Download Students discover the net forces all around them with a portable

Students discover the net
forces all around them with
a portable force indicator.
By Timothy Young and Mark Guy
S
tudents have a difficult time understanding force, especially when dealing with
a moving object. Many forces can be
acting on an object at the same time,
causing it to stay in one place or move. By
directly observing these forces, students can
better understand the effect these forces have on an
object. With a simple, student-built device called
a Fin-di (“fin dee”), students can visually see the
sum of all these forces, called the resulting force
or net force. This activity is for upper elementary
grades, but could be initiated at lower elementary
grades as a visual indication of which way pushing,
pulling, rotating, and stopping (friction) net forces
are acting on an object.
The Force Indicator (Fin-di)
An adaptation of a common physics demonstration
device, the Fin-di consists of a plastic jar containing a cork submersed in water attached to a string
(see Figure 1 for materials and instructions). The
cork in the Fin-di is buoyant in the water, similar
to a helium balloon being buoyant in air. When the
Fin-di experiences a force or acceleration, the water inside the container tends to want to stay at rest
(due to inertia). Thus, water in the moving container piles up in the back of the container, making
it denser. The cork, being lighter, is “buoyed up”
and moves away from this piled up water. Thus,
the cork moves in the direction of the motion—the
same direction as the net force and acceleration.
32 Science
Scienceand
andChildren
Children
32 Engage
To assess prior knowledge, ask students
about their experience with acceleration.
Most will say that when riding in a car or “on a
turn you get pushed to the outside.” Such experiences relate to inertia, not net forces, but are real
experiences for the students and should not be discounted. Other instances of feeling acceleration
can be introduced; for example, the feeling of being pulled off a merry-go-round, or moving back
or forward on an airplane taking off or landing.
However, once they see the Fin-di doing the opposite motion, they will be curious to understand
why. Note that the Fin-di is very sensitive, so demonstration movements should have a good separation time between starting and stopping; you
should keep the Fin-di moving as long as you can.
May the Force Be With You!
A net force is the “resulting” force or the overall force
that is acting on an object when adding individual forces. For example, in a tug-of-war, two people pull on a
rope in opposite directions. Each person applies a force
on the rope, one pulling to the left and one pulling to
the right. The net force is the resulting difference between the forces each person applies. If the person on
the left was pulling with a larger force than was the
person on the right, the net force would be to the left.
Thus, the person on the left would win, pulling the person on the right into the mud!
Two common examples of forces that can result in
net forces are pushing and pulling. The net force is
what causes the object to accelerate/decelerate, meaning there is a change in the object’s motion. A deceleration is really acceleration in the opposite direction of
motion. For example, either your hand, a motor, or a
car can push on an object forward to produce a net
force, causing an acceleration in the direction of motion. The result is the object moves forward faster and
faster. Conversely, your hand, car brakes, or friction
can produce a net force causing acceleration in the opposite direction of the motion. The result is the object
moves slower and slower. The cork in the Fin-di shows
direction of the net force which caused the change in
motion. If there is no change in the object’s speed or
motion, the cork in the Fin-di will remain upright in the
vertical position.
Show a Fin-di to the students. Describe the Fin-di
materials: peanut butter jar, cork, string, and water. Place
it on the table and ask students to predict what will happen
if you push the Fin-di forward and why. Students often
declare, “I was surprised. It moved the opposite way I
thought it would.” Repeat moving the Fin-di several
times. Have students share their initial observations, for
example:
“Wow! At the beginning, the cork goes in the direction
that you’re pushing the Fin-di.”
“If you keep speeding up the Fin-di forward, the cork
stays forward.”
“When you stop, the cork goes back.”
The Fin-di reacts to the amount of force that is applied
to it. So, if there is a small force applied by your hand the
cork will not move as much. With increasing amounts of
force, the cork can hit the side of the container.
The resulting or net force is defined as the sum of all
forces acting on an object. If all the forces exactly balance, the net force is zero, and there is no movement. If
there is a net force, then the sum of the forces is not balanced and one force will dominate. As in the tug-of-war
example, if both persons pull in opposite directions with
Figure 1.
Constructing and using the Fin-di.
photographs courtesy of the author
Teacher Background
Materials:
• Clean plastic peanut butter container (take off all
labels)
• One cork
• String
• Hot glue
• Water (Note the Fin-di also performs well with light
cooking oil; the cork moves slower and is easier to see.)
Instructions:
1. Cut cork in half, to about 2 cm. Thread one end
of the string through the center of the
half-cork. Tie knot. The teacher should
cut corks ahead of time.
2. Measure the length of the string so that the cork
will be three-quarters the height of the container
with a little extra to glue.
3. Hot glue the other end of the string to the inside of
the lid of the container.
Only a teacher or an adult should
handle a hot glue gun.
4. Fill the container to the top with water.
5. Screw on the top tightly so no water comes out!
Turn upside down.
To use the Fin-di, hold it so the container is upside
down with the cork pulling the string taut. Three simple
rules guide reading the Fin-di:
1. When the cork is in the vertical position, there is no
resulting or NET force acting on the Fin-di.
2. When the cork moves to any side of the container,
that is the direction of the resulting or NET force.
3. The degree of the movement of the cork away from
the vertical position indicates the magnitude of the
resulting or net force.
October 2011 33
Figure 2.
Figure 3.
Initial push on the Fin-di.
Continued motion.
The cork moves forward indicating a force to the right.
This net force to the right, shown in purple.
The Fin-di moves at constant velocity (i.e., no acceleration) and the cork moves to the vertical position. There
is no net force on the Fin-di.
the same force, there is no net force or movement. Only
when one person applies greater force over the other does
a net force occur resulting in movement in the direction
of the greater force.
Explore
Next, students have the opportunity to explore the actions of the Fin-di in small groups. They record their observations of the Fin-di as they move it back and forth
across the table. For open exploration purposes, the students are free to move the Fin-di in horizontal linear, circular, tilting, or rolling, and vertical motions. However,
for accurate results using the Fin-di, it must be stable in
the upright position with no tilting.
Thinking prompts for the students may include: Can
you make the cork stay forward without tilting the Fin-di?
Can you make the cork stay upright while moving?
The students convene as a class and discuss the findings
that they have experienced. Groups share a running list of
observations in class.
34 Science and Children
Explain
Students continue working with the Fin-di and complete
a data collection sheet (see NSTA Connection). Each
student pushes, with a gentle constant motion, to get the
Fin-di moving in a straight line. The students should
watch the cork closely at the instant the Fin-di moves.
The students should repeat the observation of the cork
at the beginning of the motion at least five times to understand and record on the data sheet what happens to
the cork. When a student pushes the Fin-di, the cork
will move in the direction of the applied force (Figure
2), indicating a net force in that direction. This means if
the student is pushing to the left, the cork moves to the
left, and if the motion is to the right, the cork moves to
the right.
Next, each student pushes with a gentle constant motion to get the Fin-di moving in a straight line and keep it
moving as long as possible. The students should concentrate on keeping the motion smooth, not jerking back and
forth. The students should repeat the observation of the
May the Force Be With You!
Figure 4.
Stopping.
The force of the pushing is removed. The frictional force
remains. The cork moves to the left indicating a force
to the left. The force to the left is friction, it is the net
force (arrow).
continued motion part of the movement (i.e., not the starting or stopping) at least five times. During the sustained
pushing (moving at constant speed) the Fin-di is not accelerating. The cork in the Fin-di will be vertical indicating
no acceleration, as shown in Figure 3. The student still has
to apply a force because friction is opposing the motion.
Most important, even though the Fin-di is moving, the net
force is 0. Therefore, an object in constant linear (straight
line) motion or an object at rest has no net or resulting
forces acting on it, as shown by the Fin-di.
Finally, each student pushes with a gentle constant
motion to get the Fin-di moving in a straight line motion,
and then they remove their hand so the Fin-di comes to
a stop. The students should watch the cork closely at the
instant the Fin-di slows down. The students should repeat
the observation of the cork at the end of the motion at least
five times to understand and record what happens to the
cork. As the Fin-di comes to a stop, the cork moves back,
indicating the net force is opposite the direction of motion.
This is the force of friction stopping the motion (Figure 4).
At this point, the teacher summarizes what students
have observed and possible explanations for their observations. Students expect the cork to move backward when
they push the Fin-di forward because of their experiences
with inertia, and they might be initially puzzled when the
cork quickly moves forward at the start of the motion.
One way to explain inertial “forces” is by identifying
that they are the resistance an object has to a change in its
motion or resting point. Any object or person that is accelerating will experience fictitious “forces.” These “forces” appear when the acceleration is in linear straight-line motion
or the object is moving in a circular motion (centrifugal
forces). The feeling comes from the mass or inertia that is
contained in the object. For example, the feeling of moving back in a car when accelerating from a stoplight is the
inertial force (fictitious). An observer outside the car sees
the passenger move forward faster and faster. In circular
motion, the person on a merry-go-round feels like they are
being thrown off. This is the centrifugal force (fictitious).
An observer watching from the ground sees the person
constantly changing their straight-line motion (i.e., moving in a circle) and being pulled by a force to the center.
With further demonstrations and interactive discussion, students develop a more clear understanding of the
cork’s motion as an indicator of the direction or presence of
a net force. For example, to explain that a push is a force,
the teacher could prompt students by asking, “What is a
push from a starting position called?” (a force). “Your hand
makes the Fin-di move faster and faster. The Fin-di points
in the direction you are pushing it faster and faster—the
direction of the force. When stopping, the Fin-di goes
slower and slower” “Where is the force now?” Comment
that the cork is now pointing in the opposite direction and
that the force is opposite the motion, that is why it slows
to a stop. The force causing the Fin-di to stop is the force
of friction. “Now that we have observed that pushing the
Fin-di to the right makes the cork move in the direction to
the right, how could we draw that?” If no one picks arrows,
suggest that arrows could indicate direction.
Introduce Net Force Arrows
After introducing drawn or cutout arrows to symbolize net forces, it is important to note that these
arrows do not represent speed or velocity. The
arrows represent net forces that produce a
change in speed (acceleration), meaning
the object moves “faster and faster” or
“slower and slower.” Acceleration
and force act in the same direction, either both acting to
October 2011 35
the right or both acting to the left. If a force is to the right,
the arrow should point to the right. If the force is to the
left, the arrow should point to the left. Draw on white
board or place cutout arrows on the Fin-di to indicate the
force that starts motion.
Without using the Fin-di, reiterate that pushing means
“applying a force.” This can be demonstrated by a sharp
push on a large cardboard box on the floor to get it moving. The force of pushing can be represented by an arrow
pointed in the direction of the push, this is the net force.
When the pushing stops, the box slows down. Prompt
students by asking why the box stopped. Discuss friction and explain that friction can be represented by an
arrow in the opposite direction that the students were
pushing. This is a net force in the opposite direction of
the motion. Then prompt students by saying, “We ob-
Figure 5.
Showing circular motion as a series
of pulls toward the center.
B
C
A
The blue ball is attached to a string and swung around.
If the string is released at A the blue ball will travel to
B. However in circular motion the ball travels to C. In
order to do this the ball must be pulled into the position
C. This is indicated by the force arrow in red. Circular motion is a process of continuously pulling the ball
toward the center as the ball goes around. There is a
constant force pulling the ball toward the center at all
times that it is in motion.
36 Science and Children
served that stopping the Fin-di makes the cork move in
the direction to the left, how could we draw that?” Draw
on a white board or place a cutout arrow on Fin-di to
indicate the force that stops motion.
Elaborate
In this portion of the lesson, students learn that separate,
inanimate objects “feel” the same net force as the Fin-di
when a net force is applied. The students attach the Findi with duct tape to a shoe box or other object that can
slide on the table or floor and push the box or object in a
specific straight line direction. Note that pushing on the
box, instead of the Fin-di itself, is the only change from
the previous exercise. Even though the students are not
pushing the Fin-di itself, the Fin-di feels and indicates
the net force. Students complete a data sheet (see NSTA
Connection). Teacher conclusion prompts should include, “If I take the Fin-di off the shoe box and push on
the shoe box, continue the motion, and then let it stop,
will the box feel the same net forces as we did even though
we don’t have the Fin-di attached?” “If I push any object,
continue the motion, and then let it stop, will that object
feel the same net forces the Fin-di had indicated with the
shoe box, even though the Fin-di is not attached?”
Next, students explore circular motion. Identifying net
force for circular motion may be confusing to students (and
educators). See Figure 5 for a simple explanation. With the
Fin-di, a student can visually see the direction of the net
force that is causing it, called the centripetal force. When
the Fin-di is spinning in a circle, the cork moves toward the
center, indicating there is a net force toward the center. This
can be explained by understanding the following scenario.
Suppose we are holding a string that has a ball attached.
We orbit the ball in a circle around us and the string breaks.
The ball will continue in straight-line motion from that
point on. This is true unless there is a force that acts on
it. So to keep the ball in a curved path a force is needed to
pull it toward the center. The tension in the string is what is
causing a net force to continually act on the ball. The figure
shows that if the string is cut when the ball is at position A
it will move to position B, meaning there is no net force.
But if the string holds, the ball will end up at position C,
indicating a net force toward the center.
To demonstrate this, conduct a demonstration by taping the Fin-di to one edge of a rotating stool. The students
observe Fin-di as it spins around, draw the string and cork,
and indicate the net force.
Evaluate
In this inquiry, students’ learning can be formatively assessed using a rubric (Figure 6). Students’ questions, discussion responses, and drawings of “force arrows” can all
serve as assessments of conceptual development. Specifi-
May the Force Be With You!
cally, student drawings of net force arrows indicate their
understanding of net forces associated with acceleration
of objects.
Summary
The Fin-di is a simple, easy-to-use device that instantly
gives a visual indication of the presence or absence of a
net force. The Fin-di is an engaging device that students
are drawn to immediately as they use it to investigate the
nature of forces. Students will quickly find out, much
to their amazement, that when the Fin-di experiences a
net force, the cork moves in the same direction as the net
force. They will learn that when the cork is positioned in
a straight-up, neutral position there is no net force occurring (meaning there are no individual forces or the individual forces are balanced). Students will begin to understand that the feeling that they experience during acceleration (inertia) is opposite to the direction of the net
force being applied. Teachers will witness the students’
creative thought patterns as they experiment with simple
cause-and-effect actions of the Fin-di. Teachers will also
see that students can begin to think abstractly by representing net forces with arrows that can be verified with
the Fin-di. For continued learning, teachers can ask the
students to figure out how the Fin-di works and predict
the cork’s motion in everyday movements that indicate
acceleration or no acceleration. In addition the students
can also take the Fin-di into other science classes and
present their “What do you think will happen?” findings. This amazing device will start students and teachers on a contemplation and thinking journey about why
things move or don’t move that will remain with them
for a long time. n
Connecting to the Standards
This article relates to the following National Science
Education Standards (NRC 1996):
Content Standards
Grades K–8
Standard A: Science as Inquiry
• Abilities necessary to do scientific inquiry
• Understanding about scientific inquiry
Grades 5–8
Standard B: Physical Science
• Motions and forces
National Research Council (NRC). 1996. National science education standards. Washington, DC: National
Academies Press.
Timothy Young ([email protected]) is a an associate professor of Physics and Astrophysics, and
Mark Guy is a professor of Teaching and Learning,
both at the University of North Dakota in Grand
Forks, North Dakota.
Acknowledgment
We would like to thank Brent Miller and his fifth-grade class
for piloting work with the Fin-di. We would also like to thank
Daryl Young for modeling with the Fin-di.
NSTA Connection
Download data sheets at www.nsta.org/SC1110.
Figure 6.
Assessment Rubrics for Student Drawings.
Indicating net forces with Fin-di
Target Conceptual
Development
Progressing Conceptual Development
Beginning Conceptual
Development
Net force during positive
acceleration of Fin-di
(Starting)
Position of cork and
direction of net force arrow
are drawn accurately
Either position of cork or
direction of net force arrow
are drawn inaccurately
Position of cork and net
force arrow are drawn
inaccurately
Net force during no
acceleration of Fin-di
(Continued)
Position of cork drawn
accurately. No net force
arrow is drawn.
Either position of cork or
net force arrow are drawn
inaccurately
Position of cork and net
force arrow are drawn
inaccurately
Net force during negative
acceleration of Fin-di
(Stopping)
Position of cork and
direction of net force arrow
are drawn accurately
Either position of cork or
direction of net force arrow
are drawn inaccurately
Position of cork and net
force arrow are drawn
inaccurately
October 2011 37