THE PHYSICS OF MOVING THINGS (AND NOT A MOMENTUM TOO SOON!)
Grades 7 - 9
Momentum is the fundamental principle behind anything that
moves.
From the large scale motions of the planets to the twisting and turning
of an acrobat, the rules of momentum are continuously obeyed. In this
introductory
lesson involving motion, both linear and angular momentum are explained
in a non-mathematical way.
Through the use of segmented video and stimulating hands-on activities,
students will become aware of the important properties of momentum and
learn
to appreciate its significance in a world of moving things. This lesson
is designed to take approximately 45 minutes.
"Bill Nye, the Science Guy: "Momentum"
Newton's Apple: "Angular Motion"
Students will be able to:
- list three examples of momentum found in their local environment;
- describe the importance of mass and velocity on momentum;
- determine what is necessary to produce the greatest amount of
momentum
within a particular system;
- define angular momentum
- explain how a figure skater can develop a rapid spin on ice;
- give three examples of how angular momentum is used in sports.
One each per pair of students:
- 1 basketball
- 1 tennis or ping pong ball
- 1 12" wooden ruler
- 1 quarter and 1 dime or similar sized washers for demonstrations
- 1 ballistics car (available from most science supply houses)
For demonstration
- 1 rotational inertia wheel
- 1 rotational inertia suitcase (optional) - directions enclosed
- 1 rotating stool or platform
- 1 set of dumbbell weights (approximately 5 lbs. each)
- 1 activity sheet per student
- momentum
- mass
- velocity
- rotation
- radius
- conservation
- axis
Give student teams the basketball and tennis or ping pong
ball.
Direct them to bounce these objects aw few times to determine which
object
rebounds the best, elicit responses. Now, direct the teams to place the
ping pong ball on top of the basketball and drop them together (prepare
for flying objects!). Have students write a brief summary of what just
happened
and list possible explanations. Use this time to collect the objects.
Ask teams to read their hypotheses. Bring up the term momentum here if
students
haven't already considered it in their explanations.
First Segment
The emphasis of this portion of the lesson is on linear momentum. This
section
will provide a god opportunity to introduce the basic physical concepts
related to j=objects moving in a straight line and the transfer of moving
energy from one object to another. To give students a specific focus for
viewing, direct them to watch this portion of the video carefully and to
listen for the two properties of momentum. Ask them to write these down
in the space provided on their worksheets. [Properties are speed
(velocity)
and weight (mass)]
Second Segment
Tell students to play close attention to the next portion of the video.
In this segment a ballistic car is demonstrated and the reason for
behavior
of the steel ball is explained. To give students a specific focus for
viewing,
tell them to carefully watch for the demonstration using a moving model
car and a steel ball.
Third Segment
CUE Newton's Apple: "Angular Momentum" (last segment in
the episode). Now that the concept of angular (circular) momentum has
been
introduced, the next portion of this lesson will show how angular
momentum
is conserved. It will also show how the conservation of angular momentum
is used in sports, specifically in the sport of figure skating. In this
section of video, Olympic skating champion Scott Hamilton will
demonstrate
how the conservation of angular momentum is used to execute many of the
stunts in figure skating.
To give students a specific focus for viewing, tell them to watch the
next
short section of video and listen carefully to the explanation of why
Scott
Hamilton can spin so fast on ice. Have students list the term on their
Activity
Sheets.
First Segment
START the "Bill Nye, the Science Guy" tape right after
the sign for "Bob's Rocks" appears at the beginning of the
program.
PAUSE after the words "The faster something s going, the more
it weighs, the more mass it has, the more momentum it has!" To check
for comprehension, ask students to list the most important properties
involved
in momentum.
Write the symbols for these properties on the blackboard.
(M=Mass) the term weight is used to simply this concept
(V=Velocity) Speed in a particular direction
Momentum is a function of Mass times Velocity.
Momentum = MV
Now the class will apply this knowledge to the problem of the bouncing
ping
pong ball. In the next segment of the video, the transfer of momentum is
explained. To continue their focus, as students to watch for an
explanation
of why the smaller ball went flying.
PLAY. STOP after the words "The bigger an object and
the faster it goes, the more momentum it will transfer!" Check for
understanding by asking for an explanation of what happened to the small
ball in the previous activity. The next activity will help if the
students
still have difficulty with this concept.
Second Segment
PLAY from "this little ball has weight and when it's moving
it has momentum."
STOP at the end of the segment, after the words,"...the ball
went up an came down without changing its momentum."
Third Segment
START after "Science of the Rich and Famous".
PAUSE after "...angular momentum, the spinning law of
physics!"
To check for understanding, have students list the term that applies to
the "spinning law of physics." RESUME PLAY and
PAUSE
after "...once it starts rotating, it wants to keep rotating in the
same direction, with the same speed!"
To check for comprehension, ask students if they noticed something wrong
with the last statement. REWIND to the beginning of the section
and
REPLAY if students missed the concluding statement. (In the video,
Scott Hamilton's speed increases dramatically.)
RESUME PLAY at "...but your speed did
change."
PAUSE after "...angular momentum stayed the same, it has to,
it's the law!"
Ask students whether this change in spin rate actually violates their
notion
of angular momentum.
RESUME PLAY and PAUSE after "...hey, you're not one of
these!"
It is important to have your students "feel" the law of
conservation
of angular momentum. Have one student sit on the rotation stool and give
him/her the dumbbells to hold. Starting with the dumbbells held away from
body, initiate a slow spin and then have the student bring the dumbbells
close to the body. The results are immediate. Have the students take
turns
and have them alternate the position of the dumbbells to speed up and
slow
down. Ask the students to consider themselves as part of a large wheel.
What measurement would be changed if the dumbbells represented the tire
portion of the wheel? (Answer: the radius)
Now go back to the conservation of momentum expression (mv=mv).
Ask
students to add the ingredient that was changing (r for radius) to each
side of the conservation of momentum expression. The expression now
becomes:
mvr (before) = mvr (after)
With the dumbbell's mass remaining constant, the only change in the
expression
is the velocity and radius. When the initial rotation (momentum)_ is
given,
the radius is large and the spin is low. This can be expressed as the
following:
mvR
After the students bring the dumbbells close to the body, of the spin
axis,
the radius gets smaller and the visible effect is a dramatic increase in
velocity. This part of the expression can be written as:
mVr
In keeping with the conservation of angular momentum, the expression can
be stated as:
mvR = mVr
Thus, the reason for the increased spin when the dumbbells are brought
close
to the spinning body is explained by the Law of Conservation of Angular
Momentum.
In the final section of the video, Scott Hamilton demonstrates how his
spins
on ice are a function of the changing radius of his body. Have the
students
pay close attention to each of the spins performed and see if they can
pick
out which part of the body is being used to change the radius.
RESUME PLAY. STOP after "...once again, angular
momentum
wins the day!" Check for comprehension by having students list at
least
three other applications of angular momentum in sports.
To summarize, ask students to describe, in their own words, the meaning
of momentum. Have them give examples of things around them with large
momentums.
(E.g.- the packed school bus that they took to school has a lot of
momentum!)
When momentum is started it wants to keep going. What examples can
students
cite for this principle? (Ask students if they've ever seen a speed
skater
fall?)_ Lastly, have the students write a summary of Scott Hamilton
initiates
any one of the spins shown in the video.
First Segment
Explain the activity and direct students to do the activity in groups or
pairs.
Place a ruler horizontally on a flat surface. Put the dime face down in
contact with one end of the ruler. Now, slide the quarter sharply up
against
the other end of the ruler. Momentum is transferred from the moving
quarter
to the stationary rulerdime system. Notice that the dime will move much
further than the ruler. It should be mentioned here that the amount of
momentum
given to the quarter should be approximately equal tot he total momentum
transferred tot he dime, minus some loss due to friction. It might be
helpful
to use letter size to demonstrate the magnitude of the behavior of the
dime.
The following is an example:
mv (before) = mv (after) for two objects with equal mass
M (quarter) v (quarter = m (dime) V (dime)
Have students reverse the process to see how much the quarter will move
when the dime is projected against the ruler.
Ask students to predict the behavior of the objects before performing the
experiment. When energy loss due to friction is considered, the momentum
of the quarter is transferred completely to the dime. When this happens,
momentum is said to be conserved.
Second Segment
1. Allow students to explore this concept further by actually using a
ballistic
car (easy to follow instructions come with the car). In the previous
section
of this lesson, emphasis was placed on the general principles behind
momentum.
In the next section, the emphasis will be placed on momentum that takes
a more or less circular path. This concept is known as angular
momentum.
2. To introduce this topic a fairly simple wheel, with some
modifications,
will be used. The bicycle wheel is modified by placing handles on each
side
of the axle. One of these handles should have a hole drilled through it
so that a small rope can be tied to it. In addition, the inner tube of
the
bicycle wheel has been filled with sand to add more mass to the portion
of the wheel furthest from the axis.
Holding the wheel with one hand on the rope ad the other on the rim of
the
wheel, challenge the students to balance the wheel in an upright
(vertical)
position using only the rope for support. Give them a few moments to try
to figure this problem out. The concept in this balancing act involves
momentum
similar to that of the ball in the ballistics car. Things that are in
motion
tend to continue that motion. Some students may realize that the wheel
must
be moving for the "trick" to work.
Before the students' frustration level becomes critical, have one student
hold the handle and rope while you spin the wheel. Have the student
slowly
and carefully release the handle, while continuing to hold the rope. As
long as the wheel is spinning, the wheel remains vertical. Allow the
students
to pas the wheel around so they can each feel the momentum.
Ask students to write a brief paragraph explaining why the
"trick"
would work only while the wheel was in motion. Given a certain amount of
momentum - speed in a particular direction and mass - the wheel continued
to travel the same way. Ask them to list any important applications of
this
motion.
3. Enlist the aid of your school's technology teacher to fashion a
similar
wheel inside a suitcase. Using a crank, or an electric drill, give the
wheel
inside the suitcase a rapid spin. Have an unsuspecting student carry the
suitcase to a point in the room that requires a turn in direction. The
result
s hilarious! The cause is angular momentum.
Third Segment
- Have your students work through the following questions:
Which has more momentum? A bus at rest, or the student running to catch
it?
Which has greater momentum when they move at the same speed? A truck or
a student running beside it?
- Have students analyze this statement put out by a national safety
committee. "When you are driving in your car and not wearing a
seatbelt,
your face is tailgating your windshield."
- To bring a huge oil tanker to a stop, its engines are usually cut
off at least twelve miles from port. Why is it so hard to stop or, for
that
matter, turn an oil tanker?
- Why is it easier to sit on a bike that is moving rather than one
that
is standing still?
- Why do tall animals usually walk with a slower stride than animals
with shorter legs?
- Which moves faster on a carnival merry-go-round - the inside horse
or the outside horse?
- When a spinning skater spreads his/her arms outward, does the speed
at which he/she spins decrease? Does his/her angular momentum decrease?
- Explain how a gyro is able to keep its balance.
- Take a field trip to an amusement park. Student pairs (or groups)
are to locate and describe examples of momentum and angular momentum used
for rides and games. For each right answer the teams get 1 point, 2
points
if their answer is unique and no other team has it; -1 point if the
answer
is wrong.
MATH:
Compute the changes in radius vs/ velocity for the turntable
experiment.
SOCIAL STUDIES:
Research an application of these physics principles and create a display
showing how physics is used in invention.
Master Teacher: Gib Brown
Mountain Lake Public Broadcasting/ Plattsburgh, NY
Lesson Plan Database
NTTI
Thirteen Ed Online
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