an unbalanced force
b. a frictional force
c. a balanced force
d. an inertial force
7. It requires more force to throw a basketball 10 m than to throw a tennis ball 10 m because
a. the basketball has a greater volume
b. the tennis ball is smaller
c. the basketball has more mass
d. the basketball has more surface area
8. A girl riding on a skateboard at 3 m/s suddenly hits the curb. The girl will
a. fall back with a greater speed than 3 m/s
b. fall forward at a greater speed than 3 m/s
c. fall back at a speed of 3m/s
d. fall forward at a speed of 3m/s
9. A bat strikes a ball into the outfield. If the action force is the bat hitting the ball, the
reaction force is
a. the ball hitting the bat
b. the ball hitting the ground
c. the ball flying in the air
d. the bat being dropped
10. To increase friction someone could
a. use wax on a surfboard
b. add grease to gears on a bike
c. throw sand on an icy driveway
d. oil a squeaky door
Content Benchmark P.8.B.1
Students know the effects of balanced and unbalanced forces on an object's motion. E/S
Answers to Sample Test Questions
1. (c)
2. (a)
3. (b)
4. (b)
5. (d)
6. (a)
7. (c)
8. (d)
9. (a)
10. (c)
Content Benchmark P.8.B.1
Students know the effects of balanced and unbalanced forces on an object's motion. 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. Amusement Park Physics
This website explains roller coasters and allows students to design and test their own.
The roller coasters that students create are rated for safety and fun (to factors directly
related to force and motion). The students have to select the heights of the hills, the
slopes and even the shape of the loop. This website is a great tool for discussing forces
and why certain designs will not work. Have students do the activity and print out their
ratings to share with the class!
You can access this interactive site at
http://www.learner.org/interactives/parkphysics/coaster.html
2. The Physics of Driving
Go to this site to find explanations of physics concepts such as Newton’s Laws of
Motion, momentum, and kinetic energy, and how they relate to driving. The website also
includes online quizzes, experiments and demonstrations with pictures and video clips.
Several links are provided for more physics websites.
You can access this valuable resource at http://codmanacademy.org/branches/pod105/
3. Skateboard Science
This site is great for students who love to skateboard. The students can read about the
physics of skateboarding and all the different forces involved. Arrows are used on the
real life pictures to show the different forces. The site also includes an activity to
demonstrate how riders turn in midair.
This site is found at http://www.exploratorium.edu/skateboarding/trick.html
4. Smile Program Physics Index
On this site you can find almost 200 single concept lessons. Each lesson has objectives,
materials and procedures. The lessons on this site are hands-on and can be copied and
used in the classroom. The mechanics section has many interesting lessons on force and
motion.
To access this site, go to http://www.iit.edu/~smile/physinde.html
5. The Physics Classroom Tutorial
On this website, you will find everything you need in order to gain deeper understanding
to better teach the students, animations and pictures for PowerPoint slides, as well as
misconceptions and practice questions. The tutorial is written for introductory physics
students. Each unit is broken up into lessons and sub lessons. The graphics are
informative and the language is easy to understand.
To start getting a lot of good information, go to
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/newtltoc.html